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Research article Special Issues

Uniqueness of entire solutions to quasilinear equations of p-Laplace type

  • We prove the uniqueness property for a class of entire solutions to the equation

    {divA(x,u)=σ,u0in Rn,lim inf|x|u=0,

    where σ is a nonnegative locally finite measure in Rn, absolutely continuous with respect to the p-capacity, and divA(x,u) is the A-Laplace operator, under standard growth and monotonicity assumptions of order p (1<p<) on A(x,ξ) (x,ξRn); the model case A(x,ξ)=ξ|ξ|p2 corresponds to the p-Laplace operator Δp on Rn. Our main results establish uniqueness of solutions to a similar problem,

    {divA(x,u)=σuq+μ,u0in Rn,lim inf|x|u=0,

    in the sub-natural growth case 0<q<p1, where μ,σ are nonnegative locally finite measures in Rn, absolutely continuous with respect to the p-capacity, and A(x,ξ) satisfies an additional homogeneity condition, which holds in particular for the p-Laplace operator.

    Citation: Nguyen Cong Phuc, Igor E. Verbitsky. Uniqueness of entire solutions to quasilinear equations of p-Laplace type[J]. Mathematics in Engineering, 2023, 5(3): 1-33. doi: 10.3934/mine.2023068

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  • We prove the uniqueness property for a class of entire solutions to the equation

    {divA(x,u)=σ,u0in Rn,lim inf|x|u=0,

    where σ is a nonnegative locally finite measure in Rn, absolutely continuous with respect to the p-capacity, and divA(x,u) is the A-Laplace operator, under standard growth and monotonicity assumptions of order p (1<p<) on A(x,ξ) (x,ξRn); the model case A(x,ξ)=ξ|ξ|p2 corresponds to the p-Laplace operator Δp on Rn. Our main results establish uniqueness of solutions to a similar problem,

    {divA(x,u)=σuq+μ,u0in Rn,lim inf|x|u=0,

    in the sub-natural growth case 0<q<p1, where μ,σ are nonnegative locally finite measures in Rn, absolutely continuous with respect to the p-capacity, and A(x,ξ) satisfies an additional homogeneity condition, which holds in particular for the p-Laplace operator.



    We prove the uniqueness property for a class of reachable solutions to the equation

    {Δpu=σ,u0in Rn,lim inf|x|u=0, (1.1)

    where σ0 is a locally finite Borel measure in Rn absolutely continuous with respect to the p-capacity, and Δpu=div(u|u|p2) (1<p<) is the p-Laplace operator.

    More general A-Laplace operators divA(x,u) in place of Δp, under standard growth and monotonicity assumptions of order p on A(x,ξ) (x,ξRn), are treated as well (see Section 2). All solutions u of (1.1) are understood to be A-superharmonic (or, equivalently, locally renormalized) solutions in Rn (see [19] and Section 3 below).

    We often use bilateral global pointwise estimates of solutions to (1.1) obtained by Kilpeläinen and Malý [20,21] in terms of the Havin–Maz'ya–Wolff potentials (often called Wolff potentials) W1,pσ. Criteria of existence of solutions to (1.1), which ensure that W1,pσ, can be found in [33] (see also Section 3 below).

    We remark that existence and uniqueness results are known for certain classes of solutions to quasilinear equations with A-Laplace operators similar to (1.1) in arbitrary domains ΩRn (not necessarily bounded), but with various additional restrictions on A(x,ξ) and data σ. We refer to [2] for σL1(Ω), and [25] for measures σ with finite total variation in Ω. Notice that in general only local analogues of the Kilpeläinen and Malý pointwise estimates are known for solutions in domains ΩRn. For our purposes, we need global pointwise estimates, which at the moment are available only for Ω=Rn, or in the case p=2 for linear operators div(A(x)u) in terms of positive Green's functions in domains ΩRn.

    In Section 4, we prove uniqueness of nontrivial reachable solutions to the problem

    {Δpu=σuq+μ,u0in Rn,lim inf|x|u=0, (1.2)

    in the sub-natural growth case 0<q<p1, where μ,σ are nonnegative locally finite measures in Rn absolutely continuous with respect to the p-capacity. We observe that such a uniqueness property generally fails in the case qp1.

    When we treat the uniqueness problem for solutions of equations of type (1.2), but with more general A-Laplace operators divA(x,) in place of Δp, we impose the additional homogeneity condition A(x,λξ)=λp1A(x,ξ), for all ξRn and λ>0 (see Section 4). We emphasize that our proof of uniqueness for reachable solutions of such equations relies upon the homogeneity of order p1 of the A-Laplacian, as well as homogeneity of order q of the term σuq, for 0<q<p1. Our main tool in this proof is provided by bilateral pointwise estimates for all entire solutions obtained recently in [37], which do not require the homogeneity of the A-Laplacian.

    We observe that in the case p=2 all superharmonic solutions of Eqs (1.1) and (1.2) are reachable, and hence unique. An analogue of this fact is true for more general equations with the linear uniformly elliptic A-Laplace operator div(A(x)u), with bounded measurable coefficients AL(Rn)n×n, in place of Δ. In other words, all entire A-superharmonic solutions to such equations are unique. For similar problems in domains ΩRn and linear operators with positive Green's function satisfying some additional properties (in particular, in uniform domains) the uniqueness property was obtained recently in [38].

    The uniqueness of nontrivial bounded (superharmonic) solutions for (1.2) in the case p=2 was proved earlier by Brezis and Kamin [7]. For solutions uC(¯Ω) in bounded smooth domains ΩRn and μ,σC(¯Ω), along with some more general equations involving monotone increasing, concave nonlinearities on the right-hand side, the uniqueness property was originally established by Krasnoselskii [23,Theorem 7.14].

    As shown below, for p2, all p-superharmonic solutions u to (1.1) or (1.2) are reachable, and hence unique, if, for instance, the condition lim inf|x|u=0 in (1.1) or (1.2), respectively, is replaced with lim|x|u=0. See Sections 3 and 4, where we discuss this and other conditions that ensure that all solutions are reachable.

    Existence criteria and bilateral pointwise estimates for all A-superharmonic solutions to (1.2) were obtained in [37]. (See also earlier results in [9] involving minimal solutions in the case μ=0.) In particular, it is known that the measure σ is necessarily absolutely continuous with respect to the p-capacity provided there exists a nontrivial u0 such that Δpuσuq ([9], Lemma 3.6).

    We remark that the proofs of the main existence results in [9,37] for (1.2) in the case μ=0 used a version of the comparison principle ([9], Lemma 5.2) that contained some inaccuracies. A corrected form of this comparison principle is provided in Lemma 4.1 below. The other parts of [9,37] are unaffected by this correction.

    With regards to the existence problem, we prove additionally that we can always construct a reachable solution to either (1.1) or (1.2), whenever a solution to the corresponding equation exists (see Theorem 3.10 and Remark 4.7 below).

    Let ΩRn, n2, be an open set. By M+(Ω) we denote the cone of nonnegative locally finite Borel measures in Ω, and by M+b(Ω) the subcone of finite measures in M+(Ω). For μM+(Ω), we set μM+(Ω)=μ(Ω) even if μ(Ω)=+. The space of finite signed Borel measures in Ω is denoted by Mb(Ω). By μMb(Ω) we denote the total variation of μMb(Ω).

    Let A:Rn×RnRn be a Carathéodory function in the sense that the map xA(x,ξ) is measurable for all ξRn, and the map ξA(x,ξ) is continuous for a.e. xRn. Throughout the paper, we assume that there are constants 0<αβ< and 1<p<n such that for a.e. x in Rn,

    A(x,ξ)ξα|ξ|p,|A(x,ξ)|β|ξ|p1,ξRn,[A(x,ξ1)A(x,ξ2)](ξ1ξ2)>0,ξ1,ξ2Rn,ξ1ξ2. (2.1)

    In the uniqueness results of Sec. 4, we assume additionally the homogeneity condition

    A(x,λξ)=λp1A(x,ξ),ξRn,λ>0. (2.2)

    Such homogeneity conditions are often used in the literature (see [18,21]).

    For an open set ΩRn, it is well known that every weak solution uW1,ploc(Ω) to the equation

    divA(x,u)=0inΩ (2.3)

    has a continuous representative. Such continuous solutions are said to be A-harmonic in Ω. If uW1,ploc(Ω) and

    ΩA(x,u)φdx0,

    for all nonnegative φC0(Ω), i.e., divA(x,u)0 in the distributional sense, then u is called a supersolution to (2.3) in Ω.

    A function u:Ω(,] is called A-superharmonic if u is not identically infinite in each connected component of Ω, u is lower semicontinuous, and for all open sets D such that ¯DΩ, and all functions hC(¯D), A-harmonic in D, it follows that hu on D implies hu in D.

    A typical example of A(x,ξ) is given by A(x,ξ)=|ξ|p2ξ, which gives rise to the p-Laplacian Δpu=div(|u|p2u). In this case, A-superharmonic functions will be called p-superharmonic functions.

    We recall here the fundamental connection between supersolutions of (2.3) and A-superharmonic functions discussed in [18].

    Proposition 2.1 ([18]). (ⅰ) If uW1,ploc(Ω) is such that

    divA(x,u)0inΩ,

    then there is an A-superharmonic function v such that u=v a.e. Moreover,

    v(x)=esslim infyxv(y),xΩ. (2.4)

    (ⅱ) If v is A-superharmonic, then (2.4) holds. Moreover, if vW1,ploc(Ω), then

    divA(x,v)0inΩ.

    (ⅲ) If v is A-superharmonic and locally bounded, then vW1,ploc(Ω), and

    divA(x,v)0inΩ.

    Note that if u is A-superharmonic, then the gradient of u may not exist in the sense of distributions in the case 1<p21/n. On the other hand, if u is an A-superharmonic function, then its truncation uk=min{u,k} is A-superharmonic as well, for any k>0. Moreover, by Proposition 2.1(ⅲ) we have ukW1,ploc(Ω). Using this we define the very weak gradient

    Du:=limk[min{u,k}]a.e.inΩ.

    If either uL(Ω) or uW1,1loc(Ω), then Du coincides with the regular distributional gradient of u. In general we have the following gradient estimates [20] (see also [18]).

    Proposition 2.2 ([20]). Suppose u is A-superharmonic in Ω and 1q<nn1. Then both |Du|p1 and A(,Du) belong to Lqloc(Ω). Moreover, if p>21n, then Du coincides with the distributional gradient of u.

    Note that by Proposition 2.2 and the dominated convergence theorem, we have

    divA(x,u)(φ):=ΩA(x,Du)φdx=limkΩA(x,min{u,k})φdx0,

    whenever φC0(Ω), φ0, and u is A-superharmonic in Ω. It follows from Riesz's representation theorem (see [18,Theorem 21.2]) that there exists a unique measure μ[u]M+(Ω) called the Riesz measure of u such that

    divA(x,u)=μ[u]inΩ.

    In this section, we investigate the problems of existence and uniqueness of A-superharmonic solutions in the entire space Rn to the equation

    {divA(x,u)=σ,u0in Rn,lim inf|x|u=0, (3.1)

    with measures σM+(Rn) (not necessarily finite).

    There has been a lot of work addressing the existence and uniqueness problem for quasilinear equations of the form

    {divA(x,u)=σin Ω,u=0on Ω, (3.2)

    in a bounded domain ΩRn, where σL1(Ω), or, more generally, σMb(Ω); see, e.g., [2,4,5,6,10,11,12,22,25]. For arbitrary domains, including Rn, we refer to the papers [2] (for L1 data) and [25] (for data in Mb(Ω)). In these papers one can find the notions of { entropy solutions (see [2,6,22]), SOLA (solutions obtained as limits of approximations) for L1 data (see [11]), reachable solutions (see [10]), and renormalized solutions (see [12]).

    The current state of the art on the uniqueness problem for (3.2) is that most results require that σ<<capp, i.e., σ is absolutely continuous with respect to the p-capacity in the sense that σ(K)=0 for any compact set KΩ such that capp(K)=0. The p-capacity capp() is a natural capacity associated with the p-Laplacian defined by

    capp(K):=inf{Ω|h|pdx:hC0(Ω),h1 on K},

    for any compact set KΩ.

    For later use, we now recall the following equivalent definitions of a (global) renormalized solution to Eq (3.2) (see [12]). For our purposes, we shall restrict ourselves to the case σM+b(Ω) and nonnegative solutions. Recall that we may use the decomposition σ=σ0+σs, where both σ0 and σs are nonnegative measures such that σ0<<cap0, and σs is concentrated on a set of zero p-capacity.

    Definition 3.1. Let σM+b(Ω), where ΩRn is a bounded open set. Then u0 is said to be a renormalized solution of (3.2) if the following conditions hold:

    (a) The function u is measurable and finite almost everywhere, and Tk(u) belongs to W1,p0(Ω) for every k>0, where Tk(s):=min{k,s}, s0.

    (b) The gradient Du of u satisfies |Du|p1Lq(Ω) for all q<nn1.

    (c) For any hW1,(R) with compact support, and any φW1,p(Ω)L(Ω) such that h(u)φW1,p0(Ω),

    ΩA(x,Du)(h(u)φ)dx=Ωh(u)φdσ0,

    and for any φC0b(Ω) (the space of bounded and continuous functions in Ω),

    limm1m{mu2m}A(x,Du)Duφdx=Ωφdσs.

    Definition 3.2. Let σM+b(Ω), where ΩRn is a bounded open set. Then u0 is said to be a renormalized solution of (3.2) if u satisfies (a) and (b) in Definition 3.1, and if the following condition holds:

    (c) For any hW1,(R) with such that h has compact support, and any φW1,r(Ω)L(Ω), r>n, such that h(u)φW1,p0(Ω),

    ΩA(x,Du)(h(u)φ)dx=Ωh(u)φdσ0+h(+)Ωφdσs.

    Here h(+):=lims+h(s).

    Definition 3.3. Let σM+b(Ω), where ΩRn is a bounded open set. Then u0 is said to be a renormalized solution of (3.2) if u satisfies (a) and (b) in Definition 3.1, and if the following conditions hold:

    (c) For every k>0, there exists λkM+b(Ω) concentrated on the set {u=k} such that λk<<capp, and λkσs in the narrow topology of measures in Ω as k, i.e.,

    limkΩφdλk=Ωφdσs,φC0b(Ω).

    (d) For every k>0,

    {u<k}A(x,Du)φdx={u<k}φdσ0+Ωφdλk

    for all φW1,p0(Ω)L(Ω).

    We shall also need the notion of a local renormalized (nonnegative) solution on a general open set ΩRn (not necessarily bounded) associated with a measure σM+(Ω) (not necessarily finite). We recall the following equivalent definitions (see [3]), adapted to the case of nonnegative solutions.

    Definition 3.4. Let σM+(Ω), where ΩRn is an open set. Then a nonnegative function u is said to be a local renormalized solution of the equation divA(x,u)=σ, if the following conditions hold:

    (a) The function u is measurable and finite almost everywhere, and Tk(u) belongs to W1,ploc(Ω), for every k>0, where Tk(s):=min{k,s}, s0.

    (b) The gradient Du of u satisfies |Du|p1Lqloc(Ω) for all 0<q<nn1, and up1Lsloc(Ω) for all 0<s<nnp.

    (c) For any hW1,(R) with compact support, and any φW1,p(Ω)L(Ω) with compact support in Ω such that h(u)φW1,p(Ω),

    ΩA(x,Du)(h(u)φ)dx=Ωh(u)φdσ0,

    and for any φC0b(Ω) with compact support in Ω,

    limm1m{mu2m}A(x,Du)Duφdx=Ωφdσs.

    Definition 3.5. Let σM+(Ω), where ΩRn is an open set. Then a nonnegative function u is said to be a local renormalized solution of the equation divA(x,u)=σ, if u satisfies (a) and (b) in Definition 3.4, and if the following conditions hold:

    (c) For every k>0, there exists a nonnegative measure λk<<capp, concentrated on the sets {u=k}, such that λkσs weakly as measures in Ω as k, i.e.,

    limkΩφdλk=Ωφdσs,

    for all φC0b(Ω) with compact support in Ω.

    (d) For every k>0,

    {u<k}A(x,Du)φdx={u<k}φdσ0+Ωφdλk

    for all φ in W1,p0(Ω)L(Ω) with compact support in Ω.

    We now discuss solutions of (3.1) for general measures σM+(Rn). It is known that a necessary and sufficient condition for (3.1) to admit an A-superharmonic solution is the finiteness condition

    1(σ(B(0,ρ))ρnp)1p1dρρ<+; (3.3)

    (see, e.g., [33,34]). Thus, it is possible to solve (3.1) for a wide and optimal class of measures σ satisfying (3.3) that are not necessarily finite.

    We mention that (3.3) is equivalent to the condition W1,pσ(x)<+ for some xRn (or equivalently quasi-everywhere in Rn with respect to the p-capacity), where

    W1,pσ(x):=0(σ(B(x,ρ))ρnp)1p1dρρ

    is the Havin–Maz'ya–Wolff potential of σ (often called the Wolff potential); see [17,26].

    By the fundamental result of Kilpeläinen and Malý [20,21], any A-superharmonic solution u to Eq (3.1) satisfies the following global pointwise estimates,

    1KW1,pσ(x)u(x)KW1,pσ(x),xRn, (3.4)

    where K>0 is a constant depending only on n,p and the structural constants α and β in (2.1).

    Our main goal here is to introduce a new notion of a solution to (3.1) so that existence is obtained under the natural growth condition (3.3) for σ, and uniqueness is guaranteed as long as σ<<capp (see Definition 3.8 below).

    We begin with the following result on the existence of a minimal solution to (3.1) in case the measure σ is continuous with respect to the p-capacity.

    Theorem 3.6. Let σM+(Rn), where σ<<capp. Suppose that (3.3) holds. Then there exists a minimal A-superharmonic solution to Eq (3.1).

    Proof. Condition (3.3) implies that

    1(σ(B(x,ρ))ρnp)1p1dρρ<+

    for all xRn. Thus,

    {W1,pσ=}={xRn:W11,pσ:=10(σ(B(x,ρ))ρnp)1p1dρρ=}.

    This yields

    capp({W1,pσ=})=limjcapp({W1,pσ=}Bj(0))=limjcapp({xBj(0):W11,p(σ|Bj+1(0))=})=0.

    Here we used the fact that, if μM+b(Rn), then capp({W1,pμ=})=0 (see [1,Proposition 6.3.12]). It follows that σ({W1,pσ=})=0, since σ<<capp.

    Let σk (k=1,2,) be the restriction of σ to the set Bk(0){W1,pσ<k}. We then have that σk weakly converges to σ, and

    RnW1,pσkdσkkσ(Bk(0))<+.

    Hence, σkW1,p(Bk(0)) (1/p+1/p=1), and for each k>0, there exists a unique nonnegative solution ukW1,p0(Bk(0)) to the problem

    {divA(x,uk)=σkinBk(0),uk=0onBk(0). (3.5)

    If we set uk=0 in RnBk(0), then the sequence {uk} is non-decreasing, and by [33,Theorem 2.1{ }],

    ukKW1,pσ<dσa.e.

    By [20,Theorem 1.17], it follows that the function u:=limkuk is A-superharmonic in Rn. Moreover, uKW1,pσ, and consequently

    lim inf|x|u(x)Klim inf|x|W1,pσ(x)=0.

    Thus, u is an A-superharmonic solution of (3.1).

    To show the minimality of u, let v be another A-superharmonic solution of (3.1). From the construction of u, it is enough to show that ukv for any k1. To this end, let νj, j=1,2,, be the Riesz measure of min{v,j}. Since v is A-superharmonic, it is also a local renormalized solution to divA(x,v)=σ in Rn (see [19]). Hence, by a result of [3,12] and the fact that σ<<capp, we obtain

    νj=σ|{v<j}+αj

    for αjM+(Rn) concentrated in the set {v=j}.

    Using the estimate vKW1,pσ, we deduce

    νjσ|{v<j}σ|{KW1,pσ<j}σ|{W1,pσ<k}σk,

    provided j/K>k. Since ukW1,p0(Bk(0)) and min{v,j}W1,p(Bk(0)), by the comparison principle (see [9,Lemma 5.1]), we estimate

    ukmin{v,j}v,

    provided jKk. Thus, u=limkukv. This completes the proof of the theorem.

    The proof of the minimality of u above can be modified to obtain the following comparison principle.

    Theorem 3.7 (Comparison Principle). Let σ,˜σM+(Rn), where σ˜σ and σ<<capp, 1<p<n. Then u˜u, where u is the minimal A-superharmonic solution of (3.1) and ˜u is any A-superharmonic solution of (3.1) with datum ˜σ in place of σ.

    Proof. Let σk, k=1,2,, be the restriction of σ to the set Bk(0){W1,p˜σ<k}. Since σ<<capp we have that σk weakly converges to σ. Moreover, as W1,pσkW1,pσW1,p˜σ<k on the set {W1,p˜σ<k}, it follows that

    RnW1,pσkdσkkσ(Bk(0))<+.

    Hence, σkW1,pp1(Bk(0)), and for each k>0 there exists a unique nonnegative solution ukW1,p0(Bk(0)) to the problem

    {divA(x,uk)=σkinBk(0),uk=0onBk(0).

    Letting uk=0 in RnBk(0), we have that the sequence {uk} is non-decreasing, and by [33,Theorem 2.1],

    ukKW1,pσ.

    Then uk converges pointwise to an A-superharmonic solution u of (3.1) by [20,Theorem 1.17]. On the other hand, by the comparison principle of [9,Lemma 5.1], we have

    ukuk,k1,

    where uk is defined in (3.5). Hence, letting k, we get uu, which yields u=u by the minimality of u.

    We now let ˜σj (j=1,2,) be the Riesz measure of min{˜u,j}. Recall that the Riesz measure ˜σ of ˜u can be decomposed as

    ˜σ=˜σ0+˜σs,

    where ˜σ0M+(Rn), ˜σ0<<capp, and ˜σsM+(Rn) is concentrated on a set of zero p-capacity. Then by a result of [3,12], we have

    ˜σj=˜σ0|{˜u<j}+˜αj, (3.6)

    where ˜αjM+(Rn) is concentrated in the set {˜u=j}. On the other hand, since ˜σs({˜u<})=0 (see [19,Lemma 2.9]), we can rewrite (3.6) as

    ˜σj=˜σ|{˜u<j}+˜αj. (3.7)

    Now using the estimate ˜uKW1,p˜σ and (3.7), we have

    ˜σj˜σ|{˜u<j}σ|{KW1,p˜σ<j}σ|{W1,p˜σ<k}σk,

    provided j/K>k.

    Since ukW1,p0(Bk(0)) and min{˜u,j}W1,p(Bk(0)), by the comparison principle of [9,Lemma 5.1] we find

    ukmin{˜u,j}˜u,

    provided we choose a j such that jKk. Letting k, we obtain u˜u as desired.

    Theorem 3.6 justifies the existence (and hence uniqueness) of the minimal A-superharmonic solution to (3.1) provided condition (3.3) holds and σ<<capp. It is not known if condition (3.3) alone is enough for the existence of the minimal solution. It is also not known if under condition (3.3) and σ<<capp all A-superharmonic solutions to (3.1) coincide with the minimal solution. For a partial result in this direction, see Theorem 3.12 below.

    We now introduce a new notion of a solution so that uniqueness is guaranteed for all nonnegative locally finite measures σ such that σ<<capp. Our definition is an adaptation of the notion of the reachable solution of [10,Definition 2.3].

    Definition 3.8. Let σM+(Rn). We say that a function u:Rn[0,+] is an A-superharmonic reachable solution to Eq (3.1) if u is an A-superharmonic solution of (3.1), and there exist two sequences {ui} and {σi}, i=1,2,, such that

    () Each σiM+(Rn) is compactly supported in Rn, and σiσ;

    () Each ui is an A-superharmonic solution of (3.1) with datum σi in place of σ;

    () uiu a.e. in Rn.

    Remark 3.9. The notion of reachable solution was introduced in [10] for equations over bounded domains with finite measure data. It is also related to the notion of SOLA (Solution Obtained as Limit of Approximations) of [11] for L1 data over bounded domains. By () and the weak continuity result of [36], we see that σiσ weakly as measures in Rn. The extra requirement σiσ in our definition plays an important role in the proof of uniqueness in the case when the datum σ is absolutely continuous with respect to capp.

    Theorem 3.10. Suppose σM+(Rn), and suppose (3.3) holds. Then there exists an A-superharmonic reachable solution to (3.1). Moreover, if additionally σ<<capp, then any A-superharmonic reachable solution is unique and coincides with the minimal solution.

    Proof. Existence: Suppose that (3.3) holds. Then W1,pσ<+ quasi-everywhere and hence almost everywhere. For each i=1,2,, let uji be an A-superharmonic renormalized solution (see [12]) to

    {divA(x,uji)=σ|Bi(0)inBj(0),uji=0onBj(0).

    Note that σ|Bi(0)σ and σ|Bi(0)σ weakly as measures in Rn. Also, by [33], we have

    ujiKW1,p(σ|Bi(0)).

    Hence, by [20,Theorem 1.17], there exist an A-superharmonic function ui in Rn with

    uiKW1,p(σ|Bi(0))KW1,pσ<+a.e., (3.8)

    and a subsequence {ujki}k such that ujkiu and DujkiDui a.e. as k. These estimates yield that the Riesz measure of ui is σ|Bi(0) and

    lim inf|x|ui=0.

    Using again [20,Theorem 1.17] and (3.8), we find a subsequence of {ui} that converges a.e. to an A-superharmonic reachable solution u of (3.1).

    Uniqueness: We now assume further that σ<<capp. Let u be an A-superharmonic reachable solution in the sense of Definition 3.8 with approximating sequences {ui} and {σi}. Let us fix an i{1,2,}. Then there exists a positive integer N=N() such that supp(σi)BN(0). Let v be the minimal A-superharmonic solution to (3.1). Also, let vN be the minimal A-superharmonic solution to (3.1) with datum σ|BN(0) in place of σ. We have, by Theorem 3.7,

    uvvN.

    Thus, as uju a.e., it is enough to show that

    vNui. (3.9)

    Note that since σiσ and supp(σi)BN(0) we have that

    σiσ|BN(0). (3.10)

    For R>0, let 0Θ=ΘR1 be a cutoff function such that

    ΘC0(BR(0)),Θ1onBR/2(0),and|Θ|C/R.

    For any k>0, we set

    T+k(t)={tif 0tk,kif t>k,0if t<0.

    Also, for any m>0, we define the following Lipschitz function with compact support on R:

    hm(t)={1if 0|t|m,0if |t|2m,tm+2if m<t<2m,tm+2if 2m<t<m.

    As ui and vN are both local renormalized solutions (see [3,19]), we may use

    hm(ui)hm(vn)T+k(uivN)Θ,m,k>0,

    as test functions and thus obtaining

    RnA(x,Dui)[hm(ui)hm(vN)T+k(uivN)Θ]dx=Rnhm(ui)hm(vn)T+k(uivN)Θdσi,

    and

    RnA(x,DvN)[hm(ui)hm(vN)T+k(uivN)Θ]dx=Rnhm(ui)hm(vn)T+k(uivN)Θdσ|BN(0).

    Let

    =RnA(x,Dui)[hm(ui)hm(vN)T+k(uivN)Θ]dx,

    and

    =RnA(x,DvN)[hm(ui)hm(vN)T+k(uivN)Θ]dx.

    Then by (3.10) we have

    0. (3.11)

    On the other hand, we can write

    =Rn[A(x,Dui)A(x,DvN)]T+k(uivN)hm(ui)hm(vN)Θdx+Rn[A(x,Dui)A(x,DvN)]Duihm(ui)T+k(uivN)hm(vN)Θdx+Rn[A(x,Dui)A(x,DvN)]DvNhm(vN)T+k(uivN)hm(ui)Θdx+Rn[A(x,Dui)A(x,DvN)]ΘT+k(uivN)hm(ui)hm(vN)dx.

    Thus, in view of (3.11), it follows that

    {0<uivN<k}[A(x,Dui)A(x,DvN)](DuiDvN)hm(ui)hm(vN)ΘdxRn[A(x,Dui)A(x,DvN)]Duihm(ui)T+k(uivN)hm(vN)ΘdxRn[A(x,Dui)A(x,DvN)]DvNhm(vN)T+k(uivN)hm(ui)ΘdxRn[A(x,Dui)A(x,DvN)]ΘT+k(uivN)hm(ui)hm(vN)dx=:Am+Bm+Cm.

    To estimate |Am|, we observe that |hm(t)|1 and |hm(t)|1/m. Hence,

    |Am|βkm{m<ui<2m,0<vN<2m}[|Dui|p1+|DvN|p1]|Dui|ΘdxCkm{0<ui<2m,0<vN<2m}[|Dui|p+|DvN|p]Θdx.

    On the other hand, using T+2m(ui)Θ as a test function for the equation of ui and invoking condition (2.1), we estimate

    α0<ui<2m|Dui|pΘdxRnT+2m(ui)Θdσi+βRn|Dui|p1T+2m(ui)|Θ|dx.

    Since T+2m(ui)/m2, and T+2m(ui)/m converges to zero quasi-everywhere, we deduce

    limm1m0<ui<2m|Dui|pΘdx=0.

    Similarly,

    limm1m0<vN<2m|DvN|pΘdx=0.

    Hence,

    limm|Am|=0. (3.12)

    A similar argument gives

    limm|Bm|=0.

    To estimate |Cm|, we first use the pointwise bound (3.4) to obtain

    |Cm|cRAR[|Dui|p1+|DvN|p1]min{W1,p(σ|BN(0)),k}dx,

    where AR is the annulus

    AR={R/2<|x|<R}.

    Note that for R>4N we have

    W1,p(σ|BN(0))(x)=R/4[σ(Bt(x)BN(0))tnp]1p1dttRpnp1

    for all xAR. Thus,

    |Cm|cRpnp1R1AR[|Dui|p1+|DvN|p1]dxcRpnp1R1Rnp+1[(infARui)p1+(infARvN)p1]cRpnp1R1Rnp+1Rpn=cRpnp1,

    where we used the Caccioppoli inequality and the weak Harnack inequality in the second bound. This gives

    limRlim supm|Cm|=0. (3.13)

    Since hm(ui)hm(vN)1 a.e. as m, and Θ(x)1 everywhere as R, it follows from (3.12)–(3.13) and Fatou's lemma that

    {0<uivN<k}[A(x,Dui)A(x,DvN)](DuiDvN)dx0.

    Letting k, we deduce

    {uivN>0}[A(x,Dui)A(x,DvN)](DuiDvN)dx0.

    Since the integrand is strictly positive whenever DuiDvN, we infer that Dui=DvN a.e. on the set {uivN>0}.

    We next claim that the function T+k(uivN) belongs to W1,ploc(Rn) for any k>0. To see this, for any m>k, we compute

    T+k(T+m(ui)T+m(vN))=[T+m(ui)T+m(vN)]χ{0<T+m(ui)T+m(vN)<k}=[Duiχ{0<ui<m}DvNχ{0<vN<m}]χ{0<T+m(ui)T+m(vN)<k}=[Duiχ{0<ui<m}DvNχ{0<vN<m}]χ{0<mvN<k,uim,vN<m}=DvNχ{0<vN<m}χ{0<mvN<k,um,v<m}, (3.14)

    where \chi_A is the characteristic function of a set A . Thus,

    \begin{align*} \int_{\mathbb{R}^n} |\nabla T^{+}_k(T^{+}_m(u_i)-T^{+}_m(v_N))|^p \, \Theta \, dx\leq \int_{\{ m-k < v_N < m\}} |D v_N|^p \, \Theta \, dx. \end{align*}

    On the other hand, using H_{m, k}(v_N)\Theta as a test function for the equation of v_N , where

    \begin{equation*} H_{m,k}(t) = \left\{ \begin{array}{ll} 1 \quad {\text{if }} \;0\leq |t|\leq m-k, \\ 0 \quad {\text{if }}\; |t|\geq m,\\ -\frac{t}{k} + \frac{m}{k} \quad {\text{if }}\; m-k < t < m,\\ \frac{t}{k} + \frac{m}{k} \quad {\text{if }}\; -m < t < -(m-k), \end{array} \right. \end{equation*}

    we have

    \begin{align*} \frac{\alpha}{k}\int_{\{m-k < v_N < m\}} |D v_N|^p \, \Theta \, dx &\leq \int_{\mathbb{R}^n} \Theta \, d\sigma|_{B_N(0)} +\beta \int_{\mathbb{R}^n} |D v_N|^{p-1} \, |\nabla \Theta| \, dx. \nonumber \end{align*}

    Thus, for each fixed k > 0 , the sequence \{T^{+}_k(T^{+}_m(u_i)-T^{+}_m(v_N))\}_{m} is uniformly bounded in W^{1, p}_{\rm loc}(\mathbb{R}^n) . Since T^{+}_k(T^{+}_m(u_i)-T^{+}_m(v_N))\rightarrow T_k^{+}(u_i-v_N) a.e. as m\rightarrow \infty , we see that T_k^{+}(u_i-v_N)\in W^{1, p}_{\rm loc}(\mathbb{R}^n) .

    We are now ready to complete the proof of the theorem. Since T_k^{+}(u_i-v_N) = T^{+}_k(T^{+}_m(u_i)-T^{+}_m(v_N)) a.e. on the set \{ u_i < m, v_N < m\} and the two functions belong to W^{1, p}_{\rm loc}(\mathbb{R}^n) , by (3.14) we have

    \begin{align*} \nabla T_k^{+}(u_i-v_N) = \nabla T^{+}_k(T^{+}_m(u_i)-T^{+}_m(v_N)) = 0 \end{align*}

    a.e. on the set \{ u_i < m, v_N < m\} for any m > 0 . Thus, \nabla T_k^{+}(u_i-v_N) = 0 a.e. in \mathbb{R}^n , which implies the existence of a constant \kappa\geq 0 such that

    \max \{u_i-v_N, 0\} = \kappa

    a.e. in the entire space \mathbb{R}^n . Note that if \kappa\not = 0 , then u_i = v_N+\kappa in \mathbb{R}^n , which violates the condition at infinity, \liminf_{|x|\rightarrow \infty} u_i(x) = 0 . It follows that \kappa = 0 , which yields (3.9), as desired.

    The following version of the comparison principle in \mathbb{R}^n is an immediate consequence of Theorems 3.7 and 3.10.

    Corollary 3.11. Let \sigma, \tilde{\sigma} \in {\mathcal{M}}^+(\mathbb{R}^n) , where \sigma \leq \tilde{\sigma} and \sigma < < {\rm cap}_p , 1 < p < n . Let u be an \mathcal{A} -superharmonic reachable solution of (3.1), and \tilde{u} any \mathcal{A} -superharmonic solution of (3.1) with datum \tilde{\sigma} in place of \sigma . Then u \le \tilde{u} in \mathbb{R}^n .

    For \sigma\in {\mathcal{M}}^+(\mathbb{R}^n) such that that \sigma < < {\rm cap}_p , sometimes it is desirable to know when an \mathcal{A} -superharmonic solution to (3.1) is also the \mathcal{A} -superharmonic reachable solution to (3.1), and hence also the minimal \mathcal{A} -superharmonic solution to (3.1). The following theorem provides some sufficient conditions in terms of the weak integrability of the gradient of the solution, or in terms of the finiteness of the datum \sigma .

    Theorem 3.12. Let \sigma\in {\mathcal{M}}^+(\mathbb{R}^n) , where \sigma < < {\rm cap}_p . Suppose that any one of the following conditions holds:

    (ⅰ) |D u| \in L^{\gamma, \infty}(\mathbb{R}^n) for some (p-1)n/(n-1)\leq \gamma < p , where L^{\gamma, \infty}(\mathbb{R}^n) is the weak L^\gamma space in \mathbb{R}^n ;

    (ⅱ) |D u| \in L^{p}(\mathbb{R}^n) ;

    (ⅲ) \sigma \in {\mathcal{M}}^+_b(\mathbb{R}^n) .

    Then any \mathcal{A} -superharmonic solution u to the Eq (3.1) coincides with the minimal \mathcal{A} -superharmonic solution.

    Proof. Let v be the minimal \mathcal{A} -superharmonic solution of (3.1). Our goal is to show that u\leq v a.e. Let \Theta(x) = \Theta_R(x), R > 0 , T^{+}_k(t), k > 0 , and h_m(t), m > 0 be as in the proof of Theorem 3.10. Then arguing as in the proof of Theorem 3.10, with u in place of u_i and v in place of v_N , we have

    \begin{align*} &\int_{\{ 0 < u_i -v_N < k\}} \left[\mathcal{A}(x, D u)- \mathcal{A}(x, D v)\right] \cdot (D u- D v) \, h_m(u) \, h_m(v)\, \Theta \, dx\\ &\leq A_m+B_m+C_m, \end{align*}

    where now

    \begin{align} A_m = - \int_{\mathbb{R}^n} \left[\mathcal{A}(x, D u)- \mathcal{A}(x, D v)\right] \cdot D u \, h_m'(u) \, T^{+}_k(u-v) \, h_m(v) \, \Theta \, dx, \end{align}
    \begin{align} B_m = - \int_{\mathbb{R}^n} \left[\mathcal{A}(x, D u)- \mathcal{A}(x, D v)\right] \cdot D v \, h_m'(v) \, T^{+}_k(u-v) \, h_m(u) \, \Theta \, dx, \end{align}

    and

    \begin{align} C_m = - \int_{\mathbb{R}^n} \left[\mathcal{A}(x, D u)- \mathcal{A}(x, D v)\right] \cdot \nabla \Theta \, T^{+}_k(u-v) \, h_m(u) \, h_m(v) \, dx. \end{align}

    As in the proof of Theorem 3.10, we have

    \begin{equation} \lim\limits_{m\rightarrow \infty}(|A_m|+ |B_m|) = 0. \end{equation} (3.15)

    As for C_m , we have

    \begin{align*} |C_m| \leq \frac{c}{R} \int_{A_R} \left[ |D u|^{p-1} + |D v|^{p-1} \right] \, \min\{{\bf W}_{1,p}\sigma ,k\} \, dx, \end{align*}

    where, as above, A_R is the annulus

    A_R = \{R/2 < |x| < R\}.

    Suppose now that condition (ⅰ) holds. Then |D u|^{p-1} \in L^{\frac{q}{q-1}, \infty}(\mathbb{R}^n) for some q\in (p, n] . Set

    m = \frac{n(p-1)q}{n(q-1)-(p-1)q} > 0,

    and note that {\bf W}_{1, p}\sigma \in L^{m, \infty}(\mathbb{R}^n). A proof of this fact in the 'sublinear' case (p-1)q/(q-1)\leq 1 can be found in [32].

    We have that either m\leq q or m > q . In the case m\leq q , for any \epsilon > 0 we find

    \begin{align*} &\frac{1}{R}\int_{A_R} |D u|^{p-1} \min\{ {\bf W}_{1,p}\sigma, k\} dx \\ &\qquad \leq \frac{1}{R} \left\| {|D u|^{p-1}} \right\|_{L^{\frac{q}{q-1},\infty}(A_R)} \left\| {\min\{ {\bf W}_{1,p}\sigma, k\}} \right\|_{L^{q,1}(A_R)}\\ &\qquad \leq \frac{1}{R} \left\| {|D u|^{p-1}} \right\|_{L^{\frac{q}{q-1},\infty}(\mathbb{R}^n)} \left\| {\min\{ {\bf W}_{1,p}\sigma, k\}} \right\|_{L^{q+\epsilon, \infty}(A_R)} |A_R|^{\frac{\epsilon}{q(q+\epsilon)}} \\ &\qquad \leq C k^{\frac{q+\epsilon -m}{q+\epsilon}} \left\| {|D u|^{p-1}} \right\|_{L^{\frac{q}{q-1},\infty}(\mathbb{R}^n)} \left\| { {\bf W}_{1,p}\sigma} \right\|_{L^{m, \infty}(\mathbb{R}^n)}^{\frac{m}{q+\epsilon}} R^{\frac{ n \epsilon}{q(q+\epsilon)}-1}. \end{align*}

    Here we shall choose \epsilon > 0 such that

    \frac{ n \epsilon}{q(q+\epsilon)}-1 < 0.

    In the case m > q , we have

    \begin{align} &\frac{1}{R}\int_{A_R} |D u|^{p-1} \min\{ {\bf W}_{1,p}\sigma, k\} dx \\ &\qquad \leq \frac{1}{R} \left\| {|D u|^{p-1}} \right\|_{L^{\frac{q}{q-1},\infty}(A_R)} \left\| {\min\{ {\bf W}_{1,p}\sigma, k\}} \right\|_{L^{q,1}(A_R)}\\ &\qquad \leq \frac{1}{R} \left\| {|D u|^{p-1}} \right\|_{L^{\frac{q}{q-1},\infty}(\mathbb{R}^n)} \left\| { {\bf W}_{1,p}\sigma} \right\|_{L^{m, \infty}(\mathbb{R}^n)} |A_R|^{\frac{m-q}{q m}}\\ &\qquad \leq C \left\| {|D u|^{p-1}} \right\|_{L^{\frac{q}{q-1},\infty}(\mathbb{R}^n)} \left\| { {\bf W}_{1,p}\sigma} \right\|_{L^{m, \infty}(\mathbb{R}^n)} R^{\frac{(m-q)n}{q m}-1}. \end{align} (3.16)

    Note that, since q < p ,

    \frac{(m-q)n}{mq}-1 = \frac{n(p-1) - n(q-1)+ (p-1)q}{(p-1)q} -1 < 0.

    Hence, in both cases we have, for any fixed k > 0 ,

    \begin{align} \lim\limits_{R\rightarrow \infty}\frac{1}{R}\int_{A_R} |D u|^{p-1} \min\{ {\bf W}_{1,p}\sigma, k\} dx = 0, \end{align} (3.17)

    and likewise,

    \begin{align} \lim\limits_{R\rightarrow \infty}\frac{1}{R}\int_{A_R} |D v|^{p-1} \min\{ {\bf W}_{1,p}\sigma, k\} dx = 0. \end{align} (3.18)

    On the other hand, suppose now that condition (ⅱ) holds, i.e., |D u| \in L^{p}(\mathbb{R}^n) . Then {\bf W}_{1, p}\sigma \in L^{\frac{np}{n-p}}(\mathbb{R}^n) , and as in (3.16) we have

    \begin{align*} &\frac{1}{R}\int_{A_R} |D u|^{p-1} \min\{ {\bf W}_{1,p}\sigma, k\} dx \\ &\qquad \leq C \left\| {|D u|^{p-1}} \right\|_{L^{\frac{p}{p-1}}(A_R)} \left\| { {\bf W}_{1,p}\sigma} \right\|_{L^{\frac{np}{n-p}}(A_R)}, \end{align*}

    and likewise for v . Thus (3.17) and (3.18) also hold under condition (ⅱ).

    Finally, suppose that {\rm (ⅲ)} holds. For any 1 < r < \frac{n}{n-1} and \epsilon\in(0, 1) such that {\epsilon} \frac{r}{r-1} < \frac{n(p-1)}{n-p} , we have

    \begin{align*} & \frac{1}{R} \int_{A_R} |D u|^{p-1} \min\{{\bf W}_{1,p}\sigma ,k\} dx\\ &\leq k^{1-\epsilon} \frac{1}{R} \left(\int_{B_R(0)} |Du|^{(p-1)r} dx \right)^{\frac{1}{r}} \left(\int_{B_R(0)} ({\bf W}_{1,p}\sigma)^{\frac{\epsilon\, r}{r-1}} dx \right)^{\frac{r-1}{r}}\\ &\leq C k^{1-\epsilon} R^{\frac{n}{r}-1} \left(\frac{\inf\limits_{B_R(0)} u}{R}\right)^{p-1} R^{n\frac{r-1}{r}} \left(\inf\limits_{B_R(0)} u \right)^{\epsilon}, \end{align*}

    where we used the Caccioppoli inequality and the weak Harnack inequality in the last bound (see [18,Theorem 7.46]).

    Hence, using [9,Lemma 3.1] we get

    \begin{align} & \frac{1}{R} \int_{A_R} |D u|^{p-1} \min\{{\bf W}_{1,p}\sigma ,k\} dx\\ &\leq C k^{1-\epsilon} R^{n-p} \left(\int_{R}^\infty \left(\frac{\sigma(B_t(0))}{t^{n-p}}\right)^{\frac{1}{p-1}} \frac{dt}{t}\right)^{p-1} \left(\inf\limits_{B_R(0)} u \right)^{\epsilon}\\ & \leq C k^{1-\epsilon} \sigma(\mathbb{R}^n) \left(\inf\limits_{B_R(0)} u \right)^{\epsilon}. \end{align} (3.19)

    A similar inequality holds for v in place of u . Thus, we see that (3.17) and (3.18) hold under condition (ⅲ) as well.

    Now (3.17) and (3.18) yield that, for any k > 0 , we have

    \begin{align} \lim\limits_{R\rightarrow \infty} \limsup\limits_{m\rightarrow \infty} |C_m| = 0. \end{align} (3.20)

    Using (3.15) and (3.20), we deduce

    \begin{align*} \int_{\{ 0 < u -v < k\}} \left[\mathcal{A}(x, D u)- \mathcal{A}(x, D v)\right] \cdot (D u- D v) dx \leq 0 \end{align*}

    for any k > 0 . This implies Du = Dv a.e. on the set \{u-v > 0\} and, as in the proof of Theorem 3.10, in view of the condition at infinity, we deduce u\leq v a.e. as desired.

    We now provide a criterion for reachability by requiring only the finiteness of the approximating measures \sigma_i .

    Corollary 3.13. Let u be an \mathcal{A} -superharmonic solution of (3.1), where \sigma \in {\mathcal{M}}^+(\mathbb{R}^n) , and \sigma < < {\rm cap}_p . Suppose that there exist two sequences \{u_i\} and \{\sigma_i\} , i = 1, 2, \dots , such that the following conditions hold:

    {\rm (ⅰ)} each \sigma_i \in {\mathcal{M}}^+_b(\mathbb{R}^n) , and \sigma_i \leq \sigma ;

    {\rm (ⅱ)} each u_i is an \mathcal{A} -superharmonic solution of (3.1) with datum \sigma_i in place of \sigma ;

    {\rm (ⅲ)} u_i\rightarrow u a.e. in \mathbb{R}^n .

    Then u is an \mathcal{A} -superharmonic reachable solution of (3.1), and thus coincides with the minimal solution.

    Proof. By Theorem 3.12, each u_i is a reachable solution. Thus by a diagonal process argument, we see that u is also a reachable solution. Alternatively, this can also be proved by modifying the proof of the uniqueness part in Theorem 3.10, taking into account estimates of the form (3.19).

    Theorem 3.12 formally holds under the condition |D u| \in L^{\gamma, \infty}(\mathbb{R}^n) for 0 < \gamma < (p-1)n/(n-1) as in this case \sigma = 0 . The proof of this fact, especially in the case 0 < \gamma\leq p-1 , requires some results obtained recently in [31].

    Theorem 3.14. If u is an \mathcal{A} -superharmonic function in \mathbb{R}^n such that |D u| \in L^{\gamma, \infty}(\mathbb{R}^n) for some 0 < \gamma < (p-1)n/(n-1) , then \sigma = 0 where \sigma is the Riesz measure of u .

    Proof. Let Q_r(x) , r > 0 , denote the open cube Q_r(x): = x+ (-r, r)^n with center x\in\mathbb{R}^n and side-length 2r . Using \Phi\in C_0^\infty (Q_r(x)) , \Phi\geq 0 , \Phi = 1 on Q_{r/2}(x) , and |\nabla \Phi|\leq C/r , as a test function we have

    \begin{align} \sigma(Q_{r/2}(x)) \leq \frac{C}{r}\int_{Q_{r}(x)} |D u|^{p-1} dy. \end{align} (3.21)

    Thus if \gamma\in (p-1, (p-1)n/(n-1)) , for any R > 0 we use Hölder's inequality to get

    \begin{align*} \sigma(Q_{R/2}(0)) \leq \frac{C}{R}\left\| {D u} \right\|^{p-1}_{L^{\gamma,\infty}(\mathbb{R}^n)} R^{n\frac{\gamma-p+1}{\gamma}}. \end{align*}

    Note that n\frac{\gamma-p+1}{\gamma} < 1 and thus letting R\rightarrow \infty we get \sigma = 0 .

    We now consider the case 0 < \gamma\leq p-1 . Let \gamma_1 be a fixed number in (p-1), (p-1)n/(n-1)) . By [31,Lemma 2.3], for any cube Q_\rho(x)\subset\mathbb{R}^n , we have

    On the other hand, by [31,Corollary 1.3] we find

    Note that [31,Corollary 1.3] is stated for 1 < p < 3/2 but the argument there also works for all 1 < p\leq n after taking into account the comparison estimates of [14,27,28].

    Hence, it follows that

    where we used (3.21) with r = 4 \rho in the last inequality. This allows us to employ a covering/iteration argument as in [16,Remark 6.12] to obtain that

    (3.22)

    for any \epsilon > 0 .

    Thus, if 0 < \gamma < p-1 , in view of (3.21), (3.22), and Hölder's inequality, we get

    \begin{align*} \sigma(Q_{R/2}(0)) \leq \frac{C}{R}\left\| {D u} \right\|^{\frac{p-1}{\gamma}}_{L^{\gamma,\infty}(\mathbb{R}^n)} R^{n- n\frac{p-1}{\gamma}}\rightarrow 0, \end{align*}

    as R\rightarrow \infty . Hence, \sigma = 0 . The case \gamma = p-1 is treated similarly, starting with the inequality

    \begin{align*} \sigma(Q_{R/2}(0)) \leq \frac{C}{R} \left(\int_{Q_R(0)} |D u|^{(p-1)(1+\epsilon)} dx\right)^{\frac{1}{1+\epsilon}} R^{\frac{n\epsilon}{1+\epsilon}} \end{align*}

    for a sufficiently small \epsilon > 0 .

    Due to the results of [13,14,24,30] (see also [15,29]), under some additional regularity conditions on the nonlinearity \mathcal{A}(x, \xi) , one has

    \begin{equation*} |Du(x)| \leq C \left[{\bf I}_1 \sigma(x)\right]^{\frac{1}{p-1}} \quad {\rm a.e.\; } x\in\mathbb{R}^n, \end{equation*}

    provided u is an \mathcal{A} -superharmonic solution to the Eq (3.1). This gradient estimate holds in particular for \mathcal{A}(x, \xi) = |\xi|^{p-2}\xi , i.e., the p -Laplacian \Delta_p , which yields the following corollary.

    Corollary 3.15. Let \sigma \in {\mathcal{M}}^+(\mathbb{R}^n) . Suppose that one of the following conditions holds:

    \rm (ⅰ) \sigma < < {\rm cap}_p and {\bf I}_1 \sigma \in L^{s, \infty}(\mathbb{R}^n) for some n/(n-1) < s < p/(p-1) . This holds in particular if \sigma = f\in L^{t, \infty}(\mathbb{R}^n) for some 1 < t < np/(np-n+p) ;

    \rm (ⅱ) {\bf I}_1 \sigma \in L^{p/(p-1)}(\mathbb{R}^n) , i.e., \sigma is of finite energy.

    Then any p -superharmonic solution u to the equation

    \begin{equation*} \left\{ \begin{array}{ll} - \Delta_p u = \sigma, \quad u\geq 0 \quad in\; \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, u = 0, \end{array} \right. \end{equation*}

    coincides with the minimal p -superharmonic solution.

    Finally, we show that if the condition at infinity, {\liminf\limits_{|x|\rightarrow \infty}}\, u = 0 in (3.1), is replaced with the stronger one {\lim\limits_{|x|\rightarrow \infty}}\, u = 0 , then all \mathcal{A} -superharmonic solutions are indeed reachable.

    Theorem 3.16. Suppose that u is an \mathcal{A} -superharmonic solution of the equation

    \begin{equation} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u) = \sigma, \quad u\geq 0 \quad in\; \mathbb{R}^n, \\ {\lim\limits_{|x|\rightarrow \infty}}\, u = 0, \end{array} \right. \end{equation} (3.23)

    where \sigma \in {\mathcal{M}}^+(\mathbb{R}^n) , and \sigma < < {\rm cap}_p . Then u is the unique \mathcal{A} -superharmonic solution of (3.23), which coincides with the minimal \mathcal{A} -superharmonic reachable solution of (3.1).

    Proof. First notice that the condition {\lim\limits_{|x|\rightarrow \infty}}\, u = 0 in (3.23) yields, in view of (3.4),

    \lim\limits_{|x|\rightarrow \infty} {\bf W}_{1,p}\sigma (x) = 0.

    For any \epsilon > 0 , let

    \Omega_{\epsilon}: = \{x\in\mathbb{R}^n: u(x) > {\epsilon}\},

    and

    u_\epsilon: = \max\{u, \epsilon\}-{\epsilon}.

    Clearly, {\Omega}_{\epsilon} is a bounded open set, u_{\epsilon} = u-\epsilon on \Omega_{\epsilon} , and u_{\epsilon} = 0 in \mathbb{R}^n\setminus {\Omega}_{\epsilon} .

    Let v be the minimal solution of (3.1), which is also the minimal solution of (3.23), since v \le K \, {\bf W}_{1, p}\sigma , and hence {\lim\limits_{|x|\rightarrow \infty}}\, v = 0 . It is enough to show that

    \begin{equation} u_{\epsilon}\leq v \end{equation} (3.24)

    in \Omega_{\epsilon} , as this will yield that u\leq v in \mathbb{R}^n after letting {\epsilon}\rightarrow 0^{+} .

    Now by Lemma 4.1 below, to verify (3.24), it suffices to show that u_{\epsilon} is a renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u_{\epsilon}) = \sigma \quad {\text{in }} \Omega_{\epsilon}, \\ u_{\epsilon} = 0 \quad {\text{on }} \partial{\Omega}_{\epsilon}. \end{array} \right. \end{equation*}

    Note that, for any k > 0 , T_k(u_{\epsilon}) = T_{k+\epsilon}(u)-{\epsilon} . We have T_k(u_{\epsilon})\in W^{1, p}_{\rm loc} (\mathbb{R}^n) , T_k(u_{\epsilon}) is quasi-continuous in \mathbb{R}^n , and T_k(u_{\epsilon}) = 0 everywhere in \mathbb{R}^n\setminus{\Omega}_{\epsilon} . Thus T_k(u_{\epsilon})\in W^{1, p}_0({\Omega}_{\epsilon}) (see [18,Theorem 4.5]).

    As u is a local renormalized solution in \mathbb{R}^n , for every k > 0 there exists a nonnegative measure \lambda_{k+{\epsilon}} < < {\rm cap}_p , concentrated on the sets \{u = k+{\epsilon}\} , such that \lambda_{k+{\epsilon}}\rightarrow 0 weakly as measures in \mathbb{R}^n as k\rightarrow \infty . Since {\Omega}_{\epsilon} is bounded, this implies that \lambda_{k+{\epsilon}}\rightarrow 0 in the narrow topology of measures in \Omega_{\epsilon} .

    Moreover, for k > 0 ,

    \begin{align} \int_{\{u < k+{\epsilon}\}}\mathcal{A}(x, Du)\cdot\nabla\varphi dx = \int_{\{u < k+{\epsilon}\}}\varphi d\sigma_{0} + \int_{\mathbb{R}^n}\varphi d\lambda_{k+{\epsilon}}, \end{align}

    for every \varphi\in{\rm W}_{0}^{1, \, p}(\mathbb{R}^n)\cap L^{\infty}(\mathbb{R}^n) with compact support in \mathbb{R}^n . In particular, we have

    \begin{align} \int_{\{u_{\epsilon} < k\}\cap {\Omega}_{\epsilon}}\mathcal{A}(x, Du)\cdot\nabla\varphi dx = \int_{\{u_{\epsilon} < k\}\cap {\Omega}_{\epsilon}}\varphi d\sigma_{0} + \int_{\{u_{\epsilon} = k\}\cap {\Omega}_{\epsilon}}\varphi d\lambda_{k+{\epsilon}} \end{align}

    for every \varphi\in{\rm W}_{0}^{1, \, p}({\Omega}_{\epsilon})\cap L^{\infty}({\Omega}_{\epsilon}) .

    Thus, we conclude that u_{\epsilon} is a renormalized solution in {\Omega}_{\epsilon} , as desired.

    In this section, we study solutions to the equation

    \begin{equation} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u) = \sigma u^q + \mu, \quad u\ge 0 \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, u = 0, \end{array} \right. \end{equation} (4.1)

    in the sub-natural growth case 0 < q < p-1 , with \mu, \sigma \in {\mathcal{M}}^+(\mathbb{R}^n) .

    We consider nontrivial \mathcal{A} -superharmonic solutions to (4.1) such that 0 < u < \infty d \sigma -a.e., which implies u \in L^q_{{\rm loc}} (\mathbb{R}^n, \sigma) , so that \sigma u^q + \mu \in {\mathcal{M}}^+(\mathbb{R}^n) (see [37]).

    As was noted in the Introduction, \sigma < < {\rm cap}_p whenever there exists a nontrivial solution u to (4.1), for any \mu (in particular, \mu = 0 ).

    The existence and uniqueness of nontrivial reachable \mathcal{A} -superharmonic solutions to (4.1), under the additional assumption \mu < < {\rm cap}_p , are proved below. Without this restriction on \mu , the existence of nontrivial solutions, not necessarily reachable, was obtained recently in [37], along with bilateral pointwise estimates of solutions in terms of nonlinear potentials.

    We use this opportunity to make a correction in the proof of the existence property for (4.1) in the case \mu = 0 given in [9,Theorem 1.1], which used a version of the comparison principle ([9], Lemma 5.2). It was invoked in the proof of [37,Theorem 1.1] as well. Some inaccuracies in the statement of this comparison principle and its proof are fixed in the following lemma. The rest of the proofs of [9,Theorem 1.1] and [37,Theorem 1.1] remains valid with this correction. (See the proof of Theorem 4.2 below.)

    Lemma 4.1. Let {\Omega} be a bounded open set in \mathbb{R}^n . Suppose that \mu, \nu \in {\mathcal{M}}^+_b({\Omega}) , where \mu\leq \nu and \mu < < {\rm cap}_p . If u\ge 0 is a renormalized solution of

    \begin{equation} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u) = \mu \quad in\; \, \Omega, \\ u = 0 \quad on\; \, \partial{\Omega}, \end{array} \right. \end{equation} (4.2)

    and if v\ge 0 is an \mathcal{A} -superharmonic function in {\Omega} with Riesz measure \nu such that \min\{v, k\}\in W^{1, p}({\Omega}) for any k > 0 , then u\leq v a.e.

    Proof. Let \nu_j , j > 0 , be the Riesz measure of \min\{v, j\} . Since \min\{v, j\}\in W^{1, p}({\Omega}) we see that \nu_j belongs to the dual of W^{1, p}_0({\Omega}) (see [18,Theorem 21.6]). As in (3.7), we have

    \begin{equation*} \nu_j = \nu|_{\{v < j\}} + \alpha_j \end{equation*}

    for a measure \alpha_j\in {\mathcal{M}}^+_b({\Omega}) concentrated in the set \{v = j\} . Thus the measure \mu_j: = \mu|_{\{v < j\}}\leq \nu|_{\{v < j\}} \leq \nu_j for any j > 0 . This implies that \mu_j also belongs to the dual of W^{1, p}_0({\Omega}) , and hence there exists a unique solution u_j to the equation

    \begin{equation} -{\rm div}\, {\mathcal A}(x, \nabla u_j) = \mu_j, \qquad u_j\in W^{1,p}_0({\Omega}). \end{equation} (4.3)

    Then by the comparison principle (see [9,Lemma 5.1]) we find

    0\leq u_1\leq u_2 \leq \dots \leq u_j\leq \min\{v,j\}

    for any integer j > 0 . Thus there is a function \tilde{u} on {\Omega} such that 0\leq \tilde{u}\leq v a.e. and u_j\rightarrow \tilde{u} as j\rightarrow \infty . We now claim that \tilde{u} is also a renormalized solution to Eq (4.2). If this is verified then, as \mu < < {\rm cap}_p , we must have that \tilde{u} = u a.e. (see [12,25]) and thus u\leq v a.e. as desired.

    To show that \tilde{u} is the renormalized solution of (4.2), we first use T_k^{+}(u_j) , k > 0 , as a test function for (4.3) to obtain

    \begin{equation} \alpha \int_{{\Omega}} |\nabla T_k^{+}(u_j)|^p dx \leq k\mu_j({\Omega})\leq k\mu({\Omega}). \end{equation} (4.4)

    Since T_k^{+}(u_j)\rightarrow T_k^{+}(\tilde{u}) a.e. as j\rightarrow \infty , we see that T_k^{+}(\tilde{u})\in W^{1, p}_0({\Omega}) and

    \begin{equation*} \alpha \int_{{\Omega}} |\nabla T_k^{+}(\tilde{u})|^p dx \leq k\mu({\Omega}) \end{equation*}

    for any k > 0 . By [2,Lemmas 4.1 and 4.2], this yields

    \tilde{u}\in L^{\frac{n(p-1)}{n-p},\infty}({\Omega}) \quad {\text{and}} \quad D\tilde{u}\in L^{\frac{n(p-1)}{n-1},\infty}({\Omega}).

    Moreover, arguing as in Step 4 of the proof of Theorem 3.4 in [12], we see that \{\nabla u_j\} is a Cauchy sequence in measure which converges to D\tilde{u} a.e. in {\Omega} . There is no need to take a subsequence here as the limit is independent of any subsequence.

    Moreover, for any Lipschitz function h: \mathbb{R} \rightarrow \mathbb{R} such that h' has compact support and for any function \varphi\in W^{1, r}({\Omega})\cap L^\infty({\Omega}) , r > n , such that h(\tilde{u})\varphi\in W^{1, p}_0({\Omega}) , we have

    \begin{align}\int_{\Omega} \mathcal{A}(x, \nabla u_j)\cdot D\tilde{u} \, h'(\tilde{u}) \, \varphi dx + \int_{\Omega} \mathcal{A}(x, \nabla u_j) \cdot \nabla \varphi \, h(\tilde{u})dx = \int_{\Omega} h(\tilde{u}) \, \varphi d\mu_j. \end{align}

    Thus if the support of h' is in [-M, M] , M > 0 , then, using 0\leq u_j\leq \tilde{u} , we can rewrite the above equality as

    \begin{align*} \int_{\Omega} \mathcal{A}(x, \nabla T_M^{+}(u_j)) &\cdot \nabla T_M^{+}(\tilde{u}) \, h'(\tilde{u})\, \varphi dx + \int_{\Omega} \mathcal{A}(x, \nabla u_j) \cdot \nabla \varphi \, h(\tilde{u})dx\\ &\qquad = \int_{\Omega} h(\tilde{u}) \, \varphi d\mu_j = \int_{\{0\leq v < j\}} h(\tilde{u}) \, \varphi d\mu. \end{align*}

    Note that by (4.4) and [2,Lemma 4.2], we have that \nabla u_j is uniformly bounded in L^{\frac{n(p-1)}{n-1}, \infty}({\Omega}) and \nabla T^{+}_{M}(u_j) is uniformly bounded in L^{p}({\Omega}) . Thus by the Vitali Convergence Theorem, the left-hand side of the above equality converges to

    \begin{align*} &\int_{\Omega} \mathcal{A}(x, \nabla T_M^{+}(\tilde{u})) \cdot \nabla T_M^{+}(\tilde{u}) \, h'(\tilde{u}) \, \varphi dx + \int_{\Omega} \mathcal{A}(x, D \tilde{u}) \cdot \nabla \varphi \, h(\tilde{u})dx\\ &\qquad = \int_{\Omega} \mathcal{A}(x, D \tilde{u}) \cdot D\tilde{u} \, h'(\tilde{u}) \, \varphi dx + \int_{\Omega} \mathcal{A}(x, D \tilde{u}) \cdot \nabla \varphi \, h(\tilde{u})dx. \end{align*}

    On the other hand, by the Lebesgue Dominated Convergence Theorem we have

    \begin{align} \lim\limits_{j\rightarrow \infty}\int_{\{0\leq v < j\}} h(\tilde{u}) \, \varphi d\mu = \int_{{\Omega}} h(\tilde{u}) \, \varphi d\mu. \end{align}

    Thus, we get

    \begin{align*} \int_{\Omega} \mathcal{A}(x, D\tilde{u}) \cdot D\tilde{u} \, h'(\tilde{u}) \, \varphi dx + \int_{\Omega} \mathcal{A}(x, D \tilde{u}) \cdot \nabla \varphi \, h(\tilde{u})dx = \int_{{\Omega}} h(\tilde{u}) \, \varphi d\mu, \end{align*}

    which yields that \tilde{u} is the renormalized solution of (4.2) (see Definition 3.2).

    We recall that by \varkappa = \varkappa (\mathbb{R}^n) we denote the least constant in the weighted norm inequality (see [9,37])

    \begin{equation} \left(\int_{\mathbb{R}^n} |\varphi|^q \, d \sigma\right)^{\frac 1 q} \le \varkappa \, \Vert - {\rm{div}}\, \mathcal{A}(x, \nabla \varphi)\Vert^{\frac{1}{p-1}}_{{\mathcal{M}}^+(\mathbb{R}^n)}, \end{equation} (4.5)

    for all \mathcal{A} -superharmonic functions \varphi\ge 0 in \mathbb{R}^n such that {\liminf\limits_{|x| \to \infty}} \, \varphi(x) = 0 . Notice that by estimates (3.4), K^{-1} \, \varphi \le {\bf W}_{1, p} \mu \le K \, \varphi , where \mu = - {\rm{div}}\, \mathcal{A}(x, \nabla \varphi)\in {\mathcal{M}}^{+}(\mathbb{R}^n) . Here we may assume without loss of generality that \mu \in {\mathcal{M}}_b^{+}(\mathbb{R}^n) , so that {\bf W}_{1, p} \mu\not\equiv \infty . Consequently, (4.5) is equivalent to the inequality

    \begin{align} \left(\int_{\mathbb{R}^n} ({\bf W}_{1, p} \mu)^q \, d \sigma\right)^{\frac 1 q} \le \kappa \, \Vert \mu \Vert^{\frac{1}{p-1}}_{{\mathcal{M}}^+(\mathbb{R}^n)} \qquad {\rm{for\; all}} \, \, \mu \in {\mathcal{M}}_b^+(\mathbb{R}^n), \end{align}

    where K^{-1} \, \varkappa\le \kappa \le K \, \varkappa . In particular, one can replace {\rm{div}}\, \mathcal{A}(x, \nabla \varphi) in (4.5) by \Delta_p , up to a constant which depends only on K .

    By \varkappa(B) , where B is a ball in \mathbb{R}^n , we denote the least constant in a similar localized weighted norm inequality with the measure \sigma_B in place of \sigma , where \sigma_B = \chi_B \, \sigma is the restriction of \sigma to B .

    The so-called intrinsic nonlinear potential \bf{K}_{p, q} \sigma , introduced in [9], is defined by

    \begin{align} \bf{K}_{p, q} \sigma (x) = \int_0^{\infty} \left(\frac{ \varkappa(B(x, t))^{\frac{q(p-1)}{p-1-q}}}{t^{n- p}}\right)^{\frac{1}{p-1}}\frac{dt}{t}, \qquad x \in \mathbb{R}^n. \end{align}

    Here B = B(x, t) is a ball in \mathbb{R}^n of radius t > 0 centered at x . As was noticed in [9], \bf{K}_{p, q} \sigma \not\equiv + \infty if and only if

    \begin{align} \int_1^{\infty} \left(\frac{ \varkappa(B(0, t))^{\frac{q(p-1)}{p-1-q}}}{t^{n- p}}\right)^{\frac{1}{p-1}}\frac{dt}{t} < \infty. \end{align} (4.6)

    By [9,Theorem 1.1], there exists a nontrivial \mathcal{A} -superharmonic solution to the homogeneous equation (4.1) in the case \mu = 0 if and only if \bf{W}_{1, p} \sigma \not\equiv + \infty and \bf{K}_{p, q} \sigma \not\equiv + \infty , i.e., conditions (3.3) and (4.6) hold. The next theorem shows that this solution is actually reachable.

    Theorem 4.2. Let 0 < q < p-1 , and let \sigma\in {\mathcal{M}}^+(\mathbb{R}^n) . Then the nontrivial minimal \mathcal{A} -superharmonic solution u of

    \begin{equation} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u) = \sigma u^q, \quad u\ge 0 \quad in\; \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, u = 0, \end{array} \right. \end{equation} (4.7)

    constructed in the proof of [9,Theorem 1.1] under the conditions (3.3) and (4.6), is an \mathcal{A} -superharmonic reachable solution.

    Proof. We start with the same construction as in the proof of [9,Theorem 1.1] for the minimal \mathcal{A} -superharmonic solution u , but with datum \sigma|_{B_m(0)} in place of \sigma ( m = 1, 2, \dots ).

    For a fixed m , let v_m be the minimal \mathcal{A} -superharmonic solution to the equation

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_m) = \sigma|_{B_m(0)} v_{m}^q, \quad v_m\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, v_m = 0, \end{array} \right. \end{equation*}

    We recall from the construction in [9] that

    v_m = \lim\limits_{j\rightarrow \infty}(\lim\limits_{k\rightarrow \infty} v_{j, m}^{k}),

    where v_{1, m}^k ( k = 0, 1, 2, \dots ) is the \mathcal{A} -superharmonic renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{1,m}^k) = \sigma|_{B_m(0)\cap B_{2^k}(0)} \, w_{0,m}^q \quad {\text{in }}\; B_{2^k}(0), \\ v_{1,m}^k = 0 \quad {\text{on }}\; \partial B_{2^k}(0), \end{array} \right. \end{equation*}

    with w_{0, m} = c_0 ({\bf W}_{1, p}(\sigma|_{B_m(0)}))^{\frac{p-1}{p-1-q}} , and v_{j, m}^k ( k = 0, 1, 2, \dots , j = 2, 3, \dots ) is the \mathcal{A} -superharmonic renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{j,m}^k) = \sigma|_{B_m(0)\cap B_{2^k}(0)}\, ( {\lim\limits_{i\rightarrow \infty}} v_{j-1,m}^i)^q \quad {\text{in }} B_{2^k}(0), \\ v_{j,m}^k = 0 \quad {\text{on }} \partial B_{2^k}(0). \end{array} \right. \end{equation*}

    Here c_0 is a fixed constant such that

    \begin{equation} 0 < c_0\leq \min\left\{ \left( \mathfrak{c}^{\frac{q}{p-1-q}} K^{-1} \right)^{\frac{p-1}{p-1-q}}, C K^{\frac{1-p}{p-1-q}}\right \}, \end{equation} (4.8)

    where C is the constant in (3.9) of [9], and \mathfrak{c} is the constant in (3.10) of [9] with \alpha = 1 .

    We also recall from [9] that

    u = \lim\limits_{j\rightarrow \infty}(\lim\limits_{k\rightarrow \infty} u_{j}^{k}),

    where u_{j}^{k} are the \mathcal{A} -superharmonic renormalized solutions of the corresponding problems in B_{2^k}(0) with \sigma in place of \sigma|_{B_m(0)} . In particular, \min (u_{j}^k, l) \in W^{1, p}_0 (B_{2^k}(0)) and \min (v_{j, m}^k, l) \in W^{1, p}_0 (B_{2^k}(0)) for all l > 0 .

    Thus, by the above version of the comparison principle (Lemma 4.1) we see that

    v_{j,m_1}^k\leq v_{j,m_2}^k \leq u_{j}^k\quad {\rm in} \, \, B_{2^k}(0),

    whenever m_1 \le m_2 .

    This yields

    0\leq v_1\leq v_2 \leq \dots \leq v_m\leq u \quad {\rm in} \, \, \mathbb{R}^n.

    Letting now m\rightarrow \infty , we obtain an \mathcal{A} -superharmonic reachable solution

    v: = \lim\limits_{m\to \infty} v_m

    to (4.7) such that v\leq u in \mathbb{R}^n . As u is the minimal \mathcal{A} -superharmonic solution of (4.7), we see on the other hand that u\leq v , and thus u = v , which completes the proof.

    Remark. In the proof of [9,Theorem 1.1], there is a misprint in the exponent in inequality (4.8) above for the constant c_0 . This choice of c_0 ensures the minimality of the solution u of (4.7) constructed in [9].

    We recall that, by [37,Theorem 1.1] and [37,Remark 4.3], a nontrivial \mathcal{A} -superharmonic solution of (4.1) exists if and only if \bf{W}_{1, p} \sigma\not \equiv \infty , \bf{K}_{p, q} \sigma\not \equiv \infty , and \bf{W}_{1, p} \mu \not \equiv \infty , i.e., the following three conditions hold:

    \begin{align} & \int_{1}^{\infty} \left(\frac{\sigma(B(0,\rho))}{\rho^{n-p}} \right)^{\frac{1}{p-1}}\frac{d\rho}{\rho} < +\infty, \end{align} (4.9)
    \begin{align} & \int_1^{\infty} \left(\frac{ \varkappa(B(0, \rho))^{\frac{q(p-1)}{p-1-q}}}{t^{n- p}}\right)^{\frac{1}{p-1}}\frac{d\rho}{\rho} < \infty, \end{align} (4.10)
    \begin{align} & \int_{1}^{\infty} \left(\frac{\mu(B(0,\rho))}{\rho^{n-p}} \right)^{\frac{1}{p-1}}\frac{d\rho}{\rho} < +\infty. \end{align} (4.11)

    Theorem 4.3. Let 0 < q < p-1 , and let \mu, \sigma\in {\mathcal{M}}^+(\mathbb{R}^n) , where \mu < < {\rm cap}_p . Then, under the conditions (4.9), (4.10), and (4.11), there exists a nontrivial minimal reachable \mathcal{A} -superharmonic solution of (4.1).

    Proof. Since the case \mu = 0 was treated in Theorem 4.2 above, without loss of generality we may assume that \mu\not = 0 . We recall that in the proof of [37,Theorem 1.1], a nontrivial \mathcal{A} -superharmonic solution u of (4.1), was constructed using the following iteration process. We set u_0 = 0 , and for j = 0, 1, 2, \ldots construct the iterations

    \begin{equation} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x, \nabla u_{j+1}) = \sigma u_j^q + \mu \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, u_{j+1} = 0, \end{array} \right. \end{equation} (4.12)

    where u_j \in L^q_{{\rm loc}} (\mathbb{R}^n, d \sigma) . We observe that, for each j , the solution u_{j+1} was chosen in [37] so that u_{j}\le u_{j+1} ( j = 0, 1, 2, \ldots ) by a version of the comparison principle (see [34,Lemma 3.7 and Lemma 3.9]). Then u : = \lim_{j \to \infty} u_j is a nontrivial \mathcal{A} -superharmonic solution of (4.1).

    We now modify this argument as follows to obtain a minimal nontrivial \mathcal{A} -superharmonic solution of (4.1). Notice that \mu < < {\rm cap}_p by assumption, and, as mentioned above, \sigma < < {\rm cap}_p , since a solution exists. Hence, clearly the measure \sigma u_j^q + \mu < < {\rm cap}_p as well. By Theorem 3.6, u_{j+1} can be chosen as the minimal \mathcal{A} -superharmonic solution to (4.12).

    It follows by induction that u_{j}\le u_{j+1} ( j = 0, 1, 2, \ldots ). Indeed, this is trivial when j = 0 , and then by the inductive step,

    \sigma u_{j-1}^q +\mu \le \sigma u_{j}^q +\mu, \quad j = 1,2, \ldots ,

    which is obvious when j = 1 . From this, using Theorem 3.7 we deduce u_{j}\le u_{j+1} for all j = 1, 2, \ldots .

    Similarly, if \tilde{u} is any \mathcal{A} -superharmonic solution of (4.1), then again arguing by induction and using Theorem 3.7, we deduce that u_{j+1} \le \tilde{u} ( j = 0, 1, 2, \ldots ), since

    \sigma u_{j-1}^q +\mu \le \sigma \tilde{u}^q +\mu, \quad j = 1,2, \ldots .

    Consequently, u\le \tilde{u} , i.e., u is the minimal \mathcal{A} -superharmonic solution of (4.1).

    We next show that u is a reachable solution. Using a similar iteration process with \sigma|_{B_m(0)} in place of \sigma and \mu|_{B_m(0)} in place of \mu ( m = 1, 2, \dots ), we set v_{0, m} = 0 and define v_{j, m} to be the minimal \mathcal{A} -superharmonic solution to the equation

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{j+1, m}) = \sigma|_{B_m(0)} v_{j, m}^q + \mu|_{B_m(0)}, \quad v_{j, m}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, v_{j, m} = 0, \end{array} \right. \end{equation*}

    where v_{j, m} \le v_{j+1, m} for each m = 1, 2, \ldots .

    As above, arguing by induction and using Theorem 3.7, we deduce

    v_{j, m_1} \le v_{j, m_2} \le u_j, \quad j = 1, 2, \ldots,

    whenever m_1\le m_2 . It follows that v_m : = \lim_{j\to \infty} v_{j, m}\le u ( m = 1, 2, \ldots ) is an \mathcal{A} -superharmonic solution of the equation

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x, \nabla v_{m}) = \sigma|_{B_m(0)} v_{m}^q + \mu|_{B_m(0)}, \quad v_{m}\ge 0 \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, v_{m} = 0, \end{array} \right. \end{equation*}

    where v_{m_1} \le v_{m_2} \le u if m_1\le m_2 .

    Thus, letting m\rightarrow \infty , we obtain an \mathcal{A} -superharmonic reachable solution v: = \lim\limits_{m\to \infty} v_{m} to (4.7) such that v\leq u . Since u is the minimal \mathcal{A} -superharmonic solution of (4.7), we see that u = v , which completes the proof.

    We now prove the uniqueness property for reachable solutions of (4.1).

    Theorem 4.4. Let 0 < q < p-1 , and let \mu, \sigma\in {\mathcal{M}}^+(\mathbb{R}^n) , where \mu < < {\rm cap}_p . Suppose \mathcal{A} satisfies conditions (2.1) and (2.2). Then nontrivial \mathcal{A} -superharmonic reachable solutions of (4.1) are unique.

    Proof. Let u, v be two nontrivial \mathcal{A} -superharmonic solutions of (4.1) in \mathbb{R}^n . Then by [37,Theorem 1.1] and [37,Remark 4.3], there exists a constant C\ge 1 , depending only on p , q and n , such that

    \begin{equation*} C^{-1} \, u \le v \le C \, u \quad {\rm in} \, \, \mathbb{R}^n. \end{equation*}

    Hence, clearly,

    \begin{equation*} -{\rm div}\, \mathcal{A}(x, \nabla v) = \sigma v^q + \mu \le C^q \left( \sigma u^q + \mu \right) = -{\rm div}\, \mathcal{A}(x, \nabla (C^{\frac{q}{p-1}} u)). \end{equation*}

    Notice that here by definition u, v \in L^{q}_{{\rm loc}}(\mathbb{R}^n, \sigma) . Suppose that v is a reachable solution of (4.1) in \mathbb{R}^n . Then by Corollary 3.11 with \sigma v^q + \mu in place of \sigma , and \tilde{\sigma} = C^q \left(\sigma u^q + \mu \right) , it follows that v \le C^{\frac{q}{p-1}}u .

    By iterating this argument, we deduce

    \begin{equation*} v \le C^{(\frac{q}{p-1})^j}u \quad {\rm in} \, \, \mathbb{R}^n, \qquad j = 1, 2, \ldots . \end{equation*}

    Since 0 < q < p-1 , letting j \to \infty in the preceding inequality, we obtain v \le u in \mathbb{R}^n . Interchanging the roles of u and v , we see that actually u = v in \mathbb{R}^n .

    Corollary 4.5. Nontrivial \mathcal{A} -superharmonic solutions u of (4.1) are unique under the assumptions of Theorem 4.4, provided any one of the following conditions holds:

    \rm (ⅰ) u \in L^q (\mathbb{R}^n, d \sigma) and \mu \in {\mathcal{M}}^+_b(\mathbb{R}^n) , or equivalently \varkappa (\mathbb{R}^n) < \infty and \mu \in {\mathcal{M}}^+_b(\mathbb{R}^n) ;

    \rm (ⅱ) \lim_{|x| \to \infty} u(x) = 0 ;

    \rm (ⅲ) |D u| \in L^{p}(\mathbb{R}^n) , or |D u| \in L^{\gamma, \infty}(\mathbb{R}^n) for some (p-1)n/(n-1)\leq \gamma < p .

    Proof. Suppose first that (ⅰ) holds. By [9,Theorem 4.4], \varkappa (\mathbb{R}^n) < \infty if and only if there exists a nontrivial \mathcal{A} -superharmonic solution u \in L^q(\mathbb{R}^n, d\sigma) of (4.7). In particular, since by [37,Theorem 4.1],

    u \ge C \, \left[{\bf W}_{1, p} \sigma + \left({\bf K}_{p, q} \sigma \right)^{\frac{p-1}{p-1-q}} \right],

    it follows that

    {\bf W}_{1, p} \sigma\in L^{\frac{q(p-1)}{p-1-q}}(\mathbb{R}^n, d \sigma) \quad {\rm{and}} \quad {\bf K}_{p, q} \sigma\in L^{q}(\mathbb{R}^n, d \sigma).

    Next, we denote by \varphi an \mathcal{A} -superharmonic solution to the equation

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x, \nabla \varphi) = \mu, \quad \varphi \geq 0 \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, \varphi = 0, \end{array} \right. \end{equation*}

    where \mu is the Riesz measure of \varphi . Notice that \varphi \ge K^{-1} \, {\bf W}_{1, p} \mu by the lower bound in inequality (3.4). Since \mu \in {\mathcal{M}}_b(\mathbb{R}^n) and \varkappa (\mathbb{R}^n) < \infty , using \varphi as a test function in inequality (4.5) yields {\bf W}_{1, p} \mu \in L^q (\mathbb{R}^n, d \sigma) .

    Hence, by [37,Theorem 1.1] and [37,Remark 4.3], we deduce that there exists a nontrivial \mathcal{A} -superharmonic solution of (4.1) u \in L^q (\mathbb{R}^n, d \sigma) , and, for any such a solution, \sigma u^q + \mu\in {\mathcal{M}}_b(\mathbb{R}^n) . It follows that u is a reachable \mathcal{A} -superharmonic solution of (4.1) by Theorem 3.10 and Theorem 3.12 (ⅲ).

    In case (ⅱ), by Theorem 3.16 u is a reachable solution of (4.1).

    In case (ⅲ), u is a reachable \mathcal{A} -superharmonic solution of (4.1) by Theorem 3.10 and Theorem 3.12 (ⅰ), (ⅱ).

    In all these cases, reachable \mathcal{A} -superharmonic solutions are unique by Theorem 4.4.

    Remark 4.6. Uniqueness of finite energy solutions u to (4.1) such that |D u| \in L^{p}(\mathbb{R}^n) in Corollary 4.5(ⅲ) was established in [35,Theorem 6.1] in the special case of the p -Laplace operator using a different method. (See also an earlier result [8,Theorem 5.1] in the case \mu = 0 .) Solutions of finite energy to (4.1) exist if and only if {\bf W}_{1, p} \sigma\in L^{\frac{(1+q)(p-1)}{p-1-q}}(\mathbb{R}^n, d \sigma) and {\bf W}_{1, p} \mu \in L^{1}(\mathbb{R}^n, d \mu) ([35,Theorem 1.1]).

    Remark 4.7. Under the assumptions of Theorem 4.3, but without the restriction \mu < < {\rm cap}_p , it is still possible to prove the existence of an \mathcal{A} -superharmonic reachable solution (not necessarily minimal) of (4.1). The construction of such a solution makes use of an extension of [33,Lemma 6.9] proved below.

    Proof. To prove this claim, we shall construct first a nondecreasing sequence \{u_m\}_{m\geq 1} of \mathcal{A} -superharmonic solutions of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u_{m}) = \sigma|_{B_m(0)} u_{m}^q + \mu|_{B_m(0)}, \quad u_{m}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, u_{m} = 0. \end{array} \right. \end{equation*}

    Then by [37,Theorem 1.1 and Remark 4.3],

    u_m(x) \leq C\left( {\bf W}_{1,p} \mu(x) + {\bf K}_{p,q}\sigma(x) + [{\bf W}_{1,p} \sigma(x)]^{\frac{p-1}{p-1-q}} \right), \quad x\in\mathbb{R}^n.

    It follows from [20,Theorem 1.17] that u_{m}\rightarrow u pointwise everywhere, where u is an \mathcal{A} -superharmonic reachable solution of (4.1).

    The construction of \{u_m\}_{m\geq 1} can be done as follows. It suffices to demonstrate only how to construct u_1 and u_2 such that u_2\geq u_1 , since the construction of u_m for m\ge 3 is completely analogous. Let v_1 be an \mathcal{A} -superharmonic solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{1}) = \mu|_{B_1(0)}, \quad v_{1}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, v_{1} = 0. \end{array} \right. \end{equation*}

    Here as above v_1 is an a.e. pointwise limit of a subsequence of \{v_1^{(k)}\}_{k\geq 1} , where each v_1^{(k)} is a nonnegative \mathcal{A} -superharmonic renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{1}^{(k)}) = \mu|_{B_1(0)} \quad {\text{in }} B_k(0), \\ v_{1}^{(k)} = 0 \quad {\text{on }} \partial B_k(0). \end{array} \right. \end{equation*}

    Next, for any j\geq1 , let v_{j+1} be an \mathcal{A} -superharmonic solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{j+1}) = \sigma|_{B_1(0)} v_{j}^q + \mu|_{B_1(0)}, \quad v_{j+1}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, v_{j+1} = 0. \end{array} \right. \end{equation*}

    Notice that v_{j+1} is an a.e. pointwise limit of a subsequence of \{v_{j+1}^{(k)}\}_{k\geq 1} , where each v_{j+1}^{(k)} is a nonnegative \mathcal{A} -superharmonic renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{j+1}^{(k)}) = \sigma|_{B_1(0)} v_{j}^q +\mu|_{B_1(0)} \quad {\text{in }} B_k(0), \\ v_{j+1}^{(k)} = 0 \quad {\text{on }} \partial B_k(0). \end{array} \right. \end{equation*}

    By [33,Lemma 6.9] we may assume that v_{2}^{(k)}\geq v_1^{(k)} for all k\geq 1 , and hence v_2\geq v_1 . In the same way, by induction we deduce that v_{j+1}^{(k)}\geq v_j^{(k)} for all j, k\geq 1 . It follows that v_{j+1}\geq v_j , and

    v_{j+1} \leq C\, {\bf W}_{1,p}(\sigma v_{j+1}^q) + C\, {\bf W}_{1,p}(\mu).

    Then by [37,Theorem 4.1], for any j \geq 1 , we obtain the bound

    \begin{equation} v_{j+1}(x) \leq C\left( {\bf W}_{1,p} \mu(x) + {\bf K}_{p,q}\sigma(x) + [{\bf W}_{1,p} \sigma(x)]^{\frac{p-1}{p-1-q}} \right), \quad x\in\mathbb{R}^n. \end{equation} (4.13)

    Thus, the nondecreasing sequence \{v_j\}_{j\geq 1} converges to an \mathcal{A} -superharmonic solution u_1 of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u_{1}) = \sigma|_{B_1(0)} u_{1}^q + \mu|_{B_1(0)}, \quad u_{1}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, u_{1} = 0. \end{array} \right. \end{equation*}

    To construct u_2 such that u_2\geq u_1 , let w_1 be an \mathcal{A} -superharmonic solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla w_{1}) = \mu|_{B_2(0)}, \quad w_{1}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, w_{1} = 0. \end{array} \right. \end{equation*}

    Notice that w_1 is an a.e. pointwise limit of a subsequence of \{w_1^{(k)}\}_{k\geq 1} , where each w_1^{(k)} is a nonnegative \mathcal{A} -superharmonic renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla w_{1}^{(k)}) = \mu|_{B_2(0)} \quad {\text{in }} B_k(0), \\ w_{1}^{(k)} = 0 \quad {\text{on }} \partial B_k(0). \end{array} \right. \end{equation*}

    Again, by [33,Lemma 6.9] we may assume that w_{1}^{(k)}\geq v_1^{(k)} for all k\geq 1 , and hence w_1\geq v_1 .

    Next, for any j\geq1 , let w_{j+1} be an \mathcal{A} -superharmonic solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla w_{j+1}) = \sigma|_{B_2(0)} w_{j}^q + \mu|_{B_2(0)}, \quad w_{j+1}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, w_{j+1} = 0. \end{array} \right. \end{equation*}

    Notice that w_{j+1} is an a.e. pointwise limit of a subsequence of \{w_{j+1}^{(k)}\}_{k\geq 1} , where each w_{j+1}^{(k)} is a nonnegative \mathcal{A} -superharmonic renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla w_{j+1}^{(k)}) = \sigma|_{B_2(0)} w_{j}^q +\mu|_{B_2(0)} \quad {\text{in }} B_k(0), \\ w_{j+1}^{(k)} = 0 \quad {\text{on }} \partial B_k(0). \end{array} \right. \end{equation*}

    We can ensure here that w_{j+1}^{(k)}\geq \max\{v_{j+1}^{(k)}, w_{j}^{(k)}\} for all j, k\geq 1 . Indeed, since w_1\geq v_1 and w_1\geq 0 , by Lemma 4.8 below we may assume that w_{2}^{(k)}\geq \max\{v_{2}^{(k)}, w_{1}^{(k)}\} for all k\geq 1 , and hence w_{2}\geq \max\{v_{2}, w_1\} . Repeating this argument by induction we obtain w_{j+1}^{(k)}\geq \max\{v_{j+1}^{(k)}, w_{j}^{(k)}\} for all j, k\geq 1 .

    It follows that w_{j+1}\geq \max\{v_{j+1}, w_{j}\} for all j\geq 1 . As in (4.13) we have

    \begin{equation*} w_{j+1}(x) \leq C\left( {\bf W}_{1,p} \mu(x) + {\bf K}_{p,q}\sigma(x) + [{\bf W}_{1,p} \sigma(x)]^{\frac{p-1}{p-1-q}} \right), \quad x\in\mathbb{R}^n, \end{equation*}

    and hence the nondecreasing sequence \{ w_j\} converges to an \mathcal{A} -superharmonic solution u_2 of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u_{2}) = \sigma|_{B_2(0)} u_{2}^q + \mu|_{B_2(0)}, \quad u_{2}\geq 0, \quad {\text{in }} \mathbb{R}^n, \\ {\liminf\limits_{|x|\rightarrow \infty}}\, u_{2} = 0 \end{array} \right. \end{equation*}

    such that u_2\geq u_1 , as desired.

    The following lemma invoked in the argument presented above is an extension of [33,Lemma 6.9].

    Lemma 4.8. Let {\Omega} be a bounded open set in \mathbb{R}^n and let \mu_1, \mu_2 \in \mathcal{M}_{b}^+({\Omega}) . Suppose that u_i (i = 1, 2) is a renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u_i) = \mu_{i} \quad in\; {\Omega}, \\ u_i = 0 \quad on\; \partial {\Omega}. \end{array} \right. \end{equation*}

    Then for any measure \nu \in \mathcal{M}_{b}^+({\Omega}) such that \nu\geq \mu_1 and \nu\geq \mu_2 , there is a renormalized solution v of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v) = \nu \quad in\; {\Omega}, \\ v = 0 \quad on\; \partial {\Omega}, \end{array} \right. \end{equation*}

    such that v\geq u_1 and v\geq u_2 a.e.

    Proof. For i = 1, 2 , let u_{i, k} = \min\{u_i, k\} ( k = 1, 2, \dots ). Then u_{i, k} is the bounded renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla u_{i,k}) = \mu_{i0}|_{\{u_i < k\}} + \lambda_{i,k} \quad {\text{in }} {\Omega}, \\ u_{i,k} = 0 \quad {\text{on }} \partial {\Omega}. \end{array} \right. \end{equation*}

    Here \mu_i = \mu_{i0}+\mu_{is} ( i = 1, 2 ) is the decomposition of \mu_i used in Section 3 above, where \mu_{i0}, \mu_{is} \in \mathcal{M}_{b}^+(\Omega) , \mu_{i0} < < {\rm cap}_p , and \mu_{is} is concentrated on a set of zero p -capacity. Moreover, \lambda_{i, k}\in\mathcal{M}_{b}^+(\Omega) and \lambda_{i, k} \rightarrow \mu_{is} in the narrow topology of measures as k\rightarrow \infty (see Definition 3.3).

    Now let v_k ( k = 1, 2, \dots ) be a renormalized solution of

    \begin{equation*} \left\{ \begin{array}{ll} -{\rm div}\, \mathcal{A}(x,\nabla v_{k}) = \sum\limits_{i = 1}^2 ( \mu_{i0} + \lambda_{i,k}) + (\nu-\mu_1) +(\nu-\mu_2) \quad {\text{in }} {\Omega}, \\ v_{k} = 0 \quad {\text{on }} \partial {\Omega}. \end{array} \right. \end{equation*}

    Then by [33,Lemma 6.8] we deduce v_k\geq \max\{u_{1, k}, u_{2, k}\} for all k\geq 1 . Finally, we use the stability results of [12] to find a subsequence of \{v_k\} that converges a.e. to a desired function v .

    Nguyen Cong Phuc is supported in part by the Simons Foundation, award number 426071.

    The authors declare no conflict of interest.



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