
It is well-known that the embedding of the Sobolev space of weakly differentiable functions into Hölder spaces holds if the integrability exponent is higher than the space dimension. In this paper, the embedding of the Sobolev functions into the Hölder spaces is expressed in terms of the minimal weak differentiability requirement independent of the integrability exponent. The proof is based on the generalization of the Newton-Leibniz formula to the n-dimensional rectangle and the inductive application of the Sobolev trace embedding results. The new method is applied to prove the embedding of the Sobolev spaces with dominating mixed smoothness into Hölder spaces. Counterexamples demonstrate that in terms of minimal weak regularity degree the Sobolev spaces with dominating mixed smoothness present the largest class of weakly differentiable functions with the upgrade of pointwise regularity to continuity. Remarkably, it also presents the largest class of weakly differentiable functions where the generalized Newton-Leibniz formula holds.
Citation: Ugur G. Abdulla. Generalized Newton-Leibniz formula and the embedding of the Sobolev functions with dominating mixed smoothness into Hölder spaces[J]. AIMS Mathematics, 2023, 8(9): 20700-20717. doi: 10.3934/math.20231055
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It is well-known that the embedding of the Sobolev space of weakly differentiable functions into Hölder spaces holds if the integrability exponent is higher than the space dimension. In this paper, the embedding of the Sobolev functions into the Hölder spaces is expressed in terms of the minimal weak differentiability requirement independent of the integrability exponent. The proof is based on the generalization of the Newton-Leibniz formula to the n-dimensional rectangle and the inductive application of the Sobolev trace embedding results. The new method is applied to prove the embedding of the Sobolev spaces with dominating mixed smoothness into Hölder spaces. Counterexamples demonstrate that in terms of minimal weak regularity degree the Sobolev spaces with dominating mixed smoothness present the largest class of weakly differentiable functions with the upgrade of pointwise regularity to continuity. Remarkably, it also presents the largest class of weakly differentiable functions where the generalized Newton-Leibniz formula holds.
In the present paper we study the effective conductivity of an
(λ+,λ−)∈[0,+∞[2∗≡[0,+∞[2∖{(0,0)}. |
We note that the limit case of zero conductivity corresponds to a thermal insulator. On the other hand, if the conductivity tends to
We now introduce the geometry of the problem. If
q=(q110⋯00⋱⋯0⋮⋮⋱⋮00⋯qnn), | (1) |
and
Q≡n∏j=1]0,qjj[⊆Rn. | (2) |
The set
˜Q≡]0,1[n,˜q≡In≡(10⋯00⋱⋯0⋮⋮⋱⋮00⋯1). |
Then we take
α∈]0,1[ and a bounded open connected subset Ω of Rn of class C1,α such that Rn∖¯Ω is connected. | (3) |
The symbol '
Sq[qI[ϕ]]≡⋃z∈Zn(qz+qI[ϕ]),Sq[qI[ϕ]]−≡Rn∖¯Sq[qI[ϕ]]. |
The set
With the aim of introducing the definition of the effective conductivity, we first have to introduce a boundary value problem for the Laplace equation. If
{Δu+j=0in Sq[qI[ϕ]],Δu−j=0in Sq[qI[ϕ]]−,u+j(x+qeh)=u+j(x)+δhjqjj∀x∈¯Sq[qI[ϕ]],∀h∈{1,…,n},u−j(x+qeh)=u−j(x)+δhjqjj∀x∈¯Sq[qI[ϕ]]−,∀h∈{1,…,n},λ+∂∂νqI[ϕ]u+j−λ−∂∂νqI[ϕ]u−j=0on ∂qI[ϕ],u+j−u−j=0on ∂qI[ϕ],∫∂qI[ϕ]u+jdσ=0, | (4) |
where
Definition 1.1. Let
λeff[q,ϕ,(λ+,λ−)]≡(λeffij[q,ϕ,(λ+,λ−)])i,j=1,…,n |
is the
λeffij[q,ϕ,(λ+,λ−)]≡1|Q|n{λ+∫qI[ϕ]∂∂xiu+j[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]∂∂xiu−j[q,ϕ,(λ+,λ−)](x)dx}∀i,j∈{1,…,n}. |
Remark 1.2. Under the assumptions of Definition 1.1, by applying the divergence theorem, one can verify that
λeffij[q,ϕ,(λ+,λ−)]=1|Q|n{λ+∫qI[ϕ]Du+i[q,ϕ,(λ+,λ−)](x)⋅Du+j[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]Du−i[q,ϕ,(λ+,λ−)](x)⋅Du−j[q,ϕ,(λ+,λ−)](x)dx}∀i,j∈{1,…,n}. |
Indeed, if we set
˜u+k[q,ϕ,(λ+,λ−)](x)=u+k[q,ϕ,(λ+,λ−)](x)−xk∀x∈¯Sq[qI[ϕ]]˜u−k[q,ϕ,(λ+,λ−)](x)=u−k[q,ϕ,(λ+,λ−)](x)−xk∀x∈¯Sq[qI[ϕ]]−∀k∈{1,…,n}, |
then
1|Q|n{λ+∫qI[ϕ]Du+i[q,ϕ,(λ+,λ−)](x)⋅Du+j[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]Du−i[q,ϕ,(λ+,λ−)](x)⋅Du−j[q,ϕ,(λ+,λ−)](x)dx}=1|Q|n{λ+∫qI[ϕ]Du+j[q,ϕ,(λ+,λ−)](x)⋅D(xi+˜u+i[q,ϕ,(λ+,λ−)](x))dx+λ−∫Q∖¯qI[ϕ]Du−j[q,ϕ,(λ+,λ−)](x)⋅D(xi+˜u−i[q,ϕ,(λ+,λ−)](x))dx}=1|Q|n{λ+∫qI[ϕ]Du+j[q,ϕ,(λ+,λ−)](x)⋅Dxidx+λ+∫qI[ϕ]Du+j[q,ϕ,(λ+,λ−)](x)⋅D˜u+i[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]Du−j[q,ϕ,(λ+,λ−)](x)⋅Dxidx+λ−∫Q∖¯qI[ϕ]Du−j[q,ϕ,(λ+,λ−)](x)⋅D˜u−i[q,ϕ,(λ+,λ−)](x)dx}=1|Q|n{λ+∫qI[ϕ]∂∂xiu+j[q,ϕ,(λ+,λ−)](x)dx+λ+∫qI[ϕ]Du+j[q,ϕ,(λ+,λ−)](x)⋅D˜u+i[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]∂∂xiu−j[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]Du−j[q,ϕ,(λ+,λ−)](x)⋅D˜u−i[q,ϕ,(λ+,λ−)](x)dx}. |
Therefore, in order to conclude that the two definitions are equivalent, we need to show that
λ+∫qI[ϕ]Du+j[q,ϕ,(λ+,λ−)](x)⋅D˜u+i[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]Du−j[q,ϕ,(λ+,λ−)](x)⋅D˜u−i[q,ϕ,(λ+,λ−)](x)dx=0. | (5) |
By an application of the divergence theorem for
∫qI[ϕ]Du+j[q,ϕ,(λ+,λ−)](x)⋅D˜u+i[q,ϕ,(λ+,λ−)](x)dx=∫∂qI[ϕ](∂∂νqI[ϕ]u+j[q,ϕ,(λ+,λ−)](x))˜u+i[q,ϕ,(λ+,λ−)](x)dσx | (6) |
and
∫Q∖¯qI[ϕ]Du−j[q,ϕ,(λ+,λ−)](x)⋅D˜u−i[q,ϕ,(λ+,λ−)](x)dx=∫∂Q(∂∂νQu−j[q,ϕ,(λ+,λ−)](x))˜u−i[q,ϕ,(λ+,λ−)](x)dσx−∫∂qI[ϕ](∂∂νqI[ϕ]u−j[q,ϕ,(λ+,λ−)](x))˜u−i[q,ϕ,(λ+,λ−)](x)dσx. | (7) |
By the periodicity of
∫∂Q(∂∂νQu−j[q,ϕ,(λ+,λ−)](x))˜u−i[q,ϕ,(λ+,λ−)](x)dσx=∫∂Q(∂∂νQxj)˜u−i[q,ϕ,(λ+,λ−)](x)dσx+∫∂Q(∂∂νQ˜u−j[q,ϕ,(λ+,λ−)](x))˜u−i[q,ϕ,(λ+,λ−)](x)dσx=∫∂Q(νQ(x)⋅ej)˜u−i[q,ϕ,(λ+,λ−)](x)dσx+∫∂Q(νQ(x)⋅D˜u−j[q,ϕ,(λ+,λ−)](x))˜u−i[q,ϕ,(λ+,λ−)](x)dσx=0, | (8) |
since contributions on opposite sides of
λ+∫qI[ϕ]Du+j[q,ϕ,(λ+,λ−)](x)⋅D˜u+i[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]Du−j[q,ϕ,(λ+,λ−)](x)⋅D˜u−i[q,ϕ,(λ+,λ−)](x)dx=λ+∫∂qI[ϕ](∂∂νqI[ϕ]u+j[q,ϕ,(λ+,λ−)](x))˜u+i[q,ϕ,(λ+,λ−)](x)dσx−λ−∫∂qI[ϕ](∂∂νqI[ϕ]u−j[q,ϕ,(λ+,λ−)](x))˜u−i[q,ϕ,(λ+,λ−)](x)dσx. | (9) |
Since the validity of (4) implies that
˜u+i[q,ϕ,(λ+,λ−)](x)=˜u−i[q,ϕ,(λ+,λ−)](x)∀x∈∂qI[ϕ] |
and that
λ+∂∂νqI[ϕ]u+j[q,ϕ,(λ+,λ−)](x)−λ−∂∂νqI[ϕ]u−j[q,ϕ,(λ+,λ−)](x)=0∀x∈∂qI[ϕ], |
we then deduce by (9) that (5) holds true.
As a consequence, the effective conductivity matrix of Definition 1.1 coincides with the one analyzed by Ammari, Kang, and Touibi [5,p. 121] for a periodic two-phase composite and which can be deduced by classical homogenization theory (see, e.g., Allaire [1], Bensoussan, Lions, and Papanicolaou [6], Jikov, Kozlov, and Oleĭnik [27], Milton [41]). We emphasize that the justification of the expression of the effective conductivity via homogenization theory holds for 'small' values of the periodicity parameters. For further remarks on the definition of effective conductivity we refer to Gluzman, Mityushev, and Nawalaniec [24,§2.2].
The main goal of our paper is to give an answer to the following question:
What can be said on the regularity of the map(q,ϕ,(λ+,λ−))↦λeff[q,ϕ,(λ+,λ−)]? | (10) |
We answer to the above question by proving that for all
Λij:D+n(R)×(C1,α(∂Ω,Rn)∩A˜Q∂Ω)×]−1−ε,1+ε[→R |
such that
λeffij[q,ϕ,(λ+,λ−)]=δijλ−+(λ++λ−)Λij[q,ϕ,λ+−λ−λ++λ−] | (11) |
for all
In particular, in the present paper we follow the strategy of [39] where we have studied the behavior of the longitudinal flow along a periodic array of cylinders upon perturbations of the shape of the cross section of the cylinders and the periodicity structure, when a Newtonian fluid is flowing at low Reynolds numbers around the cylinders. More precisely, we transform the problem into a set of integral equations defined on a fixed domain and depending on the set of variables
Formula (11) implies that the effective conductivity
(q,ϕ,(λ+,λ−))↦λeffij[q,ϕ,(λ+,λ−)] | (12) |
from
λeffij[qδ,ϕδ,(λ+δ,λ−δ)]=∞∑k=0ckδk | (13) |
for
Furthermore, such a high regularity result can be seen as a theoretical justification which guarantees that differential calculus may be used in order to characterize critical periodicity-shape-conductivity triples
As already mentioned, our method is based on integral equations, that are derived by potential theory. However, integral equations could also be deduced by the generalized alternating method of Schwarz (cf. Gluzman, Mityushev, and Nawalaniec [24] and Drygaś, Gluzman, Mityushev, and Nawalaniec [19]), which also allows to produce expansions in the concentration.
Incidentally, we observe that the are several contributions concerning optimization of effective parameters from many different points of view. For example, one can look for optimal lattices without confining to rectangular distributions. In this direction, Kozlov [29] and Mityushev and Rylko [44] have discussed extremal properties of hexagonal lattices of disks. On the other hand, even if, in wide generality, the optimal composite does not exist (cf. Cherkaev [13]), one can discuss the dependence on the shape under some specific restrictions. For example, one could build composites with prescribed effective conductivity as described in Lurie and Cherkaev [38] (see also Gibiansky and Cherkaev [22]). In Rylko [49], the author has studied the influence of perturbations of the shape of the circular inclusion on the macroscopic conductivity properties of 2D dilute composites. Inverse problems concerning the determination of the shape of equally strong holes in elastic structures were considered by Cherepanov [12]. For an experimental work concerning the analysis of particle reinforced composites we mention Kurtyka and Rylko [30]. Also, we mention that one could apply the topological derivative method as in Novotny and Sokołowski [46] for the optimal design of microstructures.
Let
Let
Sq[ΩQ]≡⋃z∈Zn(qz+ΩQ),Sq[ΩQ]−≡Rn∖¯Sq[ΩQ]. |
If
Ckb(¯Sq[ΩQ]−)≡{u∈Ck(¯Sq[ΩQ]−):Dγu is bounded ∀γ∈Nn s. t. |γ|≤k}, |
and we endow
‖u‖Ckb(¯Sq[ΩQ]−)≡∑|γ|≤ksupx∈¯Sq[ΩQ]−|Dγu(x)|∀u∈Ckb(¯Sq[ΩQ]−), |
where
Ck,βb(¯Sq[ΩQ]−)≡{u∈Ck,β(¯Sq[ΩQ]−):Dγu is bounded ∀γ∈Nn s. t. |γ|≤k}, |
and we endow
‖u‖Ck,βb(¯Sq[ΩQ]−)≡∑|γ|≤ksupx∈¯Sq[ΩQ]−|Dγu(x)|+∑|γ|=k|Dγu:¯Sq[ΩQ]−|β∀u∈Ck,βb(¯Sq[ΩQ]−), |
where
Ckq(¯Sq[ΩQ]−)≡{u∈Ckb(¯Sq[ΩQ]−):u is q-periodic}, |
which we regard as a Banach subspace of
Ck,βq(¯Sq[ΩQ]−)≡{u∈Ck,βb(¯Sq[ΩQ]−):u is q-periodic}, |
which we regard as a Banach subspace of
Our method is based on a periodic version of classical potential theory. In order to construct periodic layer potentials, we replace the fundamental solution of the Laplace operator by a
ΔSq,n=∑z∈Znδqz−1|Q|n, |
where
Sq,n(x)=−∑z∈Zn∖{0}1|Q|n4π2|q−1z|2e2πi(q−1z)⋅x |
in the sense of distributions in
We now introduce periodic layer potentials. Let
vq[∂ΩQ,μ](x)≡∫∂ΩQSq,n(x−y)μ(y)dσy∀x∈Rn,wq,∗[∂ΩQ,μ](x)≡∫∂ΩQνΩQ(x)⋅DSq,n(x−y)μ(y)dσy∀x∈∂ΩQ, |
for all
v+q[∂ΩQ,μ]≡vq[∂ΩQ,μ]|¯Sq[ΩQ] v−q[∂ΩQ,μ]≡vq[∂ΩQ,μ]|¯Sq[ΩQ]−. |
We collect in the following theorem some properties of
Theorem 2.1. Let
(i) The map from
(ii) Let
∂∂νΩQv±q[∂ΩQ,μ]=∓12μ+wq,∗[∂ΩQ,μ]on ∂ΩQ. |
Moreover,
∫∂ΩQwq,∗[∂ΩQ,μ]dσ=(12−|ΩQ|n|Q|n)∫∂ΩQμdσ. |
(iii) Let
Δvq[∂ΩQ,μ]=0in Rn∖∂Sq[ΩQ]. |
(iv) The operator
In order to consider shape perturbations of the inclusions of the composite, we introduce a class of diffeomorphisms. Let
A˜Q∂Ω≡{ϕ∈A∂Ω:ϕ(∂Ω)⊆˜Q},A˜Q¯Ω′≡{Φ∈A¯Ω′:Φ(¯Ω′)⊆˜Q}. | (14) |
If
We conclude this section of preliminaries with some results on problem (4). By means of the following proposition, whose proof is of immediate verification, we can transform problem (4) into a
Proposition 2.2. Let
(u+j,u−j)∈C1,αloc(¯Sq[qI[ϕ]])×C1,αloc(¯Sq[qI[ϕ]]−) |
solves problem (4) if and only if the pair
(˜u+j,˜u−j)∈C1,αq(¯Sq[qI[ϕ]])×C1,αq(¯Sq[qI[ϕ]]−) |
delivered by
˜u+j(x)=u+j(x)−xj∀x∈¯Sq[qI[ϕ]],˜u−j(x)=u−j(x)−xj∀x∈¯Sq[qI[ϕ]]−, |
solves
{Δ˜u+j=0in Sq[qI[ϕ]],Δ˜u−j=0in Sq[qI[ϕ]]−,˜u+j(x+qeh)=˜u+j(x)∀x∈¯Sq[qI[ϕ]],∀h∈{1,…,n},˜u−j(x+qeh)=˜u−j(x)∀x∈¯Sq[qI[ϕ]]−,∀h∈{1,…,n},λ+∂∂νqI[ϕ]˜u+j−λ−∂∂νqI[ϕ]˜u−j=(λ–λ+)(νqI[ϕ])jon ∂qI[ϕ],˜u+j−˜u−j=0on ∂qI[ϕ],∫∂qI[ϕ]˜u+jdσ=−∫∂qI[ϕ]yjdσy. | (15) |
Next, we show that problems (4) and (15) admit at most one solution.
Proposition 2.3. Let
(i) Problem (4) has at most one solution in
(ii) Problem (15) has at most one solution in
Proof. By the equivalence of problems (4) and (15) of Proposition 2.2, it suffices to prove statement (ⅱ), which we now consider. By the linearity of the problem, it clearly suffices to show that if
{Δ˜u+j=0in Sq[qI[ϕ]],Δ˜u−j=0in Sq[qI[ϕ]]−,˜u+j(x+qeh)=˜u+j(x)∀x∈¯Sq[qI[ϕ]],∀h∈{1,…,n},˜u−j(x+qeh)=˜u−j(x)∀x∈¯Sq[qI[ϕ]]−,∀h∈{1,…,n},λ+∂∂νqI[ϕ]˜u+j−λ−∂∂νqI[ϕ]˜u−j=0on ∂qI[ϕ],˜u+j−˜u−j=0on ∂qI[ϕ],∫∂qI[ϕ]˜u+jdσ=0, | (16) |
then
Let
∂∂νqI[ϕ]˜u−j=0on ∂qI[ϕ]. |
Accordingly, the divergence theorem implies that
0≤∫Q∖¯qI[ϕ]|D˜u−j(y)|2dy=∫∂Q˜u−j(y)∂∂νQ˜u−j(y)dσy−∫∂qI[ϕ]˜u−j(y)∂∂νqI[ϕ]˜u−j(y)dσy=0. |
Indeed, by the
∫∂Q˜u−j(y)∂∂νQ˜u−j(y)dσy=0. |
Then, there exists
Next we consider the case
∂∂νqI[ϕ]˜u+j=0on ∂qI[ϕ]. |
By the uniqueness of the solution of the interior Neumann problem up to constants, there exists
In this section, we convert problem (4) into an equivalent integral equation. As done in [39] for the longitudinal flow along a periodic array of cylinders, we do so by representing the solution in terms of single layer potentials, whose densities solve certain integral equations. Therefore, we first start with the following proposition regarding the invertibility of an integral operator that will appear in such integral formulation of problem (4).
Proposition 3.1. Let
Kγ[μ]=12μ−γwq,∗[∂qI[ϕ],μ]on ∂qI[ϕ],∀μ∈C0,α(∂qI[ϕ]). |
Then the following statements hold.
(i)
(ii)
Proof. We first consider statement (ⅰ). If
γ=γ+−γ−γ++γ−. |
Accordingly, we have to consider only the limit cases
K1[μ]=12μ−wq,∗[∂qI[ϕ],μ]=0on ∂qI[ϕ]. |
The jump formula for the normal derivative of the single layer potential of Theorem 2.1 (ⅱ) implies that
μ=∂∂νqI[ϕ]v−q[∂qI[ϕ],μ]−∂∂νqI[ϕ]v+q[∂qI[ϕ],μ]=0on ∂qI[ϕ]. |
Next, we consider the case
K−1[μ]=12μ+wq,∗[∂qI[ϕ],μ]=0on ∂qI[ϕ]. |
The jump formula for the normal derivative of the single layer potential of Theorem 2.1 (ⅱ) implies that
μ=∂∂νqI[ϕ]v−q[∂qI[ϕ],μ]−∂∂νqI[ϕ]v+q[∂qI[ϕ],μ]=0on ∂qI[ϕ]. |
Next, we consider statement (ⅱ). The Fredholm alternative theorem and the compactness of
Kγ[μ]=12μ−γwq,∗[∂qI[ϕ],μ]=0, | (17) |
then
0=∫∂qI[ϕ]Kγ[μ]dσ={12−γ(12−|qI[ϕ]||Q|)}∫∂qI[ϕ]μdσ. |
A straightforward computation shows that
We are now ready to show that problem (4) can be reformulated in terms of an integral equation which admits a unique solution.
Theorem 3.2. Let
(u+j[q,ϕ,(λ+,λ−)],u−j[q,ϕ,(λ+,λ−)])∈C1,αloc(¯Sq[qI[ϕ]])×C1,αloc(¯Sq[qI[ϕ]]−). |
Moreover
u+j[q,ϕ,(λ+,λ−)](x)=v+q[∂qI[ϕ],μj](x)−−∫∂qI[ϕ]v+q[∂qI[ϕ],μj](y)dσy−−∫∂qI[ϕ]yjdσy+xj∀x∈¯Sq[qI[ϕ]],u−j[q,ϕ,(λ+,λ−)](x)=v−q[∂qI[ϕ],μj](x)−−∫∂qI[ϕ]v−q[∂qI[ϕ],μj](y)dσy−−∫∂qI[ϕ]yjdσy+xj∀x∈¯Sq[qI[ϕ]]−, | (18) |
where
12μj−λ+−λ−λ++λ−wq,∗[∂qI[ϕ],μj]=λ+−λ−λ++λ−(νqI[ϕ])jon ∂qI[ϕ]. | (19) |
Proof. We first note that, by Proposition 2.3 (ⅱ), problem (4) has at most one solution in
(νqI[ϕ])j∈C0,α(∂qI[ϕ])0, |
Proposition 3.1 (ⅰ) implies that there exists a unique solution
λ+(−12μj+wq,∗[∂qI[ϕ],μj])−λ−(12μj+wq,∗[∂qI[ϕ],μj])=(λ–λ+)(νqI[ϕ])jon ∂qI[ϕ],v+q[∂qI[ϕ],μj]−−∫∂qI[ϕ]v+q[∂qI[ϕ],μj]dσ−v−q[∂qI[ϕ],μj]+−∫∂qI[ϕ]v−q[∂qI[ϕ],μj]dσ=0on ∂qI[ϕ]. |
Accordingly, the properties of the single layer potential (see Theorem 2.1) together with Proposition 2.2 imply that the pair of functions defined by (18) solves problem (4).
The previous theorem provides an integral equation formulation of problem (4) and a representation formula for its solution. We conclude this section by writing the effective conductivity in a form which makes use of the density
∫qI[ϕ]∂∂xiu+j[q,ϕ,(λ+,λ−)](x)dx=∫∂qI[ϕ]u+j[q,ϕ,(λ+,λ−)](y)(νqI[ϕ](y))idσy=∫∂qI[ϕ](v+q[∂qI[ϕ],μj](y)−−∫∂qI[ϕ]v+q[∂qI[ϕ],μj](z)dσz−−∫∂qI[ϕ]zjdσz+yj)(νqI[ϕ](y))idσy=∫∂qI[ϕ]v+q[∂qI[ϕ],μj](y)(νqI[ϕ](y))idσy−∫∂qI[ϕ](νqI[ϕ](y))idσy−∫∂qI[ϕ]v+q[∂qI[ϕ],μj](z)dσz−∫∂qI[ϕ](νqI[ϕ](y))idσy−∫∂qI[ϕ]zjdσz+δij|qI[ϕ]|n. |
Similarly, we have
∫Q∖¯qI[ϕ]∂∂xiu−j[q,ϕ,(λ+,λ−)](x)dx=∫∂Qu−j[q,ϕ,(λ+,λ−)](y)(νQ(y))idσy−∫∂qI[ϕ]u−j[q,ϕ,(λ+,λ−)](y)(νqI[ϕ](y))idσy=δij|Q|n−∫∂qI[ϕ]v−q[∂qI[ϕ],μj](y)(νqI[ϕ](y))idσy+∫∂qI[ϕ](νqI[ϕ](y))idσy−∫∂qI[ϕ]v−q[∂qI[ϕ],μj](z)dσz+∫∂qI[ϕ](νqI[ϕ](y))idσy−∫∂qI[ϕ]zjdσz−δij|qI[ϕ]|n. |
Indeed
∫∂Q(v−q[∂qI[ϕ],μj](y)−−∫∂qI[ϕ]v−q[∂qI[ϕ],μj](z)dσz−−∫∂qI[ϕ]zjdσz+yj)(νQ(y))idσy=∫∂Qyj(νQ(y))idσy=δij|Q|n. |
Moreover, by the divergence theorem, we have
∫∂qI[ϕ](νqI[ϕ](y))idσy=0∀i∈{1,…,n}. |
Accordingly, by the continuity of the single layer potential, we have that
λeffij[q,ϕ,(λ+,λ−)]=1|Q|n{λ+∫qI[ϕ]∂∂xiu+j[q,ϕ,(λ+,λ−)](x)dx+λ−∫Q∖¯qI[ϕ]∂∂xiu−j[q,ϕ,(λ+,λ−)](x)dx}=1|Q|n{δijλ−|Q|n+(λ+−λ−)(∫∂qI[ϕ]vq[∂qI[ϕ],μj](y)(νqI[ϕ](y))idσy−∫∂qI[ϕ](νqI[ϕ](y))idσy−∫∂qI[ϕ]vq[∂qI[ϕ],μj](z)dσz−∫∂qI[ϕ](νqI[ϕ](y))idσy−∫∂qI[ϕ]zjdσz+δij|qI[ϕ]|n)}=δijλ−+(λ++λ−){1|Q|n(λ+−λ−)(λ++λ−)(∫∂qI[ϕ]vq[∂qI[ϕ],μj](y)(νqI[ϕ](y))idσy+δij|qI[ϕ]|n)}. | (20) |
Thanks to Theorem 3.2, the study of problem (4) can be reduced to the study of the boundary integral equation (19). Therefore, our first step in order to study the dependence of the solution of problem (4) upon the triple
Before starting with this plan, we note that equation (19) is defined on the
Lemma 4.1. Let
12θj(t)−λ+−λ−λ++λ−∫qϕ(∂Ω)DSq,n(qϕ(t)−s)⋅νqI[ϕ](qϕ(t))(θj∘ϕ(−1))(q−1s)dσs=λ+−λ−λ++λ−(νqI[ϕ](qϕ(t)))j∀t∈∂Ω, | (21) |
if and only if the function
μj(x)=(θj∘ϕ(−1))(q−1x)∀x∈∂qI[ϕ] | (22) |
solves equation (19). Moreover, equation (21) has a unique solution in
Proof. The equivalence of equation (21) in the unknown
Inspired by Lemma 4.1, for all
Mj:D+n(R)×(C1,α(∂Ω,Rn)∩A˜Q∂Ω)×]−2,2[×C0,α(∂Ω)→C0,α(∂Ω) |
by setting
Mj[q,ϕ,γ,θ](t)≡12θ(t)−γ∫qϕ(∂Ω)DSq,n(qϕ(t)−s)⋅νqI[ϕ](qϕ(t))(θ∘ϕ(−1))(q−1s)dσs−γ(νqI[ϕ](qϕ(t)))j∀t∈∂Ω, | (23) |
for all
Mj[q,ϕ,λ+−λ−λ++λ−,θ]=0 on ∂Ω. | (24) |
Our aim is to recover the regularity of the solution
Lemma 4.2. Let
(i) The map from
V[q,ϕ,θ](t)≡∫qϕ(∂Ω)Sq,n(qϕ(t)−s)(θ∘ϕ(−1))(q−1s)dσs∀t∈∂Ω, |
is real analytic.
(ii) The map from
W∗[q,ϕ,θ](t)≡∫qϕ(∂Ω)DSq,n(qϕ(t)−s)⋅νqI[ϕ](qϕ(t))(θ∘ϕ(−1))(q−1s)dσs∀t∈∂Ω, |
is real analytic.
Next, we state the following technical lemma about the real analyticity upon the diffeomorphism
Lemma 4.3. Let
(i) For each
∫ϕ(∂Ω)w(s)dσs=∫∂Ωw∘ϕ(y)˜σ[ϕ](y)dσy,∀ω∈L1(ϕ(∂Ω)). |
Moreover, the map
(ii) The map from
We are now ready to prove that the solutions of (24) depend real analytically upon the triple 'periodicity-shape-contrast'. We do so by means of the following.
Proposition 4.4. Let
(i) For each
Mj[q,ϕ,γ,θj]=0on ∂Ω, |
and we denote such a function by
(ii) There exist
D+n(R)×(C1,α(∂Ω,Rn)∩A˜Q∂Ω)×]−1−ε,1+ε[ |
to
θj[q,ϕ,γ]=Θj[q,ϕ,γ]∀(q,ϕ,γ)∈D+n(R)×(C1,α(∂Ω,Rn)∩A˜Q∂Ω)×[−1,1]. |
Proof. The proof of statement (ⅰ) is a straightforward modification of the proof of Lemma 4.1. Indeed, it suffices to replace
Next we turn to consider statement (ⅱ). As a first step we have to study the regularity of the map
∂θMj[q,ϕ,γ,θj[q,ϕ,γ]](ψ)(t)=12ψ(t)−γ∫qϕ(∂Ω)DSq,n(qϕ(t)−s)⋅νqI[ϕ](qϕ(t))(ψ∘ϕ(−1))(q−1s)dσs∀t∈∂Ω, |
for all
In this section we prove our main result that answers to question (10) on the behavior of the effective conductivity upon the triple 'periodicity-shape-conductivity'. To this aim, we exploit the representation formula in (20) of the effective conductivity and the analyticity result of Proposition 4.4.
Theorem 5.1. Let
λeffij[q,ϕ,(λ+,λ−)]≡δijλ−+(λ++λ−)Λij[q,ϕ,λ+−λ−λ++λ−] | (25) |
for all
Proof. Let
Λij[q,ϕ,γ]≡1|Q|nγ{∫∂qI[ϕ]vq[∂qI[ϕ],(Θj[q,ϕ,γ]∘ϕ(−1))(q−1⋅)](y)(νqI[ϕ](y))idσy+δij|qI[ϕ]|n} |
for all
Λij[q,ϕ,γ]=1|Q|nγ{∫∂ΩV[q,ϕ,Θj[q,ϕ,γ]](y)(νqI[ϕ](qϕ(y)))i˜σ[qϕ](y)dσy+δij|qI[ϕ]|n} |
for all
|Q|n=n∏l=1qll∀q∈D+n(R), |
clearly
|qI[ϕ]|n=∫qI[ϕ]1dy=|Q|n∫I[ϕ]1dy=|Q|n1n∫ϕ(∂Ω)y⋅νI[ϕ](y)dσy=|Q|n1n∫∂Ωϕ(y)⋅νI[ϕ](ϕ(y))˜σ[ϕ](y)dσy. |
Then, by taking into account that the pointwise product in Schauder spaces is bilinear and continuous, and that the integral in Schauder spaces is linear and continuous, Lemma 4.3 implies that the map from
In the present paper we considered the effective conductivity of a two or three dimensional periodic two-phase composite material. The composite is obtained by introducing into a homogeneous matrix a periodic set of inclusions of a large class of sufficiently smooth shapes. We proved a regularity result for the effective conductivity of such a composite upon perturbations of the periodicity structure, of the shape of the inclusions, and of the conductivities of each material. Namely, we showed the real analytic dependence of the effective conductivity as a functional acting between suitable Banach spaces.
The consequences of our result are twofold. First, this high regularity result represents a theoretical justification to guarante that differential calculus may be used in order to characterize critical periodicity-shape-conductivity triples
λeffij[qδ,ϕδ,(λ+δ,λ−δ)]=∞∑k=0ckδk |
for
Both the authors are members of the 'Gruppo Nazionale per l'Analisi Matematica, la Probabilità e le loro Applicazioni' (GNAMPA) of the 'Istituto Nazionale di Alta Matematica' (INdAM) and acknowledge the support of the Project BIRD191739/19 'Sensitivity analysis of partial differential equations in the mathematical theory of electromagnetism' of the University of Padova. P.M. acknowledges the support of the grant 'Challenges in Asymptotic and Shape Analysis - CASA' of the Ca' Foscari University of Venice. The authors wish to thank the anonymous referees for many valuable comments that have improved the presentation of the paper.
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