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Research article

Exact traveling wave solutions of the stochastic Wick-type fractional Caudrey-Dodd-Gibbon-Sawada-Kotera equation

  • In this work, we test the intgrability of the stochastic Wick-type fractional Caudrey-Dodd-Gibbon-Sawada-Kotera (CDGSK) equation on the Painlevé test and construct new Wick-type and nob-Wick-type versions of exact traveling wave solutions of the stochastic Wick-type fractional CDGSK equation by employing the Hermit transformation, the conformable fractional derivative and the sub-equations method. Moreover, we obtain exact traveling wave solutions of the fractional Sawada-Kotera (SK) equation and the fractional Caudrey-Dodd-Gibbon (CDG) equation as well. It is note that physical illustration may be useful to predict internal structure of the considered equations. The results confirm that sub-equations method is very effective and efficient to find exact traveling wave solutions of Wick-type fractional nonlinear evolution equations.

    Citation: Jin Hyuk Choi, Hyunsoo Kim. Exact traveling wave solutions of the stochastic Wick-type fractional Caudrey-Dodd-Gibbon-Sawada-Kotera equation[J]. AIMS Mathematics, 2021, 6(4): 4053-4072. doi: 10.3934/math.2021240

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  • In this work, we test the intgrability of the stochastic Wick-type fractional Caudrey-Dodd-Gibbon-Sawada-Kotera (CDGSK) equation on the Painlevé test and construct new Wick-type and nob-Wick-type versions of exact traveling wave solutions of the stochastic Wick-type fractional CDGSK equation by employing the Hermit transformation, the conformable fractional derivative and the sub-equations method. Moreover, we obtain exact traveling wave solutions of the fractional Sawada-Kotera (SK) equation and the fractional Caudrey-Dodd-Gibbon (CDG) equation as well. It is note that physical illustration may be useful to predict internal structure of the considered equations. The results confirm that sub-equations method is very effective and efficient to find exact traveling wave solutions of Wick-type fractional nonlinear evolution equations.



    We consider a singular no-sign obstacle problem of the type

    {div(xa1u)=xa1f(x)χ{u0}in B+1,u=0on B1{x1=0}, (1.1)

    where a>1, χD is the characteristic function of D, B1Rn is the unit ball and B+1=B1{x1>0}. The equation is considered in the weak form,

    B+1xa1uφdx=B+1xa1f(x)φχ{u0}dx,

    for all φW1,20(B+1). This problem, when the non-negativity assumption u0 is imposed, is already studied in [9]. The above no-sign problem, as a general semilinear PDE with non-monotone r.h.s., introduces certain difficulties and to some extent some challenges. To explain this we shall give a very short review of the existing results and methods for similar type of problems (see also the book [6] and Caffarelli's review of the classical obstacle problem [2]). The general methodology of approaching such problems lies in using the so-called ACF-monotonicity formula (see [8]) or alternatively using John Andersson's dichotomy (see [1] or [3]). Although there are still some chances that both these methods will work for our problem above, we shall introduce a third method here which relies on a softer version of a monotonicity formula (which has a wider applicability) in combination with some elaborated analysis. We refer to this as a Weiss-type monotonicity formula, see (2.1) below.

    For clarity of exposition we shall introduce some notations and definitions here that are used frequently in the paper. Throughout this paper, Rn will be equipped with the Euclidean inner product xy and the induced norm |x|, Br(x0) will denote the open n-dimensional ball of center x0, radius r with the boundary Br(x0). In addition, Br=Br(0) and Br=Br(0). Rn+ stands for half space {xRn:x1>0} as well as B+r=BrRn+. Moreover, in the text we use the n-dimensional Hausdorff measure Hn. For a multi-index μ=(μ1,,μn)Zn+, we denote the partial derivative with μu=μ1x1μnxnu and |μ|1=μ1++μn.

    For a domain ΩRn+ and 1p<, we use the notation Lp(Ω) and Wm,p(Ω) as the standard spaces. However, we need some new notation for the weighted spaces

    Lp(Ω;xθ1):={u:Ωxθ1|u(x)|pdx<},

    where θR. For mN, we define the weighted Sobolev space Wm,p(Ω;xθ1) as the closure of C(¯Ω) with the following norm,

    uWm,p(Ω;xθ1):=uLp(Ω;xθ1)+x1DuLp(Ω;xθ1)++xm1DmuLp(Ω;xθ1).

    It is noteworthy that for θ=0, we have Lp(Ω;1)=Lp(Ω) but Wm,p(Ω;1)Wm,p(Ω). Generally, the trace operator has no sense for θ>1, while functions in W1,p(Ω;xθ1) have zero traces on {x1=0} for θ1. (Theorem 6 in [7]).

    We consider uW1,p(B+1,xθ1) for some θ<n and n<p to be a weak solution of (1.1). This condition provides the continuity of x(θ+n)/p1u up to the boundary according to Sobolev embedding Theorem 3.1 in [5]. First, we prove the following a priori regularity result.

    Proposition 1.1. (Appendix A) Let uW1,p(B+1,xθ1) be a solution of (1.1) for some θ<n, n<p and fL(B+1). Then for each max{0,1+θ+np}<β<1 there exists C=C(β,n,a) such that for r1/2,

    supB+r(x0)|xβ11u|Cr2β,

    for all x0{x1=0}.

    In Appendix A we will prove this proposition. Our main result in this paper concerns the optimal growth rate of solution u of (1.1) at touching free boundary points, which is stated in the following theorem.

    Theorem 1.2. Suppose uW1,p(B+1,xθ1) is a solution of (1.1) for some θ<n, n<p and x0{u=0}{x1=0}B+1/4. Moreover, if fCα(¯B+1) for some α(0,1), then

    |u(x)|Cx21((|xx0|+x1x1)(n+a+4)/2+1),

    for a universal constant C=C(a,n,[f]0,α).

    Our main tool in proving optimal decay for solutions from the free boundary points is Weiss-monotonicity formula, combined with some elaborated techniques. We define the balanced energy functional

    Φx0(r,u)=rn2aB+r(x0)(xa1|u|2+2xa1f(x0)u)dx2rn3aBr(x0)Rn+xa1u2dHn1. (2.1)

    Considering the scaling ur,x0=ur(x)=u(rx+x0)r2, Φx0(r,u)=Φ0(1,ur). In what follows we prove almost-monotonicity of the energy.

    Lemma 2.1 (Almost-Monotonicity Formula). Let u solve (1.1) and be as in Proposition 1.1 and assume that u(x0)=0 for some x0{x1=0} and fCα(¯B+1) for some α(0,1). Then u satisfies, for rr0 such that B+r0(x0)B+1,

    ddrΦx0(r,u)2rB1Rn+xa1(rur)2dHn1Crα+β2,

    where C depends only on fCα(¯B+1) and the constant C(β,n,a) in Proposition 1.1.

    Proof. Let ur(x):=u(rx+x0)r2, then

    =12ddrΦx0(r,u)=12ddr[B+1(xa1|ur|2+2xa1f(x0)ur)dx2B1Rn+xa1u2rdHn1]=12[B+1(2xa1urrur+2xa1f(x0)rur)dx4B1Rn+xa1urrurdHn1]=B+1div(xa1rurur)rurdiv(xa1ur)+xa1f(x0)rurdx2B1Rn+xa1urrurdHn1=B+1(f(x0)f(rx+x0)χ{ur0})xa1rurdx+B+1xa1rururνdHn12B1Rn+xa1urrurdHn1=B+1(f(x0)f(rx+x0)χ{ur0})xa1rurdx+rB1Rn+xa1(rur)2dHn1=B+1(f(x0)f(rx+x0))χ{ur0}xa1rurdx+f(x0)B+1{ur=0}xa1rurdx+rB1Rn+xa1(rur)2dHn1.

    Note that the second integral

    B+1{ur=0}xa1rurdx=0

    as |{ur=0ur0}|=0 and rur=0 on {ur=0ur=0}. Since |rur|Crβ2 we infer that

    B+1(f(x0)f(rx+x0))χ{ur0}xa1rurdxCrα+β2

    and conclude that

    12ddrΦx0(r,u)rB1Rn+xa1(rur)2dHn1Crα+β2.

    Definition 2.2. Let HP2 stand for the class of all two-homogeneous functions PW1,2(B+1;xa21) satisfying div(xa1P)=0 in Rn+ with boundary condition P=0 on x1=0. We also define the operator Π(v,r,x0) to be the projection of vr,x0 onto HP2 with respect to the inner product

    vw=B1n+xa1vwdHn1.

    We will use the following extension of [10,Lemma 4.1].

    Lemma 2.3. Assume that div(xa1w)=0 in B+1 with boundary condition w=0 on x1=0, and w(0)=|w(0)|=0. Then

    B+1xa1|w|2dx2B1Rn+xa1w2dHn10,

    and equality implies that wHP2, i.e., it is homogeneous of degree two.

    Proof. We define an extension of the Almgren frequency,

    rN(w,r):=rB+rxa1|w|2dxB+rxa1w2dHn1,
    12N(w,r)N(w,r)=B+rxa1(νw)2dHn1B+rxa1wνwdHn1B+rxa1wνwdHn1B+rxa1w2dHn10.

    Moreover, if N(w,r)=κ for ρ<r<σ, it implies that w is homogeneous of degree κ in BσBρ.

    Now supposing towards a contradiction that N(w,s)<2 for some s(0,1], and defining wr(x):=w(rx)w(rx)L2(B+1,xb1), we infer from N(w,s)<2 that wr is bounded in L2(B+1;xa1) and so wrmw0 weakly in L2(B+1;xa1) and wrmw0 strongly in L2(B+1;xa1) as a sequence rm0. Consequently, w0 satisfies div(xa1w0)=0 in B+1, w0(0)=|w0(0)|=0 and w0=0 on x1=0 as well as w0L2(B+1;xa1)=1. Furthermore, for all r(0,1) we have

    N(w0,r)=limrm0N(wrm,r)=limrm0N(w,rrm)=N(w,0+),

    and so w0 must be a homogeneous function of degree κ:=N(w,0+)<2. Note that for every multi-index μ{0}×Zn1+, the higher order partial derivative ζ=μw0 satisfies the equation div(xa1ζ)=0 in Rn+. From the integrability and homogeneity we infer that μw00 for κ|μ|1<n2, otherwise

    B+1|μw0|2dx=(10r2(κ|μ|1)+n1dr)B1Rn+|μw0|2dHn1

    can not be bounded. Thus xw0(x1,x) is a polynomial, and we can write w0(x1,x)=xκ1p(xx1). Consider the multi-index μ such that |μ|1=degp, so μw0=xκ|μ|11μp is a solution of div(xa1ζ)=0 in Rn+. Therefore, μw0W1,2(B+1;xθ1) for 1<θ according to Proposition A.1, which implies that 2(κ|μ|1)+θ>1. So, degp<κ+θ+12.

    Substituting w0(x)=xκ11(αx1+x) for κ>1 in the equation and comparing with w0(0)=|w0(0)|=0 we arrive at the only nonzero possible case being κ+a=2, which contradicts a>1. The case κ<1 leads to degp=0 and w0(x)=αxκ1, which implies κ+a=1 and a contradiction to a>1.

    Proposition 3.1. Let fCα(¯B+1) and u be solution of (1.1) satisfying the condition in Proposition 1.1. Then the function

    rrn3aB+r(x0)xa1u2(x)dHn1

    is bounded on (0,1/8), uniformly in x0{u=0}{x1=0}B1/8.

    Proof. Let us divide the proof into steps.

    Step 1 We claim that there exists a constant C1< such that for all x0{u=0}{x1=0}B1/8 and r1/8,

    f(x0)B+1xa1ux0,r(x)dxC1,

    where ux0,r:=u(rx+x0)r2. To prove this we observe that w:=ux0,r satisfies

    div(xa1w)=xa1fr(x):=xa1f(x0+rx)χ{ux0,r0},in B+1.

    Moreover, for ϕ(ρ):=ρna+1B+ρxa1w(x)dHn1 we have

    ϕ(ρ)=B+1xa1w(ρx)xdHn1=ρna+1B+ρdiv(xa1w(x))dx=ρna+1B+ρxa1fr(x)dx.

    If f(x0)18α[f]0,α then fr0 for r1/8. Therefore ϕ is increasing and ϕ(ρ)ϕ(0)=0 (recall that w(0)=0). Similarly, if f(x0)18α[f]0,α, we obtain that ϕ(ρ)0. Therefore the claim is true for C1=0 in these cases.

    In the case |f(x0)|18α[f]0,α, then |fr(x)|28α[f]0,α and then

    |ϕ(ρ)|213α[f]0,αρna+1B+ρxa1dx213α[f]0,αρ.

    So, |ϕ(ρ)|23α[f]0,αρ2 and

    |f(x0)B+1xa1ux0,r(x)dx|=|f(x0)10ρn+a1ϕ(ρ)dρ|26α[f]20,αn+a+2=:C1.

    Step 2 We claim that there exists a constant C2< such that

    distL2(B1Rn+;xa1)(ux0,r,HP2)C2,

    for every x0{u=0}{x1=0}B1/8, r1/8. Suppose towards a contradiction that this is not true, then there exists a sequence um, xmˉx and rm0 such that

    Mm=umxm,rmΠ(um,rm,xm)L2(B1Rn+;xa1),m.

    Let um:=umxm,rm and pm=Π(um,rm,xm) and wm=umpmMm. Then, since um(0)=|um(0)|=0 and by the monotonicity formula and the result of previous step, we find that

    B+1xa1|wm|2dx2B+1xa1w2mdHn1=1M2m[Φxm(rm)2B+1f(xm)xa1umdx]+1M2mB+1xa1(pmpmν2umpmν2p2m+4umpm)dHn11M2m(Φxm(rm)+2C1)1M2m(Φxm(12)+2C1)0,m. (3.1)

    Passing to a subsequence such that wmw in {\mathcal L}^2(B^+_{1};x_1^a) as m\to\infty , the compact embedding on the boundary implies that \|w_\|{ {\mathcal L}^{2}(\partial B_{1}\cap \mathbb{R}_+^n; x_1^a)} = 1 , and

    \begin{equation} \int_{B_{1}^+}x_1^a|\nabla w|^{2}dx\le2\int_{\partial B_{1}}x_1^aw^{2}{\mathop{}\!d} {\mathcal H}^{n-1} \end{equation} (3.2)

    and that

    \begin{equation} \int_{\partial B_{1}}wp{\mathop{}\!d} {\mathcal H}^{n-1} = 0,\qquad\forall p\in\mathbb{HP}_{2}. \end{equation} (3.3)

    Since \operatorname{div}(x_1^a\nabla w_{m}) = \frac{x_1}{M_{m}}f(x^m+r\cdot)\chi_{\{u_m\neq0\}} , it follows that \operatorname{div}(x_1^a\nabla w) = 0 in B_{1}^+ . Moreover, we obtain from L^{p} -theory that w_{m}\to w in C_{\text{loc}}^{1, \alpha}(B^+_{1}) for each \alpha\in(0, 1) as m\to\infty . Consequently w(0) = \lvert\nabla w(0)\rvert = 0 . Thus we can apply Lemma 2.3 and obtain from (3.2) that w is homogeneous of degree 2, contradicting (3.3) and \|w_\|{ {\mathcal L}^{2}(\partial B_{1})} = 1. This proves the claim.

    Step 3 We will show that there exists constant C_2 such that for all x^0\in\partial\{u = 0\}\cap\{x_1 = 0\} satisfying

    \begin{equation} \liminf\limits_{r\to0^{+}}\frac{|B_r^+(x^0)\cap\{u = 0\}| }{|B_r^+|} = 0, \end{equation} (3.4)

    we have

    \Phi_{x^0}(0+)-\int_{B_{1}^+}x_1^{a}f(x^0)u_{x^0,r}\,dx\geq -C_2r^\alpha |f(x^0)|.

    In order to see this, we can observe that

    \begin{align*} \int_{B_{1}^+}x_1^af(x^0)u_{x^0,r}(x)dx & = f(x^0)\int_{0}^{1}\int_{0}^{1}\partial_{s}\Big[\int_{\partial B_{\rho}^+}(sx_1)^au_{x^0,r}(sx){\mathop{}\!d} {\mathcal H}^{n-1}(x)\Big] {\mathop{}\!d} s{\mathop{}\!d}\rho\\ & = f(x^0)\int_{0}^{1}\rho\int_{0}^{1}\int_{\partial B_{\rho}^+}(sx_1)^a\nabla u_{x^0,r}(sx)\cdot\nu{\mathop{}\!d} {\mathcal H}^{n-1}{\mathop{}\!d} s{\mathop{}\!d}\rho\\ & = f(x^0)\int_{0}^{1}\rho\int_{0}^{1}s\int_{B_{\rho}^+} \operatorname{div}((sx_1)^a \nabla u_{x^0,r}(sx)){\mathop{}\!d} x{\mathop{}\!d} s{\mathop{}\!d}\rho\\ & = f(x^0)\int_{0}^{1}\rho\int_{0}^{1}s\int_{B_{\rho}^+}(sx_1)^{a}f(rsx)\chi_{\Omega_{x^0,r}}(sx){\mathop{}\!d} x{\mathop{}\!d} s{\mathop{}\!d}\rho\\ & = f(x^0)^{2}\int_{0}^{1}\rho^{1+a+n}\int_{0}^{1}s^{1+a}\int_{B_{1}^+}x_1^{a}{\mathop{}\!d} x{\mathop{}\!d} s{\mathop{}\!d}\rho\\ &\qquad + f(x^0)\int_{0}^{1}\rho^{1+a+n}\int_{0}^{1}s^{1+a}\int_{B_{1}^+}x_1^{a}\left(f(rs\rho x)-f(x^0)\right){\mathop{}\!d} x{\mathop{}\!d} s{\mathop{}\!d}\rho\\ &\le \frac{f(x^0)^{2}}{(n+a+2)(a+2)} \int_{B_{1}^+}x_1^{b}{\mathop{}\!d} x+C_2r^\alpha |f(x^0)|. \end{align*}

    Now by condition (3.4), consider a sequence r_m \rightarrow0 such that \frac{|B_{r_m}^+(x^0)\cap\{v = 0\}| }{|B_{r_m}^+|} \rightarrow0 and assume that \nabla(u_{x^0, r_m}-p_{x^0, r_m})\rightharpoonup \nabla w in {\mathcal L}^2(B_1^+; x_1^a) as m \rightarrow\infty . Observe now \operatorname{div}(x_1^a w) = f(x^0) in B_1^+ and by similar calculation as above we will have

    \int_{B_{1}^+}x_1^af(x^0)w\,dx = \frac{f(x^0)^{2}}{(n+a+2)(a+2)} \int_{B_{1}^+}x_1^{a}{\mathop{}\!d} x.

    On the other hand,

    \begin{align*} \Phi_{x^0}(0+) & = \lim\limits_{m\to\infty}\Phi_{x^0}(r_{m})\\ & \ge\int_{B_{1}^+}(x_1^a\lvert\nabla w\rvert^{2}+2f(x^0)x_1^aw)dx-2\int_{\partial B_{1}^+}x_1^aw^{2}{\mathop{}\!d} {\mathcal H}^{n-1}\\ & = \int_{B_{1}^+}(-w \operatorname{div}(x_1^a \nabla w)+2f(x^0)x_1^aw)dx\\ & = \int_{B_{1}^+}x_1^af(x^0)w\,dx = \frac{f(x^0)^{2}}{(n+a+2)(a+2)} \int_{B_{1}^+}x_1^{a}{\mathop{}\!d} x. \end{align*}

    Step 4 In this step, we prove the proposition for the points satisfying condition (3.4). For these points, we have

    \begin{align*} &\frac12\partial_{r}\Big[\int_{\partial B_{1}^+}x_1^au_{x^0,r}^{2}\,d {\mathcal H}^{n-1}\Big] = \int_{\partial B_{1}^+}x_1^au_{x^0,r}\partial_r u_{x^0,r}\,d {\mathcal H}^{n-1} = \frac{1}{r}\int_{\partial B_{1}^+}x_1^au_{x^0,r}(\nabla u_{x^0,r}\cdot x-2u_{x^0,r}){\mathop{}\!d} {\mathcal H}^{n-1}\\ & = \frac{1}{r}\int_{B_{1}^+}(x_1^a\lvert \nabla u_{x^0,r}\rvert^{2}+u_{x^0,r} \operatorname{div}(x_1^a \nabla u_{x^0,r}))dx -\frac{2}{r}\int_{\partial B_{1}^+}x_1^au_{x^0,r}^{2}{\mathop{}\!d} {\mathcal H}^{n-1}\\ & = \frac{1}{r}\left(\Phi_{x^0}(r)-2\int_{B_{1}^+} f(x^0)x_1^au_{x^0,r}(x)\,dx\right. \left.+\int_{B_{1}^+}x_1^af(rx)u_{x^0,r}(x)\,dx\right)\\ & = \frac{1}{r}\left(\Phi_{x^0}(r)-\int_{B_{1}^+}f(x^0)x_1^au_{x^0,r}dx\right) +\frac{1}{r}\left(\int_{B_{1}^+}(f(rx)-f(x^0))x_1^au_{x^0,r}(x)\,dx\right)\\ & \ge\frac{1}{r}\left(\Phi_{x^0}(0^{+})-\int_{B_{1}^+}f(x^0)x_1^au_{x^0,r}dx\right)-Cr^{\alpha+\beta-2}-C_3r^{\alpha+\beta-2} \ge-C_{2}r^{\alpha-1}-Cr^{\alpha+\beta-2}-C_3r^{\alpha+\beta-2} \end{align*}

    Thus r\mapsto\partial_{r}\left[\int_{\partial B_{1}^+}x_1^au_{x^0, r}^{2}{\mathop{}\!d} {\mathcal H}^{n-1}\right] is integrable and we obtain uniform boundedness of \int_{\partial B_{1}^+}x_1^au_{x^0, r}^{2}d {\mathcal H}^{n-1} = r^{-n-3-b}\int_{\partial B_{r}(x^0)^+}x_1^au^{2}{\mathop{}\!d} {\mathcal H}^{n-1} for all points with property (3.4). It follows that the boundedness holds uniformly on the closure of those points x^0 .

    Step 5 We now consider the case

    \liminf\limits_{r\to0^{+}}\frac{|B_r^+(x^0)\cap\{u = 0\}| }{|B_r^+|} \gt 0.

    Let us assume towards a contradiction that there are sequences u^m , r_{m} and x^m such that and M_{m} = \|{u_{x^m, r_m}}\|_{ {\mathcal L}^{2}(\partial B_{1}^+)}\to+\infty as m\to\infty . Setting w_{m} = \frac{u^m_{x^m, r_{m}}}{M_{m}} we obtain, as in Step 2, that a subsequence of w_{m} converges weakly in W^{1, 2}(B_{1}^+; x_1^{a-2}) to a function w , with \|w_\|{ {\mathcal L}^{2}(\partial B_{1}^+; x_1^a)} = 1 , w(0) = \lvert\nabla w(0)\rvert = 0 , \operatorname{div}(x_1^a\nabla w) = 0 and

    \int_{B_{1}^+}x_1^a\lvert\nabla w|^{2}dx\le2\int_{\partial B_{1}^+}x_1^aw^{2}{\mathop{}\!d} {\mathcal H}^{n-1}.

    According to Lemma 2.3, w\in\mathbb{HP}_2 . In addition we now know that

    \int_{B_{1}^+}\chi_{\{w = 0\}}\ge\limsup\limits_{r_{m}\to0^{+}}\int_{B_{1}^+}\chi_{\{u_{x^0,r_{m}} = 0\}} \gt 0.

    This however contradicts the analyticity of w inside B_1^+ , knowing that \|w_\|{ {\mathcal L}^{2}(\partial B_{1};x_1^a)} = 1 .

    Now we are ready to prove the main result of the article.

    Proof of Theorem 1.2. From Theorem 8.17 in [4], we know that if \operatorname{div}(b(x)\nabla w) = g such that 1\leq b(x)\leq 5^a , then there exists a universal constant C = C(a, n) such that

    \|w\|_{ {\mathcal L}^\infty(B_{R/2})}\leq C(a,n)\left(R^{-n/2}\|w\|_{ {\mathcal L}^2(B_R)}+R^{2}\|g\|_{ {\mathcal L}^\infty(B_R)}\right).

    Now for x^0 \in \partial\{u = 0\}\cap\{x_1 = 0\}\cap B_{1/8}^+ and an arbitrary point y\in \partial B_r^+(x^0) , we apply the above estimate for R = 2\delta/3 , w = (\delta/3)^au and equation \operatorname{div}\left(b(x)\nabla w\right) = x_1^af\chi_{\{u\neq0\}} , where \delta = y_1 and b(x) = \frac{x_1^a}{(\delta/3)^a} . Note that 1\leq b(x)\leq 5^a in B_{2\delta/3}(y) and

    |u(y)|\leq C(a,n)\left((2\delta/3)^{-n/2}\|u\|_{ {\mathcal L}^2(B_{2\delta/3}(y))}+(2\delta/3)^{2}5^a\|f\|_{ {\mathcal L}^\infty(B_{2\delta/3}(y))}\right).

    According to Proposition 3.1,

    \begin{align*} \|u\|^2_{ {\mathcal L}^2(B_{2\delta/3}(y))}\leq & \Big(\frac3\delta\Big)^a\int_{B_{2\delta/3}(y)}x_1^a|u|^2\,dx\\ \leq & \Big(\frac3\delta\Big)^a\int_{B_{r+2\delta/3}(x^0)}x_1^a|u|^2\,dx \\ \leq &C\Big(\frac3\delta\Big)^a(r+2\delta/3)^{n+a+4}. \end{align*}

    Hence,

    \begin{equation*} |u(y)|\leq C\left(\delta^{-(n+a)/2}(r+\delta)^{(n+a+4)/2}+\delta^{2}\right)\leq Cy_1^2\left(\Big(\frac{r+y_1}{y_1}\Big)^{(n+a+4)/2}+1\right). \end{equation*}

    From this theorem it follows that solutions have quadratic growth inside cones.

    Corollary 3.2. Suppose u is a solution of (1.1) satisfying the condition in Proposition 1.1 and x^0\in \partial\{u = 0\}\cap\{x_1 = 0\}\cap B_{1/8}^+ . Then, for every constant \tau > 0 ,

    \sup\limits_{B_r(x^0)\cap {\mathcal C}}|u|\leq C\left(\left(\frac{1}{\tau}+ 1\right)^{\frac{n+a+4}{2}}+1\right) r^2,

    where {\mathcal C} : = \{x: x_1\geq \tau |x-x^0|\} .

    This paper was prepared while M. Fotouhi was visiting KTH Royal Institute of Technology. A. Minne was supported by the Knut and Alice Wallenberg Foundation. H. Shahgholian was supported by Swedish Research Council.

    The authors declare no conflict of interest.

    Let u be a solution of (1.1) for f\in {\mathcal L}^\infty(B_1^+) . We are going to show a priori regularity for solutions to (1.1). Consider the operator {\mathcal L}_{a, c}u: = x_1^2\Delta u+ax_1\partial_1u-cu . The following proposition is the regularity result related to this operator which has been proven by Krylov [5,Theorem 2.7,Theorem 2.8].

    Proposition A.1. i) For any a\in {\mathbb R}, p > 1 and \theta\in {\mathbb R} there exists a constant c_0 > 0 such that for any c\geq c_0 the operator {\mathcal L}_{a, c} is a bounded one-to-one operator from W^{2, p}({\mathbb R}^n_+; x_1^\theta) onto {\mathcal L}^p({\mathbb R}^n_+; x_1^\theta) and its inverse is also bounded, in particular for any u\in W^{2, p}({\mathbb R}^n_+; x_1^\theta)

    \|u\|_{W^{2,p}( {\mathbb R}^n_+;x_1^\theta)}\leq C\| {\mathcal L}_{a,c}u\|_{ {\mathcal L}^p( {\mathbb R}^n_+;x_1^\theta)},

    where C is independent of u and c .

    ii) The statement in i) is valid for the operator {\mathcal L}_{a, 0} when -1 < \theta < a-2 and a > 1 or either a-2 < \theta < -1 and a < 1 .

    Now we can deduce a priori regularity result for u as follows.

    Proof of Proposition 1.1. Notice that x_1^{\beta-1}u\in C(\overline{B_1^+}) due to Sobolev embedding Theorem 3.1 in [5]. Then if the statement of proposition fails, there exists a sequence u_j of solutions (1.1), x^j\in \{x_1 = 0\} and r_j\rightarrow0 such that

    \sup\limits_{B_r^+(x^j)}|x_1^{\beta-1}u_j|\leq jr^{1+\beta/2},\quad \forall r\geq r_j, \qquad \sup\limits_{B_{r_j}^+(x^j)}|x_1^{\beta-1}u_j| = jr_j^{1+\beta/2}.

    In particular, the function \tilde u_j(x) = \frac{u_j(x^j+r_jx)}{jr_j^{1+\beta/2}} , satisfies

    \begin{equation} \sup\limits_{B_R^+}|x_1^{\beta-1}\tilde u_j|\leq R^{1+\beta/2}, \qquad\text{for }1\leq R\leq\frac1{r_j}, \end{equation} (A.1)

    and with equality for R = 1 , along with

    \begin{equation} {\mathcal L}_{a,c_0}\tilde u_j = \frac{r_j^{1-\beta/2}}{j}f(x^j+r_jx)-c_0\tilde u_j, \end{equation} (A.2)

    where c_0 is defined in Proposition A.1. According to (A.1), the right hand side of (A.2) is uniformly bounded in {\mathcal L}^p(B_R^+; x_1^\theta) for p(\beta-1)-1 < \theta\leq-1 . From here and Proposition A.1 we conclude that \{\tilde u_j\} is bounded in W^{2, p}(B_{R}^+; x_1^\theta) for some \theta\leq-1 and there is a convergent subsequence, tending to a function u_0 with properties

    \begin{equation} \sup\limits_{B_R^+}|x_1^{\beta-1} u_0|\leq R^{1+\beta/2},\quad\forall R\geq1,\quad\sup\limits_{B_1^+}|x_1^{\beta-1} u_0| = 1,\quad \operatorname{div}(x_1^a \nabla u_0) = 0, \end{equation} (A.3)

    as well as the condition \theta\leq-1 insures that the trace operator is well defined and u_0 is zero on \{x_1 = 0\} . The Liouville type theorem in [9,Lemma 20]) implies that

    |D^2u_0(x^0)|\leq \frac C{R^2}\sup\limits_{B_R^+(x^0)}|u_0|\leq \frac{C(R+|x_0|)^{2-\beta/2}}{R^2}\rightarrow0.

    Therefore, u_0 is a linear function, which contradicts (A.3).



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