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RETRACTED ARTICLE: A remark on the aminimally thin sets associated with the Schrödinger operator
Boundary Value Problems volume 2014, Article number: 133 (2014)
Abstract
The aim of this paper is to give a new criterion for aminimally thin sets at infinity with respect to the Schrödinger operator in a cone, which supplement the results obtained by T. Zhao.
1 Introduction and results
Let R and {\mathbf{R}}_{+} be the set of all real numbers and the set of all positive real numbers, respectively. We denote by {\mathbf{R}}^{n} (n\ge 2) the ndimensional Euclidean space. A point in {\mathbf{R}}^{n} is denoted by P=(X,{x}_{n}), X=({x}_{1},{x}_{2},\dots ,{x}_{n1}). The Euclidean distance between two points P and Q in {\mathbf{R}}^{n} is denoted by PQ. Also PO with the origin O of {\mathbf{R}}^{n} is simply denoted by P. The boundary and the closure of a set S in {\mathbf{R}}^{n} are denoted by ∂S and \overline{S}, respectively. Further, intS, diamS, and dist({S}_{1},{S}_{2}) stand for the interior of S, the diameter of S, and the distance between {S}_{1} and {S}_{2}, respectively.
We introduce a system of spherical coordinates (r,\mathrm{\Theta}), \mathrm{\Theta}=({\theta}_{1},{\theta}_{2},\dots ,{\theta}_{n1}), in {\mathbf{R}}^{n} which are related to cartesian coordinates ({x}_{1},{x}_{2},\dots ,{x}_{n1},{x}_{n}) by {x}_{n}=rcos{\theta}_{1}.
Let D be an arbitrary domain in {\mathbf{R}}^{n} and {\mathcal{A}}_{a} denote the class of nonnegative radial potentials a(P), i.e. 0\le a(P)=a(r), P=(r,\mathrm{\Theta})\in D, such that a\in {L}_{\mathrm{loc}}^{b}(D) with some b>n/2 if n\ge 4 and with b=2 if n=2 or n=3 (see [[1], p.354] and [2]).
If a\in {\mathcal{A}}_{a}, then the stationary Schrödinger operator
where Δ is the Laplace operator and I is the identical operator, can be extended in the usual way from the space {C}_{0}^{\mathrm{\infty}}(D) to an essentially selfadjoint operator on {L}^{2}(D) (see [[1], Ch. 11]). We will denote it Sc{h}_{a} as well. This last one has a Green afunction {G}_{D}^{a}(P,Q). Here {G}_{D}^{a}(P,Q) is positive on D and its inner normal derivative \partial {G}_{D}^{a}(P,Q)/\partial {n}_{Q}\ge 0, where \partial /\partial {n}_{Q} denotes the differentiation at Q along the inward normal into D.
We call a function u\not\equiv \mathrm{\infty} that is upper semicontinuous in D a subfunction with respect to the Schrödinger operator Sc{h}_{a} if its values belong to the interval [\mathrm{\infty},\mathrm{\infty}) and at each point P\in D with 0<r<r(P) we have the generalized meanvalue inequality (see [[1], Ch. 11])
satisfied, where {G}_{B(P,r)}^{a}(P,Q) is the Green afunction of Sc{h}_{a} in B(P,r) and d\sigma (Q) is a surface measure on the sphere S(P,r)=\partial B(P,r). If −u is a subfunction, then we call u a superfunction (with respect to the Schrödinger operator Sc{h}_{a}).
The unit sphere and the upper half unit sphere in {\mathbf{R}}^{n} are denoted by {\mathbf{S}}^{n1} and {\mathbf{S}}_{+}^{n1}, respectively. For simplicity, a point (1,\mathrm{\Theta}) on {\mathbf{S}}^{n1} and the set \{\mathrm{\Theta};(1,\mathrm{\Theta})\in \mathrm{\Omega}\} for a set Ω, \mathrm{\Omega}\subset {\mathbf{S}}^{n1}, are often identified with Θ and Ω, respectively. For two sets \mathrm{\Xi}\subset {\mathbf{R}}_{+} and \mathrm{\Omega}\subset {\mathbf{S}}^{n1}, the set \{(r,\mathrm{\Theta})\in {\mathbf{R}}^{n};r\in \mathrm{\Xi},(1,\mathrm{\Theta})\in \mathrm{\Omega}\} in {\mathbf{R}}^{n} is simply denoted by \mathrm{\Xi}\times \mathrm{\Omega}. By {C}_{n}(\mathrm{\Omega}), we denote the set {\mathbf{R}}_{+}\times \mathrm{\Omega} in {\mathbf{R}}^{n} with the domain Ω on {\mathbf{S}}^{n1}. We call it a cone. We denote the set I\times \mathrm{\Omega} with an interval on R by {C}_{n}(\mathrm{\Omega};I).
From now on, we always assume D={C}_{n}(\mathrm{\Omega}). For the sake of brevity, we shall write {G}_{\mathrm{\Omega}}^{a}(P,Q) instead of {G}_{{C}_{n}(\mathrm{\Omega})}^{a}(P,Q). We shall also write {g}_{1}\approx {g}_{2} for two positive functions {g}_{1} and {g}_{2}, if and only if there exists a positive constant c such that {c}^{1}{g}_{1}\le {g}_{2}\le c{g}_{1}.
Let Ω be a domain on {\mathbf{S}}^{n1} with smooth boundary. Consider the Dirichlet problem
where {\mathrm{\Lambda}}_{n} is the spherical part of the Laplace operata {\mathrm{\Delta}}_{n}
We denote the least positive eigenvalue of this boundary value problem by λ and the normalized positive eigenfunction corresponding to λ by \phi (\mathrm{\Theta}). In order to ensure the existence of λ and a smooth \phi (\mathrm{\Theta}), we put a rather strong assumption on Ω: if n\ge 3, then Ω is a {C}^{2,\alpha}domain (0<\alpha <1) on {\mathbf{S}}^{n1} surrounded by a finite number of mutually disjoint closed hypersurfaces (e.g. see [[3], pp.8889] for the definition of {C}^{2,\alpha}domain).
For any (1,\mathrm{\Theta})\in \mathrm{\Omega}, we have (see [[4], pp.78])
where P=(r,\mathrm{\Theta})\in {C}_{n}(\mathrm{\Omega}) and \delta (P)=dist(P,\partial {C}_{n}(\mathrm{\Omega})).
Solutions of an ordinary differential equation (see [[5], p.217])
It is well known (see, for example, [6]) that if the potential a\in {\mathcal{A}}_{a}, then equation (2) has a fundamental system of positive solutions \{V,W\} such that V and W are increasing and decreasing, respectively.
We will also consider the class {\mathcal{B}}_{a}, consisting of the potentials a\in {\mathcal{A}}_{a} such that there exists the finite limit {lim}_{r\to \mathrm{\infty}}{r}^{2}a(r)=k\in [0,\mathrm{\infty}), and, moreover, {r}^{1}{r}^{2}a(r)k\in L(1,\mathrm{\infty}). If a\in {\mathcal{B}}_{a}, then the (sub)superfunctions are continuous (see [7]). In the rest of paper, we assume that a\in {\mathcal{B}}_{a} and we shall suppress this assumption for simplicity.
Denote
then the solutions to equation (2) have the asymptotic (see [3])
It is well known that the Martin boundary of {C}_{n}(\mathrm{\Omega}) is the set \partial {C}_{n}(\mathrm{\Omega})\cup \{\mathrm{\infty}\}, each of which is a minimal Martin boundary point. For P\in {C}_{n}(\mathrm{\Omega}) and Q\in \partial {C}_{n}(\mathrm{\Omega})\cup \{\mathrm{\infty}\}, the Martin kernel can be defined by {M}_{\mathrm{\Omega}}^{a}(P,Q). If the reference point P is chosen suitably, then we have
for any P=(r,\mathrm{\Theta})\in {C}_{n}(\mathrm{\Omega}).
In [[8], p.67], Zhao introduce the notations of athin (with respect to the Schrödinger operator Sc{h}_{a}) at a point, apolar set (with respect to the Schrödinger operator Sc{h}_{a}) and aminimal thin sets at infinity (with respect to the Schrödinger operator Sc{h}_{a}). A set H in {\mathbf{R}}^{n} is said to be athin at a point Q if there is a fine neighborhood E of Q which does not intersect H\mathrm{\setminus}\{Q\}. Otherwise H is said to be not athin at Q on {C}_{n}(\mathrm{\Omega}). A set H in {\mathbf{R}}^{n} is called a polar set if there is a superfunction u on some open set E such that H\subset \{P\in E;u(P)=\mathrm{\infty}\}. A subset H of {C}_{n}(\mathrm{\Omega}) is said to be aminimal thin at Q\in \partial {C}_{n}(\mathrm{\Omega})\cup \{\mathrm{\infty}\} on {C}_{n}(\mathrm{\Omega}), if there exists a point P\in {C}_{n}(\mathrm{\Omega}) such that
where {\stackrel{\u02c6}{R}}_{{M}_{\mathrm{\Omega}}^{a}(\cdot ,Q)}^{H} is the regularized reduced function of {M}_{\mathrm{\Omega}}^{a}(\cdot ,Q) relative to H (with respect to the Schrödinger operator Sc{h}_{a}).
Let H be a bounded subset of {C}_{n}(\mathrm{\Omega}). Then {\stackrel{\u02c6}{R}}_{{M}_{\mathrm{\Omega}}^{a}(\cdot ,\mathrm{\infty})}^{H}(P) is bounded on {C}_{n}(\mathrm{\Omega}) and hence the greatest aharmonic minorant of {\stackrel{\u02c6}{R}}_{{M}_{\mathrm{\Omega}}^{a}(\cdot ,\mathrm{\infty})}^{H} is zero. When by {G}_{\mathrm{\Omega}}^{a}\mu (P) we denote the Green apotential with a positive measure μ on {C}_{n}(\mathrm{\Omega}), we see from the Riesz decomposition theorem that there exists a unique positive measure {\lambda}_{H}^{a} on {C}_{n}(\mathrm{\Omega}) such that
for any P\in {C}_{n}(\mathrm{\Omega}) and {\lambda}_{H}^{a} is concentrated on {I}_{H}, where
The Green aenergy {\gamma}_{\mathrm{\Omega}}^{a}(H) (with respect to the Schrödinger operator Sc{h}_{a}) of {\lambda}_{H}^{a} is defined by
Also, we can define a measure {\sigma}_{\mathrm{\Omega}}^{a} on {C}_{n}(\mathrm{\Omega})
In [[8], Theorem 5.4.3], Long gave a criterion that characterizes aminimally thin sets at infinity in a cone.
Theorem A A subset H of {C}_{n}(\mathrm{\Omega}) is aminimally thin at infinity on {C}_{n}(\mathrm{\Omega}) if and only if
where {H}_{j}=H\cap {C}_{n}(\mathrm{\Omega};[{2}^{j},{2}^{j+1})) and j=0,1,2,\dots .
In recent work, Zhao (see [[2], Theorems 1 and 2]) proved the following results. For similar results in the half space with respect to the Schrödinger operator, we refer the reader to the papers by Ren and Su (see [9, 10]).
Theorem B The following statements are equivalent.

(I)
A subset H of {C}_{n}(\mathrm{\Omega}) is aminimally thin at infinity on {C}_{n}(\mathrm{\Omega}).

(II)
There exists a positive superfunction v(P) on {C}_{n}(\mathrm{\Omega}) such that
\underset{P\in {C}_{n}(\mathrm{\Omega})}{inf}\frac{v(P)}{{M}_{\mathrm{\Omega}}^{a}(P,\mathrm{\infty})}=0(5)and
H\subset \{P\in {C}_{n}(\mathrm{\Omega});v(P)\ge {M}_{\mathrm{\Omega}}^{a}(P,\mathrm{\infty})\}. 
(III)
There exists a positive superfunction v(P) on {C}_{n}(\mathrm{\Omega}) such that even if v(P)\ge c{M}_{\mathrm{\Omega}}^{a}(P,\mathrm{\infty}) for any P\in H, there exists {P}_{0}\in {C}_{n}(\mathrm{\Omega}) satisfying v({P}_{0})<c{M}_{\mathrm{\Omega}}^{a}({P}_{0},\mathrm{\infty}).
Theorem C If a subset H of {C}_{n}(\mathrm{\Omega}) is aminimally thin at infinity on {C}_{n}(\mathrm{\Omega}), then we have
Remark From equation (3), we immediately know that equation (6) is equivalent to
This paper aims to show that the sharpness of the characterization of an aminimally thin set in Theorem C. In order to do this, we introduce the Whitney cubes in a cone.
A cube is the form
where j, {l}_{1},\dots ,{l}_{n} are integers. The Whitney cubes of {C}_{n}(\mathrm{\Omega}) are a family of cubes having the following properties:

(I)
{\bigcup}_{k}{W}_{k}={C}_{n}(\mathrm{\Omega}).

(II)
int{W}_{j}\cap int{W}_{k}=\mathrm{\varnothing} (j\ne k).

(III)
diam{W}_{k}\le dist({W}_{k},{\mathbf{R}}^{n}\mathrm{\setminus}{C}_{n}(\mathrm{\Omega}))\le 4diam{W}_{k}.
Theorem 1 If H is a union of cubes from the Whitney cubes of {C}_{n}(\mathrm{\Omega}), then equation (7) is also sufficient for H to be aminimally thin at infinity with respect to {C}_{n}(\mathrm{\Omega}).
From the Remark and Theorem 1, we have the following.
Corollary 1 Let v(P) be a positive superfunction on {C}_{n}(\mathrm{\Omega}) such that equation (5) holds. Then we have
Corollary 2 Let H be a Borel measurable subset of {C}_{n}(\mathrm{\Omega}) satisfying
If v(P) is a nonnegative superfunction on {C}_{n}(\mathrm{\Omega}) and c is a positive number such that v(P)\ge c{M}_{\mathrm{\Omega}}^{a}(P,\mathrm{\infty}) for all P\in H, then v(P)\ge c{M}_{\mathrm{\Omega}}^{a}(P,\mathrm{\infty}) for all P\in {C}_{n}(\mathrm{\Omega}).
2 Lemmas
To prove our results, we need some lemmas.
Lemma 1 Let {W}_{k} be a cube from the Whitney cubes of {C}_{n}(\mathrm{\Omega}). Then there exists a constant c independent of k such that
Proof If we apply a result of Long (see [[8], Theorem 6.1.3]) for compact set {\overline{W}}_{k}, we obtain a measure μ on {C}_{n}(\mathrm{\Omega}), supp\mu \subset {\overline{W}}_{k}, \mu ({\overline{W}}_{k})=1 such that
for any P\in {\overline{W}}_{k}. Also there exists a positive measure {\lambda}_{{\overline{W}}_{k}}^{a} on {C}_{n}(\mathrm{\Omega}) such that
for any P\in {C}_{n}(\mathrm{\Omega}).
Let {P}_{k}=({r}_{k},{\mathrm{\Theta}}_{k}), {\rho}_{k}, {t}_{k} be the center of {W}_{k}, the diameter of {W}_{j}, the distance between {W}_{k} and \partial {C}_{n}(\mathrm{\Omega}), respectively. Then we have {\rho}_{k}\le {t}_{k}\le 4{\rho}_{k} and {\rho}_{k}\le {r}_{k}. Then from equation (1) we have
for any P\in {\overline{W}}_{k}. We can also prove that
for any P\in {\overline{W}}_{k} and any Q\in {\overline{W}}_{k}. Hence we obtain
from equations (8), (9), (10), and (11). Since
from equations (3), (9), and (10), we have from (12)
Since
we obtain from equation (13)
On the other hand, we have from equation (1)
which, together with equation (14), gives the conclusion of Lemma 1. □
3 Proof of Theorem 1
Let \{{W}_{k}\} be a family of cubes from the Whitney cubes of {C}_{n}(\mathrm{\Omega}) such that H={\bigcup}_{k}{W}_{k}. Let \{{W}_{k,j}\} be a subfamily of \{{W}_{k}\} such that {W}_{k,j}\subset ({H}_{j1}\cup {H}_{j}\cup {H}_{j+1}), where j=1,2,3,\dots .
Since {\gamma}_{\mathrm{\Omega}}^{a} is a countably subadditive set function (see [[8], p.49]), we have
for j=1,2,\dots . Hence for j=1,2,\dots we see from Lemma 1
which, together with equation (1), gives
for j=1,2,\dots . Thus equations (15), (16), and (17) give
for j=1,2,\dots . Finally we obtain from equation (1)
which shows with Theorem A that H is aminimally thin at infinity with respect to {C}_{n}(\mathrm{\Omega}).
Change history
25 May 2021
A Correction to this paper has been published: https://doi.org/10.1186/s13661021015309
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Acknowledgements
This work was supported by the National Natural Science Foundation of China under Grants Nos. 11301140 and U1304102.
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Xue, G. RETRACTED ARTICLE: A remark on the aminimally thin sets associated with the Schrödinger operator. Bound Value Probl 2014, 133 (2014). https://doi.org/10.1186/168727702014133
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DOI: https://doi.org/10.1186/168727702014133