## Abstract

Our main aim in this paper is to obtain a new type of boundary integral behaviors of harmonic functions in a smooth cone. As an application, the least harmonic majorant of a nonnegative subharmonic function is also given.

## Introduction

Let \(B(P,R)\) denote the open ball with center at *P* and radius *R* in \(\mathbf{R}^{n}\), where \(\mathbf{R}^{n}\) is the *n*-dimensional Euclidean space, \(P\in\mathbf{R}^{n}\) and \(R>0\). Let \(B(P)\) denote the neighborhood of *P* and \(S_{R}=B(O,R)\) for simplicity. The unit sphere and the upper half unit sphere in \(\mathbf{R}^{n}\) are denoted by \(\mathbf{S}_{1}\) and \(\mathbf{S}_{1}^{+}\), respectively. For simplicity, a point \((1,\Theta)\) on \(\mathbf{S}_{1}\) and the set \(\{\Theta; (1,\Theta)\in\Gamma\}\) for a set Γ, \(\Gamma\subset\mathbf{S}_{1}\), are often identified with Θ and Γ, respectively. Let \(\Lambda\times\Gamma\) denote the set \(\{(r,\Theta)\in \mathbf{R}^{n}; r\in\Lambda,(1,\Theta)\in\Gamma\}\), where \(\Lambda\subset\mathbf{R}_{+}\) and \(\Gamma\subset\mathbf{S}_{1}\). We denote the set \(\mathbf{R}_{+}\times\mathbf{S}_{1}^{+}=\{(X,x_{n})\in\mathbf{R}^{n}; x_{n}>0\}\) by \(\mathbf{T}_{n}\), which is called the half space.

We shall also write \(h_{1}\approx h_{2}\) for two positive functions \(h_{1}\) and \(h_{2}\) if and only if there exists a positive constant *a* such that \(a^{-1}h_{1}\leq h_{2}\leq ah_{1}\). We denote \(\max\{u(r,\Theta),0\}\) and \(\max\{-u(r,\Theta),0\}\) by \(u^{+}(r,\Theta)\) and \(u^{-}(r,\Theta)\), respectively.

The set \(\mathbf{R}_{+}\times\Gamma\) in \(\mathbf{R}^{n}\) is called a cone. We denote it by \(\mathfrak{C}_{n}(\Gamma)\), where \(\Gamma\subset\mathbf{S}_{1}\). The sets \(I\times\Gamma\) and \(I\times \partial{\Gamma}\) with an interval on **R** are denoted by \(\mathfrak {C}_{n}(\Gamma;I)\) and \(\mathfrak{S}_{n}(\Gamma;I)\), respectively. We denote \(\mathfrak {C}_{n}(\Gamma)\cap S_{R}\) and \(\mathfrak{S}_{n}(\Gamma; (0,+\infty))\) by \(\mathfrak {S}_{n}(\Gamma; R)\) and \(\mathfrak{S}_{n}(\Gamma)\), respectively.

Furthermore, we denote by *dσ* (resp. \(dS_{R}\)) the \((n-1)\)-dimensional volume elements induced by the Euclidean metric on \(\partial{\mathfrak{C}_{n}(\Gamma)}\) (resp. \(S_{R}\)) and by *dw* the elements of the Euclidean volume in \(\mathbf{R}^{n}\).

It is known (see, e.g., [1], p.41) that

where \(\Delta^{*}\) is the Laplace-Beltrami operator. We denote the least positive eigenvalue of this boundary value problem (1.1) by *λ* and the normalized positive eigenfunction corresponding to *λ* by \(\varphi(\Theta)\), \(\int_{\Gamma}\varphi^{2}(\Theta)\,dS_{1}=1\).

We remark that the function \(r^{\aleph^{\pm}}\varphi(\Theta)\) is harmonic in \(\mathfrak{C}_{n}(\Gamma)\), belongs to the class \(C^{2}(\mathfrak{C}_{n}(\Gamma )\backslash\{O\})\) and vanishes on \(\mathfrak{S}_{n}(\Gamma)\), where

For simplicity we shall write *χ* instead of \(\aleph^{+}-\aleph^{-}\).

For simplicity we shall assume that the boundary of the domain Γ is twice continuously differentiable, \(\varphi\in C^{2}(\overline{\Gamma})\) and \(\frac{\partial\varphi}{\partial n}>0\) on *∂*Γ. Then (see [2], pp.7-8)

where \(\Theta\in\Gamma\).

Let \(\delta(P)=\operatorname{dist}(P,\partial{\mathfrak{C}_{n}(\Gamma)})\), we have

for any \(P=(1,\Theta)\in\Gamma\) (see [3, 4]).

Let \(u(r,\Theta)\) be a function on \(\mathfrak{C}_{n}(\Gamma)\). For any given \(r\in\mathbf{R}_{+}\), the integral

is denoted by \(\mathcal{N}_{u}(r)\) when it exists. The finite or infinite limit

is denoted by \(\mathscr{U}_{u}\) when it exists.

### Remark 1

A function \(g(t)\) on \((0,\infty)\) is \(\mathbb{A}_{d_{1},d_{2}}\)-convex if and only if \(g(t)t^{d_{2}}\) is a convex function of \(t^{d}\) (\(d=d_{1}+d_{2}\)) on \((0,\infty)\) or, equivalently, if and only if \(g(t)t^{-d_{1}}\) is a convex function of \(t^{-d}\) on \((0,\infty)\).

### Remark 2

\(\mathcal{N}_{u}(r)\) is \(\mathbb{A}_{\aleph^{+},\gamma-1}\)-convex on \((0,\infty)\), where *u* is a subharmonic function on \(\mathfrak{C}_{n}(\Gamma)\) such that

for any \(Q\in \partial{\mathfrak{C}_{n}(\Gamma)}\) (see [5]).

The function

is called the ordinary Poisson kernel, where \(\mathbb{G}_{\mathfrak{C}_{n}(\Gamma)}\) is the Green function.

The Poisson integral of *g* relative to \(\mathfrak{C}_{n}(\Gamma)\) is defined by

where *g* is a continuous function on \(\partial{\mathfrak{C}_{n}(\Gamma)}\) and \(\frac{\partial}{\partial n_{Q}}\) denotes the differentiation at *Q* along the inward normal into \(\mathfrak{C}_{n}(\Gamma)\).

We set functions *f* satisfying

where \(p>0\) and

Further, we denote by \(\mathcal{A}_{\Gamma}\) the class of all measurable functions \(g(t,\Phi)\) (\(Q=(t,\Phi)=(Y, y_{n})\in \mathfrak{C}_{n}(\Gamma)\)) satisfying the following inequality:

and the class \(\mathcal{B}_{\Gamma}\) consists of all measurable functions \(h(t,\Phi)\) (\((t,\Phi)=(Y, y_{n})\in\mathfrak{S}_{n}(\Gamma)\)) satisfying

We will also consider the class of all continuous functions \(u(t,\Phi)\) (\((t,\Phi)\in\overline{\mathfrak{C}_{n}(\Gamma)}\)) harmonic in \(\mathfrak{C}_{n}(\Gamma)\) with \(u^{+}(t,\Phi)\in \mathcal{A}_{\Gamma}\) (\((t,\Phi)\in\mathfrak{C}_{n}(\Gamma)\)), and \(u^{+}(t,\Phi)\in\mathcal{B}_{\Gamma}\) (\((t,\Phi)\in\mathfrak {S}_{n}(\Gamma)\)) is denoted by \(\mathcal{C}_{\Gamma}\).

### Remark 3

If we denote \(\Gamma=S_{1}^{+}\) in (1.6) and (1.7), we have

Recently Zhao and Yamada (see [6]) obtained the following result.

### Theorem A

*Let**g**be a measurable function on*
\(\partial{T_{n}}\)*such that*

*Then the harmonic function*
\(\mathbb{PI}_{T_{n}}[g]\)*satisfies*
\(\mathbb{PI}_{T_{n}}[g](P)=o(r \sec^{n-1}\theta_{1})\)*as*
\(r\rightarrow\infty\)*in*
\(T_{n}\).

Recently Wang and Qiao (see [7]) generalized Theorem A to the conical case.

### Theorem B

*Let**g**be a continuous function on*
\(\partial{\mathfrak{C}_{n}(\Gamma)}\)*satisfying* (1.5) *with*
\(p=1\)*and*
\(\gamma=-\aleph^{-}+1\). *Then*

## Results

Our main aim in this paper is to give the least harmonic majorant of a nonnegative subharmonic function on \(\mathfrak{C}_{n}(\Gamma)\). For related results, we refer the reader to the papers [8, 9].

### Theorem 1

*If**u**is a subharmonic function on a domain containing*
\(\overline{\mathfrak{C}_{n}(\Gamma)}\), \(u\geq0\)*on*
\(\mathfrak{C}_{n}(\Gamma)\)*and*
\(u'=u\vert\partial{\mathfrak{C}_{n}(\Gamma )}\) (*the restriction of**u**to*
\(\partial{\mathfrak{C}_{n}(\Gamma)}\)) *satisfies* (1.5), *then the limit*
\(\mathscr{U}_{u}\) (\(0\leq\mathscr{U}_{u}\leq+\infty\)) *exists*. *Further*, *if*
\(\mathscr{U}_{u}<+\infty\), *then*

*where*
\(h_{u}(P)\)*is the least harmonic majorant of**u**on*
\(\mathfrak{C}_{n}(\Gamma)\).

## Main lemmas

### Lemma 1

*Let**u**be a function subharmonic on*
\(\mathfrak{C}_{n}(\Gamma)\)*satisfying* (1.4) *for any*
\(Q\in\partial {\mathfrak{C}_{n}(\Gamma)}\). *Then the limit*
\(\mathscr{U}_{u}\) (\(-\infty<\mathscr{U}_{u}\leq+\infty\)) *exists*.

### Proof

It suffices to prove that the limit \(\lim_{r\rightarrow0}r^{\gamma-1}\mathcal{N}_{u}(r)\) exists, then apply it to the function

where \(K: (r,\Theta)\rightarrow(r^{-1},\Theta)\) is the Kelvin transform (see [10], pp.36-37). Consider the auxiliary function

on \((a^{-\chi},+\infty)\). Then, from Remarks 1 and 2, \(I(s)\) is a convex function on \((a^{-\chi},+\infty)\). Hence

exists. □

### Lemma 2

*Let**u**be a nonnegative subharmonic function on*
\(\mathfrak{C}_{n}(\Gamma)\)*satisfying* (1.4) *for any*
\(Q\in \partial{\mathfrak{C}_{n}(\Gamma)}\)*and*

*Then*

*for any*
\((r,\Theta)\in\mathfrak{C}_{n}(\Gamma)\), *where**M**is a positive constant*.

### Proof

Take any \((r,\Theta)\in\mathfrak{C}_{n}(\Gamma)\) and any pair of numbers \(R_{1}\), \(R_{2}\) (\(0<2R_{1}<r<\frac{1}{2}R_{2}<+\infty\)). We define a boundary function on \(\partial{\mathfrak{C}_{n}(\Gamma;(R_{1},R_{2}))}\) by

This is an upper semi-continuous function which is bounded above. If we denote Perron-Wiener-Brelot solution of the Dirichlet problem on \(\mathfrak{C}_{n}(\Gamma;(R_{1},R_{2}))\) with *ν* by \(H_{\nu}((r,\Theta);\mathfrak{C}_{n}(\Gamma;(R_{1},R_{2})))\), then we have

which gives that

As \(R_{1}\rightarrow0\) and \(R_{2}\rightarrow+\infty\) in (3.2), we complete the proof by (3.1). □

### Lemma 3

*Let**g**be a locally integrable function on*
\(\partial{\mathfrak{C}_{n}(\Gamma)}\)*satisfying* (1.5) *and**u**be a subharmonic function on*
\(\mathfrak{C}_{n}(\Gamma)\)*satisfying*

*and*

*for any*
\(Q\in\partial{\mathfrak{C}_{n}(\Gamma)}\). *Then the limits*
\(\mathscr{U}_{u}\)*and*
\(\mathscr{U}_{u^{+}}\) (\(-\infty<\mathscr{U}_{u}\leq+\infty\), \(0\leq\mathscr{U}_{u^{+}}\leq+\infty\)) *exist*, *and if* (3.1) *is satisfied*, *then*

*where**M**is a positive constant and*
\(P=(r,\Theta)\in\mathfrak {C}_{n}(\Gamma)\).

### Proof

Consider two subharmonic functions

on \(\mathfrak{C}_{n}(\Gamma)\). From (3.3) and (3.4) we have

for any \(Q\in\partial{\mathfrak{C}_{n}(\Gamma)}\). Hence it follows from Lemma 1 that the limits \(\mathscr{U}_{U}\) and \(\mathscr{U}_{U'}\) (\(-\infty<\mathscr{U}_{U}\leq+\infty\), \(0\leq\mathscr{U}_{U'}\leq+\infty\)) exist. Since

Theorem B (Theorem 1 will be proved in the next section) gives the existences of the limits \(\mathscr{U}_{u}\), \(\mathscr{U}_{u^{+}}\),

Since \(0\leq U^{+}(P)\leq u^{+}(P)+(\mathbb{PI}_{\mathfrak{C}_{n}(\Gamma )}[g])^{-}(P)\) on \(\mathfrak{C}_{n}(\Gamma)\), it also follows from Theorem B and (3.1) that

Hence, by applying Lemma 2 to \(U(P)\), we obtain the conclusion from (3.6). □

### Lemma 4

*Let**g**be a nonnegative lower semi*-*continuous function on*
\(\partial{\mathfrak{C}_{n}(\Gamma)}\)*satisfying* (1.5) *and**u**be a nonnegative subharmonic function on*
\(\mathfrak{C}_{n}(\Gamma )\)*such that*

*for any*
\(Q\in\partial{\mathfrak{C}_{n}(\Gamma)}\). *Then the limit*
\(\mathscr{U}_{u}\) (\(0\leq\mathscr{U}_{u}\leq+\infty\)) *exists*, *and if*
\(\mathscr{U}_{u}<+\infty\), *then*

*for any*
\(P=(r,\Theta)\in\mathfrak{C}_{n}(\Gamma)\).

### Proof

Since −*g* is an upper semi-continuous function \(\partial{\mathfrak{C}_{n}(\Gamma)}\), it follows from [11], p.3, that

for any \(Q\in \partial{\mathfrak{C}_{n}(\Gamma)}\). We see from (3.7) and (3.8) that

for any \(Q\in \partial{\mathfrak{C}_{n}(\Gamma)}\), which gives (3.3). Since *g* and *u* are nonnegative, (3.4) also holds. Thus we obtain the conclusion from Lemma 3. □

### Lemma 5

*Let**u**be subharmonic on a domain containing*
\(\overline{\mathfrak{C}_{n}(\Gamma)}\)*such that*
\(u'=u\vert\partial{\mathfrak{C}_{n}(\Gamma)}\)*satisfies* (1.5) *and*
\(u\geq0\)*on*
\(\mathfrak{C}_{n}(\Gamma)\). *Then*
\(\mathbb{PI}_{\mathfrak{C}_{n}(\Gamma )}[u'](P)\leq h(P)\)*on*
\(\mathfrak{C}_{n}(\Gamma)\), *where*
\(h(P)\)*is any harmonic majorant of**u**on*
\(\mathfrak{C}_{n}(\Gamma)\).

### Proof

Take any \(P'=(r',\Theta')\in \mathfrak{C}_{n}(\Gamma)\). Let *ϵ* be any positive number. In the same way as in the proof of Lemma 2, we can choose *R* such that

Further, take an integer *j* (\(j>R\)) such that

Since

for any \(P\in\mathfrak{C}_{n}(\Gamma;(0,j))\), we have from (3.9) and (3.10) that (see [12])

Here note that \(H_{u}(P;\mathfrak{C}_{n}(\Gamma;(0,j)))\) is the least harmonic majorant of *u* on \(\mathfrak{C}_{n}(\Gamma;(0,j))\) (see [13], Theorem 3.15). If *h* is a harmonic majorant of *u* on \(\mathfrak{C}_{n}(\Gamma)\), then

Thus we obtain from (3.11) that

which gives the conclusion of Lemma 5. □

## Proof of Theorem 1

Let \(P=(r,\Theta)\) be any point of \(\mathfrak{C}_{n}(\Gamma)\) and *ϵ* be any positive number. By the Vitali-Carathéodory theorem (see [10], p.56), we can find a lower semi-continuous function \(g'(Q)\) on \(\partial{\mathfrak {C}_{n}(\Gamma)}\) such that

and

Since

for any \(Q\in\partial{\mathfrak{C}_{n}(\Gamma)}\) from (4.1), it follows from Lemma 4 that the limit \(\mathscr{U}_{u}\) exists (see [11]), and if \(\mathscr{U}_{u}<+\infty\), then

Hence we have from (4.2) and (4.3) that (2.1) holds.

Next we shall assume that \(h_{u}(P)\) is the least harmonic majorant of *u* on \(\mathfrak{C}_{n}(\Gamma)\). Set \(h''(P)\) is a harmonic function on \(\mathfrak{C}_{n}(\Gamma)\) such that

Consider the harmonic function

Since

Theorem B gives that \(\mathscr{U}_{{h^{\ast}}^{+}}<+\infty\). Further, from Lemma 2 we see that

for any \(Q\in \partial{\mathfrak{C}_{n}(\Gamma)}\). From Theorem B and (4.4) we know

We see from Lemma 2 that \(h^{\ast}(P)\leq0\) on \(\mathfrak {C}_{n}(\Gamma)\), which shows that \(h_{u}(P)\) is the least harmonic majorant of \(u(P)\) on \(\mathfrak{C}_{n}(\Gamma)\). Theorem 1 is proved.

## Conclusions

In this article, we have obtained a new type of boundary integral behaviors of harmonic functions in a smooth cone. As an application, we also gave the least harmonic majorant of a nonnegative subharmonic function.

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## List of abbreviations

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## Change history

### 22 January 2020

The Editors-in-Chief have retracted this article [1] because of overlap with articles from different authors that were simultaneously under consideration with another journal [2, 3]. Additionally, this article shows evidence of both authorship and peer review manipulation. The authors have not responded to any correspondence regarding this retraction.

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## Acknowledgements

The authors would like to thank the editor, the associate editor and the anonymous referees for their careful reading and constructive comments which have helped us to significantly improve the presentation of the paper.

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### Authors’ contributions

GX drafted the manuscript. MH helped to draft the manuscript and revised the written English. JS helped to draft the manuscript and revised it according to the referee reports. All authors read and approved the final manuscript.

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The Editors-in-Chief have retracted this article [1] because of overlap with articles from different authors that were simultaneously under consideration with another journal [2, 3]. Additionally, this article shows evidence of both authorship and peer review manipulation. The authors have not responded to any correspondence about this retraction.

[1] Han, M., Sun, J. and Xue, G. Bound Value Probl (2017) 2017: 67. https://doi.org/10.1186/s13661-017-0798-5

[2] Wan, L. Further result on Dirichlet-Sch type inequality and its application. J Inequal Appl 2017, 104 (2017) doi:10.1186/s13660-017-1381-4

[3] Shu, C., Chen, L. and Vargas-De-Teón, R. Poisson type inequalities with respect to a cone and their applications. J Inequal Appl 2017, 114 (2017) doi:10.1186/s13660-017-1387-y

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Han, M., Sun, J. & Xue, G. RETRACTED ARTICLE: A new application of boundary integral behaviors of harmonic functions to the least harmonic majorant.
*Bound Value Probl* **2017, **67 (2017). https://doi.org/10.1186/s13661-017-0798-5

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### Keywords

- boundary integral behavior
- subharmonic function
- harmonic majorant