具De Giorgi型非线性分数阶方程解的单调性

Monotonicity of solutions for fractional equations with De Giorgi type nonlinearities

本文发展了一套适用于分数阶Laplace算子的滑动(sliding)方法.首先建立滑动方法中用到的两个重要定理:狭窄区域原理和无界区域极值原理.基于这两个定理,本文说明了如何利用滑动方法得到半线性分数阶方程解在有界区域和全空间的单调性,其中采用了一些新的想法.第一点是利用s-下调和函数的Poisson积分表示来建立极大值原理,第二点是沿着一列极大值点列利用平均值不等式来估计与分数阶Laplace算子相关的奇异积分.相信这些新的结果可以成为分析分数阶方程的重要工具.

In this paper, we develop a sliding method for the fractional Laplacian. We first obtain the key ingredients needed in the sliding method either in a bounded domain or in the whole space, such as narrow region principles and maximum principles in unbounded domains. Then using semi-linear equations involving the fractional Laplacian in both bounded domains and in the whole space, we illustrate how this new sliding method can be employed to obtain monotonicity of solutions. Some new ideas are introduced, among which, one is to use the Poisson integral representation of s-subharmonic functions in deriving the maximum principle, and the other is to estimate the singular integrals defining the fractional Laplacians along a sequence of approximate maximum points by using a generalized average inequality. We believe that this new inequality will become a useful tool in analyzing fractional equations.