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arXiv 2006.04770math.AP
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关于自由边界问题解的唯一性和单调性

On the uniqueness and monotonicity of solutions of free boundary problems

Daniele Bartolucci, Aleks Jevnikar

摘要:

对于任何$Ω\subset \mathbb{R}^N$光滑有界区域,我们证明了在$Ω$上等离子体物理中产生的自由边界问题的正解的唯一性,其正解的唯一性仅取决于Sobolev嵌入$H^{1}_0(Ω)\hookrightarrow L^{2p}(Ω)$,$p\in [1,\frac{N}{N-2})$的最佳常数,并证明了边界密度和适当定义的能量具有普遍的单调性.至少据我们所知,对于$p>1$,这是关于非二维球的域的唯一性的第一个结果,特别是关于解的单调性的第一个结果,这似乎是新的,即使对于$p=1$。对于$p=1$是尖锐的阈值产生保证在$Ω$内部没有自由边界的新条件。作为推论,在相同的范围内,我们解决了一个长期存在的开放问题(可以追溯到1980年的工作Berestycki-Brezis)变分解的唯一性。在二维球上,我们描述了正解的全分支,即证明了正解沿曲线的单调性,直到边界密度为零。

英文摘要:

For any $Ω\subset \mathbb{R}^N$ smooth and bounded domain, we prove uniqueness of positive solutions of free boundary problems arising in plasma physics on $Ω$ in a neat interval depending only by the best constant of the Sobolev embedding $H^{1}_0(Ω)\hookrightarrow L^{2p}(Ω)$, $p\in [1,\frac{N}{N-2})$ and show that the boundary density and a suitably defined energy share a universal monotonic behavior. At least to our knowledge, for $p>1$, this is the first result about the uniqueness for a domain which is not a two-dimensional ball and in particular the very first result about the monotonicity of solutions, which seems to be new even for $p=1$. The threshold, which is sharp for $p=1$, yields a new condition which guarantees that there is no free boundary inside $Ω$. As a corollary, in the same range, we solve a long-standing open problem (dating back to the work of Berestycki-Brezis in 1980) about the uniqueness of variational solutions. Moreover, on a two-dimensional ball we describe the full branch of positive solutions, that is, we prove the monotonicity along the curve of positive solutions until the boundary density vanishes.

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