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arXiv 2408.15071math.MGmath.FA
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通过度量测度空间中的链定义的Sobolev空间

Sobolev spaces via chains in metric measure spaces

Emanuele Caputo, Nicola Cavallucci

AI总结:

本文在度量测度空间上引入链Sobolev空间,证明其在完备情形与经典定义等价,在非完备情形与局部Lipschitz常数松弛定义等价,并应用于Poincaré不等式的等价表述。

摘要:

我们在可能非完备的度量测度空间上,基于链上梯度定义了链Sobolev空间。在此背景下,$\varepsilon$-链是有限个点的集合,其中相邻点之间的距离至多为$\varepsilon$。它们扮演着曲线离散版本的角色。相应地定义了链上梯度,并通过令尺寸参数$\varepsilon$趋于零来定义链Sobolev空间。在完备情形下,我们证明了链Sobolev空间等于经典的上梯度松弛或Lipschitz函数的局部Lipschitz常数意义下的Sobolev空间。这一事实的证明受到Eriksson-Bique最近发展的技术的启发。在可能非完备的情形下,我们证明了链Sobolev空间等于通过Lipschitz函数局部Lipschitz常数松弛定义的Sobolev空间,而一般来说它们与沿曲线的上梯度定义的空间不同。我们将论文中发展的理论应用于证明Poincaré不等式的等价表述,这些表述涉及$\varepsilon$-上梯度的逐点估计、连接点的链模的下界以及非完备情形下用Minkowski含量度量的分离集的大小。在此过程中,我们讨论了弱$\varepsilon$-上梯度的概念以及沿链积分的非对称概念。

英文摘要:

We define the chain Sobolev space on a possibly non-complete metric measure space in terms of chain upper gradients. In this context, $\varepsilon$-chains are a finite collection of points with distance at most $\varepsilon$ between consecutive points. They play the role of discrete versions of curves. Chain upper gradients are defined accordingly and the chain Sobolev space is defined by letting the size parameter $\varepsilon$ going to zero. In the complete setting, we prove that the chain Sobolev space is equal to the classical notions of Sobolev spaces in terms of relaxation of upper gradients or of the local Lipschitz constant of Lipschitz functions. The proof of this fact is inspired by a recent technique developed by Eriksson-Bique. In the possible non-complete setting, we prove that the chain Sobolev space is equal to the one defined via relaxation of the local Lipschitz constant of Lipschitz functions, while in general they are different from the one defined via upper gradients along curves. We apply the theory developed in the paper to prove equivalent formulations of the Poincaré inequality in terms of pointwise estimates involving $\varepsilon$-upper gradients, lower bounds on modulus of chains connecting points and size of separating sets measured with the Minkowski content in the non-complete setting. Along the way, we discuss the notion of weak $\varepsilon$-upper gradients and asymmetric notions of integral along chains.

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