B.M. Brown, M. Langer and K.M. Schmidt,
The HELP inequality on trees,
Proc. Sympos. Pure Math. 77 (2008), 337–354

Abstract:
We establish analogues of Hardy and Littlewood's integro-differential equation for Schrödinger-type operators on metric and discrete trees, based on a generalised strong limit-point property of the graph Laplacian.

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