Harmonic functions with finite <i>p</i>-energy on lamplighter graphs are constant

Gournay, Antoine

doi:10.1016/j.crma.2016.04.015

Group theory/Combinatorics

Harmonic functions with finite p-energy on lamplighter graphs are constant
[Sur les graphes d'allumeur de réverbères, les fonctions harmoniques de p-énergie finie sont constantes]

Présenté par : the Editorial Board

Gournay, Antoine ¹

¹ TU Dresden, Fachrichtung Mathematik, 01062 Dresden, Germany

Comptes Rendus. Mathématique, Tome 354 (2016) no. 8, pp. 762-765.

Résumé
Abstract

Le but de cette note est de montrer que plusieurs graphes d'allumeurs de réverbères, où le graphe d'espace est infini avec au plus deux bouts et le graphe des lampes a au plus deux bouts, ne possèdent pas de fonction harmonique non constante à gradient $ℓ^{p}$ (i.e. une p-energie finie) qu'importe le $p \in [1, \infty [$ (et, de manière équivalente, que leur cohomologie $ℓ^{p}$ réduite est triviale en degré un). Des arguments similaires permettent aussi de conclure pour plusieurs produits directs de graphes (y compris tous les graphes de Cayley). Les démonstrations reposent sur un théorème de Thomassen [16] sur les lignes couvrantes dans le carré des graphes.

The aim of this note is to show that lamplighter graphs where the space graph is infinite and at most two-ended and the lamp graph is at most two-ended do not admit harmonic functions with gradients in $ℓ^{p}$ (i.e. finite p-energy) for any $p \in [1, \infty [$ except constants (and, equivalently, that their reduced $ℓ^{p}$ cohomology is trivial in degree one). Similar arguments are then applied to many direct products of graphs to conclude the same (including all direct products of Cayley graphs). The proof relies on a theorem of Thomassen [16] on spanning lines in squares of graphs.

Reçu le : 2015-12-07
Accepté le : 2016-04-18
Publié le : 2016-05-25

DOI : 10.1016/j.crma.2016.04.015

Affiliations des auteurs :

Gournay, Antoine ¹

¹ TU Dresden, Fachrichtung Mathematik, 01062 Dresden, Germany

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Gournay, Antoine. Harmonic functions with finite p-energy on lamplighter graphs are constant. Comptes Rendus. Mathématique, Tome 354 (2016) no. 8, pp. 762-765. doi : 10.1016/j.crma.2016.04.015. http://www.numdam.org/articles/10.1016/j.crma.2016.04.015/

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