Uniformity seminorms on ℓ∞ and applications

Bernard Host*, Bryna Kra

*Corresponding author for this work

Research output: Contribution to journalArticle

39 Scopus citations

Abstract

A key tool in recent advances in understanding arithmetic progressions and other patterns in subsets of the integers is certain norms or seminorms. One example is the norms on ℤ/Nℤ introduced by Gowers in his proof of Szemerédi's Theorem, used to detect uniformity of subsets of the integers. Another example is the seminorms on bounded functions in a measure preserving system (associated to the averages in Furstenberg's proof of Szemerédi's Theorem) defined by the authors. For each integer k ≥ 1, we define seminorms on ℓ analogous to these norms and seminorms. We study the correlation of these norms with certain algebraically defined sequences, which arise from evaluating a continuous function on the homogeneous space of a nilpotent Lie group on a orbit (the nilsequences). Using these seminorms, we define a dual norm that acts as an upper bound for the correlation of a bounded sequence with a nilsequence. We also prove an inverse theorem for the seminorms, showing how a bounded sequence correlates with a nilsequence. As applications, we derive several ergodic theoretic results, including a nilsequence version of the Wiener-Wintner ergodic theorem, a nil version of a corollary to the spectral theorem, and a weighted multiple ergodic convergence theorem.

Original languageEnglish (US)
Pages (from-to)219-276
Number of pages58
JournalJournal d'Analyse Mathematique
Volume108
Issue number1
DOIs
StatePublished - Sep 1 2009

ASJC Scopus subject areas

  • Analysis
  • Mathematics(all)

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