TY - JOUR

T1 - Nonconventional ergodic averages and multiple recurrence for von neumann dynamical systems

AU - Austin, Tim

AU - Eisner, Tanja

AU - Tao, Terence

N1 - Copyright:
Copyright 2011 Elsevier B.V., All rights reserved.

PY - 2011

Y1 - 2011

N2 - The Furstenberg recurrence theorem (or equivalently Szemerédi's theorem) can be formulated in the language of von Neumann algebras as follows: given an integer k ≥ 2, an abelian finite von Neumann algebra (M, τ) with an automorphism α: M→M, and a nonnegative a ∈ M with τ (a) > 0, one has lim inf N→∞ N-1 ∑Nn=1 Re τ(aαn (a) · · · α(K-1)n (a))> 0; a later result of Host and Kra shows this limit exists. In particular, Re τ(aαn (a) · · · α(K-1)n (a)) is positive for all n in a set of positive density. From the von Neumann algebra perspective, it is natural to ask to what remains of these results when the abelian hypothesis is dropped. All three claims hold for k = 2, and we show that all three claims hold for all k when the von Neumann algebra is asymptotically abelian, and that the last two claims hold for k = 3 when the von Neumann algebra is ergodic. However, we show that the first claim can fail for k=3 even with ergodicity, the second claim can fail for k ≥ 4 even when assuming ergodicity, and the third claim can fail for k = 3 without ergodicity, or k ≥ 5 and odd assuming ergodicity. The second claim remains open for nonergodic systems with k = 3, and the third claim remains open for ergodic systems with k = 4.

AB - The Furstenberg recurrence theorem (or equivalently Szemerédi's theorem) can be formulated in the language of von Neumann algebras as follows: given an integer k ≥ 2, an abelian finite von Neumann algebra (M, τ) with an automorphism α: M→M, and a nonnegative a ∈ M with τ (a) > 0, one has lim inf N→∞ N-1 ∑Nn=1 Re τ(aαn (a) · · · α(K-1)n (a))> 0; a later result of Host and Kra shows this limit exists. In particular, Re τ(aαn (a) · · · α(K-1)n (a)) is positive for all n in a set of positive density. From the von Neumann algebra perspective, it is natural to ask to what remains of these results when the abelian hypothesis is dropped. All three claims hold for k = 2, and we show that all three claims hold for all k when the von Neumann algebra is asymptotically abelian, and that the last two claims hold for k = 3 when the von Neumann algebra is ergodic. However, we show that the first claim can fail for k=3 even with ergodicity, the second claim can fail for k ≥ 4 even when assuming ergodicity, and the third claim can fail for k = 3 without ergodicity, or k ≥ 5 and odd assuming ergodicity. The second claim remains open for nonergodic systems with k = 3, and the third claim remains open for ergodic systems with k = 4.

KW - Multiple recurrence

KW - Nonconventional ergodic averages

KW - Szemerédi's theorem

KW - Von Neumann algebras

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U2 - 10.2140/pjm.2011.250.1

DO - 10.2140/pjm.2011.250.1

M3 - Article

AN - SCOPUS:79953792794

VL - 250

SP - 1

EP - 60

JO - Pacific Journal of Mathematics

JF - Pacific Journal of Mathematics

SN - 0030-8730

IS - 1

ER -