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Multiple correlation sequences not approximable by nilsequences

Published online by Cambridge University Press:  16 July 2021

JOP BRIËT
Affiliation:
Centrum Wiskunde & Informatica (CWI), Science Park 123, 1098XG, Amsterdam, The Netherlands (e-mail: j.briet@cwi.nl)
BEN GREEN*
Affiliation:
Mathematical Institute, Andrew Wiles Building, Radcliffe Observatory Quarter, Woodstock Rd, OxfordOX2 6QW, UK

Abstract

We show that there is a measure-preserving system $(X,\mathscr {B}, \mu , T)$ together with functions $F_0, F_1, F_2 \in L^{\infty }(\mu )$ such that the correlation sequence $C_{F_0, F_1, F_2}(n) = \int _X F_0 \cdot T^n F_1 \cdot T^{2n} F_2 \, d\mu $ is not an approximate integral combination of $2$ -step nilsequences.

Type
Original Article
Copyright
© The Author(s), 2021. Published by Cambridge University Press

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