Approximate cohomology

David Kazhdan, Tamar Ziegler*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

10 Scopus citations


Let k be a field, G be an abelian group and r∈ N. Let L be an infinite dimensional k-vector space. For any m∈ End k(L) we denote by r(m) ∈ [ 0 , ∞] the rank of m. We define by R(G, r, k) ∈ [ 0 , ∞] the minimal R such that for any map A: G→ End k(L) with r(A(g+ g′ ′) - A(g) - A(g′ ′)) ≤ r, g, g′ ′∈ G there exists a homomorphism χ: G→ End k(L) such that r(A(g) - χ(g)) ≤ R(G, r, k) for all g∈ G. We show the finiteness of R(G, r, k) for the case when k is a finite field, G= V is a k-vector space V of countable dimension. We actually prove a generalization of this result. In addition we introduce a notion of Approximate Cohomology groups HFk(V,M) [which is a purely algebraic analogue of the notion of ϵ-representation (Kazhdan in Isr. J. Math. 43:315–323, 1982)] and interperate our result as a computation of the group HF1(V,M) for some V-modules M.

Original languageAmerican English
Pages (from-to)499-509
Number of pages11
JournalSelecta Mathematica, New Series
Issue number1
StatePublished - 1 Mar 2018

Bibliographical note

Publisher Copyright:
© 2017, Springer International Publishing.


  • Algebraic rank
  • Bias
  • Cohomology
  • Exponential sums
  • Gowers norms


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