Norm Convergence of Multiple Ergodic Averages for Commuting Transformations

Let T1, . . . , Tl : X → X be commuting measure-preserving transformations on a probability space (X,X , μ). We show that the multiple ergodic averages 1 N PN−1 n=0 f1(T n 1 x) . . . fl(T n l x) are convergent in L2(X,X , μ) as N → ∞ for all f1, . . . , fl ∈ L (X,X , μ); this was previously established for l = 2 by Conze and Lesigne [2] and for general l assuming some additional ergodicity hypotheses on the maps Ti and TiT −1 j by Frantzikinakis and Kra [3] (with the l = 3 case of this result established earlier in [29]). Our approach is combinatorial and finitary in nature, inspired by recent developments regarding the hypergraph regularity and removal lemmas, although we will not need the full strength of those lemmas. In particular, the l = 2 case of our arguments are a finitary analogue of those in [2].