Abstract. This article gives an affirmative solution to the problem whether the ergodic Cesáro averages generated by a positive Dunford-Schwartz operator in a noncommutative space L p (M, τ ), 1 ≤ p < ∞, converge almost uniformly (in Egorov's sense). This problem goes back to the original paper of Yeadon [21], published in 1977, where bilaterally almost uniform convergence of these averages was established for p = 1.
IntroductionAlmost uniform (a.u.) convergence in Egorov's sense in a von Neumann algebra M was considered by Lance [9], where a breakthrough noncommutative individual ergodic theorem was established for a positive state preserving automorphism of M. Later Lance's result was generalized, while the proofs were simplified; see [8,4,6].For a semifinite von Neumann algebra M with a faithful normal semifinte trace τ , Yeadon [21] introduced the so-called bilaterally almost uniform (b.a.u.) convergence in Egorov's sense to prove a noncommutative individual ergodic theorem for a positive Dunford-Schwartz operator in the space L 1 (M, τ ) of τ −integrable operators affiliated with M.Since b.a.u. convergence is generally weaker than a.u. convergence, serious attempts have been made to show that there is a.u. convergence in Yeadon's seminal result. But the problem persisted, and a significant number of noncommutative individual ergodic theorems concerning b.a.u. convergence of ergodic averages have been established; see, for example, [15,5,3,11,7,18,12,2].It was derived in [7] that if 1 < p < 2 (2 ≤ p < ∞), then for a positive DunfordSchwartz operator in a noncommutative space L p (M, τ ), the corresponding ergodic Cesáro averages converge b.a.u. (respectively, a.u.). Later, in [10] (see also [2]), it was shown that this result can be obtained directly from Yeadon's maximal inequality for L 1 (M, τ ) established in [21]. In particular, it was shown that a.u. convergence for p ≥ 2 follows easily due to Kadison's inequality. But the case 1 ≤ p < 2 still remained open.The aim of this article is to prove that there is a.u. convergence for all 1 ≤ p < ∞, which is given in Theorem 2.3. Note that this result was not known even for a finite trace. Lemma 3.1 is crucial. The main finding of the article is Lemma 3.2 where the matrix {e k,n } of projections in M is constructed. Also, the notion of (bilaterally) Date: June 28, 2017. 2010 Mathematics Subject Classification. 47A35(primary), 46L52(secondary). Key words and phrases. Semifinite von Neumann algebra; Dunford-Schwartz operator; individual ergodic theorem; bilaterally uniform equicontinuity in measure at zero; almost uniform convergence.