Applications of von Neumann Algebras to Rigidity Problems of (2-step) Riemannian (Nil-)Manifolds

In this paper, basic notions of von Neumann algebra and its direct analogous in the realm of groupoids and measure spaces have been considered. By recovering the action of a locally compact Lie group from a crossed product of a von Neumann algebra, other proof of one of a geometric proposition of O'Neil and an extension of it has been proposed. Also, using the advanced exploration of nilmanifolds in measure spaces and their corresponding automorphisms (Lie algebraic derivations) a different proof of an analytic proposition of Gordon and Mao has been attained. These two propositions are of the most important ones for rigidity problems of Riemannian manifolds especially 2-step nilmanifolds. Keywords. Von Neumann algebras, 2-step nilmanifolds, Free and ergodic actions, Derivations, Automorphisms.
1. Introduction Motivation. Let M is a simply connected 2-step nilpotent Lie group with a left invariant metric and [LAMBDA] is a cocompact discrete subgroup of isometries of M. In [...]