Flat cyclic Fr\'echet modules, amenable Fr\'echet algebras, and approximate identities

Abstract

Let A be a locally m-convex Fr\'echet algebra. We give a necessary and sufficient condition for a cyclic Fr\'echet A-module X=A_+/I to be strictly flat, generalizing thereby a criterion of Helemskii and Sheinberg. To this end, we introduce a notion of "locally bounded approximate identity" (a locally b.a.i. for short), and we show that X is strictly flat if and only if the ideal I has a right locally b.a.i. Next we apply this result to amenable algebras and show that a locally m-convex Fr\'echet algebra A is amenable if and only if A is isomorphic to a reduced inverse limit of amenable Banach algebras. We also extend a number of characterizations of amenability obtained by Johnson and by Helemskii and Sheinberg to the setting of locally m-convex Fr\'echet algebras. As a corollary, we show that Connes and Haagerup's theorem on amenable C^*-algebras and Sheinberg's theorem on amenable uniform algebras hold in the Fr\'echet algebra case. We also show that a quasinormable locally m-convex Fr\'echet algebra has a locally b.a.i. if and only if it has a b.a.i. On the other hand, we give an example of a commutative, locally m-convex Fr\'echet-Montel algebra which has a locally b.a.i., but does not have a b.a.i. Some of the results of this paper were announced in ArXiv preprint math.FA/0511132.