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Analytic Isomorphisms of compressed local algebras

In this paper we consider Artin local K-algebras with maximal length in the class of Artin algebras with given embedding dimension and socle type. They have been widely studied by several authors, among others by Iarrobino, Fröberg and Laksov. If the local K-algebra is Gorenstein of socle degree 3, then the authors proved that it is canonically graded, i.e. analytically isomorphic to its associated graded ring. This unexpected result has been extended to compressed level K-algebras of socle degree 3 by A. De Stefani. In this paper we end the investigation proving that extremal Artin Gorenstein local K-algebras of socle degree s \le 4 are canonically graded, but the result does not extend to extremal Artin Gorenstein local rings of socle degree 5 or to compressed level local rings of socle degree 4 and type >1. As a consequence we present results on Artin compressed local K-algebras having a specified socle type.