Extending the idea of compressed algebra to arbitrary socle-vectors, II: cases of non-existence

Abstract

This paper is the continuation of the previous work on generalized compressed algebras (GCA's). First we exhibit a new class of socle-vectors $s$ which admit a GCA (whose $h$-vector is lower than the upper-bound $H$ of Theorem A of the previous paper). In particular, it follows that for every socle-vector $s$ of type 2 there exists a GCA (in any codimension $r$). The main result of this paper is the following: there exist pairs $(r,s)$ which do not admit a GCA. Moreover, the way this pathology occurs may be "arbitrarily bad" (even in codimension 3). Finally, we start considering the difficult problem of characterizing the pairs $(r,s)$ which admit a GCA, focusing on a particular class of socle-vectors of codimension 3.