The Scarf complex and betti numbers of powers of extremal ideals

Abstract

This paper is concerned with finding bounds on betti numbers and describing combinatorially and topologically (minimal) free resolutions of powers of ideals generated by a fixed number q of square-free monomials. Among such ideals, we focus on a specific ideal Eq, which we call extremal, and which has the property that for each r≥1 the betti numbers of Eqr are an upper bound for the betti numbers of Ir for any ideal I generated by q square-free monomials (in any number of variables). We study the Scarf complex of the ideals Eqr and use this simplicial complex to extract information on minimal free resolutions. In particular, we show that Eqr has a minimal free resolution supported on its Scarf complex when q≤4 or when r≤2, and we describe explicitly this complex. For any q and r, we also show that β1(Eqr) is the smallest possible, or in other words equal to the number of edges in the Scarf complex. These results lead to effective bounds on the betti numbers of Ir, with I as above. For example, we obtain that pd(Ir)≤5 for all ideals I generated by 4 square-free monomials and any r≥1.