Ideals generated by pfaffians are of interest in commutative algebra and algebraic geometry, as well as in combinatorics. In this article we compute multiplicity and Castelnuovo-Mumford regularity of pfaffian ideals of ladders. We give explicit formu
las for some families of ideals, and indicate a procedure that allows to recursively compute the invariants of any pfaffian ideal of ladder. Our approach makes an essential use of liaison theory.
A set of polynomials G in a polynomial ring S over a field is said to be a universal Gru007foebner basis, if G is a Gru007foebner basis with respect to every term order on S. Twenty years ago Bernstein, Sturmfels, and Zelevinsky proved that the set o
f the maximal minors of a matrix X of variables is a universal Gru007foebner basis. Boocher recently proved that any initial ideal of the ideal of maximal minors of X has a linear resolution. In this paper we give a quick proof of the results mentioned above. Our proof is based on a specialization argument. Then we show that similar statements hold in a more general setting, for matrices of linear forms satisfying certain homogeneity conditions. More precisely, we show that the set of maximal minors of a matrix L of linear forms is a universal Gru007foebner basis for the ideal I that it generates, provided that L is column-graded. Under the same assumption we show that every initial ideal of I has a linear resolution. Furthermore, the projective dimension of I and of its initial ideals is n-m, unless I=0 or a column of L is identically 0. Here L is a matrix of size m times n, and m is smaller than or equal to n. If instead L is row-graded, then we prove that I has a universal Gru007foebner basis of elements of degree m and that every initial ideal of I has a linear resolution, provided that I has the expected codimension. The proofs are based on a rigidity property of radical Borel fixed ideals in a multigraded setting: We prove that if two Borel fixed ideals I and J have the same multigraded Hilbert series and I is radical, then I = J. We also discuss some of the consequences of this rigidity property.
