A GENERAL FORMULA FOR THE INDEX OF DEPTH STABILITY OF EDGE IDEALS

By a classical result of Brodmann, the function depthR/I-t is asymptotically a constant, i.e. there is a number s such that depthR/I-t=depth R/I-s for t>s. One calls the smallest number s with this property the index of depth stability of I and denotes it by dstab(I). This invariant remains mysterious til now. The main result of this paper gives an explicit formula for dstab(I) when I is an arbitrary ideal generated by squarefree monomials of degree 2. That is the first general case where one can characterize dstab(I) explicitly. The formula expresses dstab(I) in terms of the associated graph. The proof involves new techniques which relate different topics such as simplicial complexes, systems of linear inequalities, graph parallelizations, and ear decompositions. It provides an effective method for the study of powers of edge ideals.