On the multigraded Hilbert and Poincaré–Betti series and the Golod property of monomial rings

In this paper we study the multigraded Hilbert and Poincaré–Betti series of image, where S is the ring of polynomials in n indeterminates divided by the monomial ideal image. There is a conjecture about the multigraded Poincaré–Betti series by Charalambous and Reeves which they proved in the case where the Taylor resolution is minimal. We introduce a conjecture about the minimal A-free resolution of the residue class field and show that this conjecture implies the conjecture of Charalambous and Reeves and, in addition, gives a formula for the Hilbert series. Using Algebraic Discrete Morse theory, we prove that the homology of the Koszul complex of A with respect to x1,…,xn is isomorphic to a graded commutative ring of polynomials over certain sets in the Taylor resolution divided by an ideal image of relations. This leads to a proof of our conjecture for some classes of algebras A. We also give an approach for the proof of our conjecture via Algebraic Discrete Morse theory in the general case. The conjecture implies that A is Golod if and only if the product (i.e. the first Massey operation) on the Koszul homology is trivial. Under the assumption of the conjecture we finally prove that a very simple purely combinatorial condition on the minimal monomial generating system of image implies Golodness for A.