Non-Noetherian Bass and Betti numbers

Abstract

This paper investigates the vanishing and non-vanishing of Betti and Bass numbers for non-finitely generated modules. We prove that for \(d\)-dimensional Cohen--Macaulay local rings, every non-zero \(\mathfrak{m}\)-torsion module satisfies \(β_d(M)\neq 0\), and we establish the Betti number behavior of the injective hull \(E_R(k)\). We study tor-rigidity for \(H^d_{\mathfrak{m}}(R)\). We also provide partial positive answers to Schoutens' question on whether the vanishing of some Betti number of a big Cohen--Macaulay algebra forces the Cohen--Macaulay property of \(R\). For the absolute integral closure \(R^+\), we establish both Tor and Ext results. On the Tor side, we prove that \(β_i(R^+)=0\) for some \(i>0\) implies regularity in a series cases. On the Ext side, we prove that \(μ_i(R^+)=0\) for some \(i> d\) forces regularity for Gorenstein domains of prime characteristic, and we obtain analogous results for graded normal domains of dimension \(2\) and also for quotient and isolated singularities in any dimension. Also $μ_i(R^\infty)=0$ forces regularity for F-pure rings.