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A note on resolutions mod 1. (English) Zbl 0922.13010

Let \(A\) be a Noetherian local ring with maximal ideal \(\mathfrak m\), \(I\) a proper ideal and \(M\) a finitely generated \(A/I\)-module. Then the following inequality holds: \[ \beta_p^{A/I}(M) \leq \begin{cases} \beta_p^A(M) & (p = 0, 1) \\ \beta_p^A(M) + \sum_{i = 0}^{p-2} \beta_{p-i-2}^A(I) \beta_i^{A/I}(M) & (p \geq 2). \end{cases} \tag{1} \] Here \(\beta_p^A(M)\) denotes the \(p\)-th Betti number of \(M\) over \(A\). We say that \((M, I)\) is a Golod pair if equality in (1) holds. In this case the author constructed the minimal free resolution of \(M\) over \(A/I\) by using ones of \(M\) and \(I\) over \(A\) [D. Gokhale, Commun. Algebra 22, No. 3, 989-1030 (1994; Zbl 0796.13010)]. We know that \((A/\mathfrak m, I)\) is a Golod pair if \(I\) is the product of a proper ideal and an \(A\)-non zero divisor [see L. L. Avramov, J. Algebra 50, 400-453 (1978; Zbl 0395.13005); proposition 4.4]. In the present paper the author improves this. Let \(J\) be a proper ideal and \(x\) an \(A\)-non zero divisor. Then \((M, xJ)\) is a Golod pair for any finitely generated \(A/xJ\)-module \(M\).

MSC:

13D03 (Co)homology of commutative rings and algebras (e.g., Hochschild, André-Quillen, cyclic, dihedral, etc.)
Full Text: DOI

References:

[1] DOI: 10.1016/0021-8693(78)90163-1 · Zbl 0395.13005 · doi:10.1016/0021-8693(78)90163-1
[2] DOI: 10.1080/00927879408824889 · Zbl 0796.13010 · doi:10.1080/00927879408824889
[3] Gover E.H., Math. Scand 46 pp 5– (1980)
[4] DOI: 10.1016/0022-4049(85)90017-9 · Zbl 0585.13005 · doi:10.1016/0022-4049(85)90017-9
[5] DOI: 10.1016/0021-8693(69)90023-4 · Zbl 0189.04004 · doi:10.1016/0021-8693(69)90023-4
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