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arXiv · 1801.02260

Divisibility classes are seldom closed under flat covers

Michal Hrbek
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Abstract

It is well-known that a class of all modules, which are torsion-free with respect to a set of ideals, is closed under injective envelopes. In this paper, we consider a kind of a dual to this statement - are the divisibility classes closed under flat covers? - and argue that this is seldom the case. More precisely, we show that the class of all divisible modules over an integral domain R is closed under flat covers if and only if R is almost perfect. Also, we show that if the class of all s-divisible modules, where s is a regular element of a commutative ring R, is closed under flat covers then the quotient ring R/sR satisfies some rather restrictive properties. The question is motivated by the recent classification [11] of tilting classes over commutative rings.

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Michal Hrbek. 2018-01-07. Divisibility classes are seldom closed under flat covers. https://arxiv.org/abs/1801.02260

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