arXiv 1007.49772010-07math.RAmath.AG

Flat Mittag-Leffler modules over countable rings

Silvana Bazzoni, Jan Stovicek

备注：: 7 pages; version 2: minor changes, more explanation added in the proof of Theorem 6 and Lemma 7, references added and updated
发表信息：: Proc. Amer. Math. Soc. 140 (2012), 1527-1533
DOI：: 10.1090/S0002-9939-2011-11070-0

英文摘要：

We show that over any ring, the double Ext-orthogonal class to all flat Mittag-Leffler modules contains all countable direct limits of flat Mittag-Leffler modules. If the ring is countable, then the double orthogonal class consists precisely of all flat modules and we deduce, using a recent result of Šaroch and Trlifaj, that the class of flat Mittag-Leffler modules is not precovering in Mod-R unless R is right perfect.

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