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 Saroch and Trlifaj, that the class of flat Mittag-Leffler modules is not precovering in Mod -R unless R is right perfect.
Flat Mittag-Leffler modules over countable rings
BAZZONI, SILVANA;
2012
Abstract
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 Saroch 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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