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NCTS Seminar in Algebraic Geometry
14:00 - 15:00, October 25, 2019 (Friday)
R201, Astronomy-Mathematics Building, NTU
(台灣大學天文數學館 201室)
The Deligne-Illusie Theorem and Exceptional Enriques Surfaces
Stefan Schroeer (Heinrich-Heine-Universität Düsseldorf)


Building on the results of Deligne and Illusie on liftings to truncated Witt vectors, we give a criterion for non-liftability that involves only the dimension of certain cohomology groups of vector bundles arising from the Frobenius pushforward of the de Rham complex. Using vector bundle methods, we apply this to show that exceptional Enriques surfaces, a class introduced by Ekedahl and Shepherd-Barron, do not lift to truncated Witt vectors, yet the base of the miniversal formal deformation over the Witt vectors is regular. These results hinge on some observations in homological algebra that relates splittings in derived categories to Yoneda extensions and certain diagram completions.


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