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May 10, 2014 at 16:14 vote accept Puzzled
Oct 5, 2013 at 9:43 answer added Darius Math timeline score: 3
Oct 5, 2013 at 3:28 comment added Sasha And moreover, the kernel of $df$ is zero in this case.
Oct 5, 2013 at 1:33 comment added Piotr Achinger The sequence exists, but it won't be exact on the right. You always have the map $df:T_X\to f^* T_Y$, and you define $T_{X/Y}$ to be the kernel, but of course $df$ will not be surjective where $f$ is not smooth (in fact, smoothness of $f$ is equivalent to surjectivity).
Oct 4, 2013 at 20:57 history asked Puzzled CC BY-SA 3.0