Conormal injectivity for a generically smooth Cartier divisor

ID: conormal-injectivity-for-a-generically-smooth-cartier-divisor

Let be an integral effective Cartier divisor in an integral variety over a perfect field, and suppose is not contained in the singular locus of . On a dense open subset both varieties are smooth. The Zariski tangent space of there has codimension one in that of , so the differential of the local defining equation is nonzero. Thus the conormal sheaf map is injective generically. Its kernel is a subsheaf of a line bundle on the integral variety and is therefore a torsion-free sheaf; generic vanishing implies zero everywhere. Right exactness is the Conormal exact sequence for Kähler differentials.

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