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Saturday, 20 January 2007

ag.algebraic geometry - Normal Varieties

I would like to risk an answer that does not use the language of algebraic geometry. For a pair (complex analytic variety X; closed analytic subvariety Y), U=XsetminusY,
there exists a triangulation such that Y is a subcomplex (see, for example Triangulations of algebraic sets - Hironaka 1974, can be found with google books). In other words X is a simplicial complex, and Y
is a subcomplex. Now, if X is normal its singularities are in real codimension at least 4. I.e. X is a PL manifold in codimension 4.



In order to show that the fundamental group of XsetminusY surjects onto the fundamental group of X, it is sufficient to show that every loop in X can be homotoped into XsetminusY. Since Y it is contained in the simplicial subcomplex of codimension 2 it is enough to show that any loop in X can be homotoped so it does not touch any simplex of codim 2, but this is true for every PL space that is a manifold in codim 2.

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