Limits of Tangents, Whitney Stratifications and a Plücker Type Formula

Lê Dũng Tráng () and Bernard Teissier ()
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Lê Dũng Tráng: University of Aix-Marseille
Bernard Teissier: Université Paris Cité and Sorbonne Université, CNRS, IMJ-PRG

Chapter Chapter 1 in Handbook of Geometry and Topology of Singularities IV, 2023, pp 1-44 from Springer

Abstract: Abstract Let X denote a purely d-dimensional reduced complex analytic space. If it has singularities, it has no tangent bundle, which makes many classical and fundamental constructions impossible directly. However, there is a unique proper map ν X : N X → X $$\nu _X\colon NX\to X$$ which has the property that it is an isomorphism over the non-singular part X 0 $$X^0$$ of X and the tangent bundle T X 0 $$T_{X^0}$$ lifted to NX by this isomorphism extends uniquely to a vector bundle on NX. For x ∈ X $$x\in X$$ , the set-theoretical fiber | ν X − 1 ( x ) | $$\vert \nu _X^{-1}(x)\vert $$ is the set of limit directions of tangent spaces to X 0 $$X^0$$ at points approaching x. The space NX is reduced and equidimensional, but in general singular. If X is a closed analytic subspace of an open set U of C N $${\mathbf C}^N$$ , the space NX is a closed analytic subspace of X × G ( d , N ) $$X\times \mathbf G(d,N)$$ , where G ( d , N ) $$\mathbf G(d,N)$$ denotes the Grassmannian of d-dimensional vector subspaces of C N $${\mathbf C}^N$$ . The rich geometry of the Grassmannian makes it complicated to study the geometry of the map ν X $$\nu _X$$ using intersection theory. There is an analogous construction where tangent spaces are replaced by tangent hyperplanes, and the map ν X $$\nu _X$$ is replaced by the conormal map κ X : C ( X ) → X $$\kappa _X\colon C(X)\to X$$ , where C ( X ) $$C(X)$$ denotes the conormal space, which is a subspace of X × P ̌ N − 1 $$X\times \check {\mathbf {P}}^{N-1}$$ , where P ̌ N − 1 $$\check {\mathbf {P}}^{N-1}$$ is the space of hyperplanes of P N $${\mathbf {P}}^N$$ , the dual projective space, so that the intersection theory is simpler. This paper is devoted to these two constructions, their applications to stratification theory in the sense of Whitney and to a general Plücker type formula for projective varieties.

Date: 2023
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DOI: 10.1007/978-3-031-31925-9_1

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