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A general calculation of the number of vanishing cycles. (English) Zbl 0824.32013

Let \(X\) be a reduced complex analytic space containing an open neighborhood \(U\) of the origin in \(\mathbb{C}^ n\) and let \(f : (U,0) \to (\mathbb{C})\) be an analytic function. Suppose that \(\mathbb{F}^ \bullet\) is a bounded complex of sheaves on \(X\) which is constructible with respect to a Whitney stratification \(\{S_ \alpha\}\) of \(X\). Let \(H\) be a hypersurface in \(U\) which transversely intersects the strata of \(\{S_ \alpha\}\) except, perhaps, at the origin. The author obtains an algebraic formula for calculating the difference between the Euler characteristics of the stalks cohomology at the origin of the sheaves of vanishing cycles along \(f\) and along the restriction \(f \mid_ H\). Thus, he generalizes a result of Le Dung Trang [Ann. Inst. Fourier 23(1973), No. 4, 261- 270 (1974; Zbl 0293.32013)]. In the case where \(\mathbb{F}^ \bullet\) is a perverse sheaf on a complete intersection the formula (5.11a) of E. J. N. Looijenga [‘Isolated singular points on complete intersections’, Lond. Math. Soc. Lectures Note Series 77, Cambridge (1984; Zbl 0552.14002)] is also generalized. As an application it is proved that the complex link of any isolated complete intersection singularity is never contractible.

MSC:

32S60 Stratifications; constructible sheaves; intersection cohomology (complex-analytic aspects)
32S55 Milnor fibration; relations with knot theory
32S30 Deformations of complex singularities; vanishing cycles
14B05 Singularities in algebraic geometry
14F43 Other algebro-geometric (co)homologies (e.g., intersection, equivariant, Lawson, Deligne (co)homologies)
57M25 Knots and links in the \(3\)-sphere (MSC2010)
57Q45 Knots and links in high dimensions (PL-topology) (MSC2010)
Full Text: DOI

References:

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