Picard–Lefschetz theory


In mathematics, Picard–Lefschetz theory studies the topology of a complex manifold by looking at the critical points of a holomorphic function on the manifold. It was introduced by Émile Picard for complex surfaces in his book, and extended to higher dimensions by. It is a complex analog of Morse theory that studies the topology of a real manifold by looking at the critical points of a real function. extended Picard–Lefschetz theory to varieties over more general fields, and Deligne used this generalization in his proof of the Weil conjectures.

Picard–Lefschetz formula

The Picard–Lefschetz formula describes the monodromy at a critical point.
Suppose that f is a holomorphic map from an '-dimensional projective complex manifold to the projective line P1. Also suppose that all critical points are non-degenerate and lie in different fibers, and have images x1,...,xn in P1. Pick any other point x in P1. The fundamental group π1 is generated by loops wi going around the points xi, and to each point xi there is a vanishing cycle in the homology Hk of the fiber at x. Note that this is the middle homology since the fibre has complex dimension k, hence real dimension 2k.
The monodromy action of π1 on Hk is described as follows by the Picard–Lefschetz formula. The monodromy action of a generator wi of the fundamental group on ' ∈ Hk is given by
where δi is the vanishing cycle of xi. This formula appears implicitly for k = 2 in. gave the explicit formula in all dimensions.

Example

Consider the projective family of hyperelliptic curves of genus defined by
where is the parameter and. Then, this family has double-point degenerations whenever. Since the curve is a connected sum of tori, the intersection form on of a generic curve is the matrix
we can easily compute the Picard-Lefschetz formula around a degeneration on. Suppose that are the -cycles from the -th torus. Then, the Picard-Lefschetz formula reads
if the -th torus contains the vanishing cycle. Otherwise it is the identity map.