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 ''x''₁,...,''x''_''n'' in P¹. Pick any other point ''x'' in P¹. The fundamental group is generated by loops ''w''_''i'' going around the points ''x''_''i'', and to each point ''x''_''i'' there is a vanishing cycle in the homology ''H''_''k''(''Y''_''x'') of the fiber at ''x''. Note that this is the middle homology since the fibre has complex dimension ''k'', hence real dimension 2''k''.
The monodromy action of on ''H''_''k''(''Y''_''x'') is described as follows by the Picard–Lefschetz formula. (The action of monodromy on other homology groups is trivial.) The monodromy action of a generator ''w''_''i'' of the fundamental group on is given by

:$w_i(\gamma) = \gamma+(-1)^\langle \gamma,\delta_i\rangle \delta_i$

where δ_''i'' is the vanishing cycle of ''x''_''i''. This formula appears implicitly for ''k'' = 2 (without the explicit coefficients of the vanishing cycles δ_''i'') in . gave the explicit formula in all dimensions.

Example

Consider the projective family of hyperelliptic curves of genus $g$ defined by
:$y^2 = (x-t)(x-a_1)\cdots(x-a_k)$
where $t \in \mathbb^1$ is the parameter and $k=2g+1$. Then, this family has double-point degenerations whenever $t = a_i$. Since the curve is a connected sum of $g$ tori, the intersection form on $H_1$ of a generic curve is the matrix
:$\begin 0 & 1 \\ 1 & 0 \end^ = \begin 0 & 1 & 0 & 0 & \cdots & 0 & 0 \\ 1 & 0 & 0 & 0 & \cdots & 0 & 0 \\ 0 & 0 & 0 & 1 & \cdots & 0 & 0 \\ 0 & 0 & 1 & 0 & \cdots & 0 & 0\\ \vdots & \vdots & \vdots & \vdots & \cdots & \vdots & \vdots \\ 0 & 0 & 0 & 0 & \cdots & 0 & 1 \\ 0 & 0 & 0 & 0 & \cdots & 1 & 0 \end$
we can easily compute the Picard-Lefschetz formula around a degeneration on $\mathbb^1_t$. Suppose that $\gamma, \delta$ are the $1$-cycles from the $j$-th torus. Then, the Picard-Lefschetz formula reads
:$w_j(\gamma) = \gamma - \delta$
if the $j$-th torus contains the vanishing cycle. Otherwise it is the identity map.

See also

* Lefschetz pencil

References

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{{DEFAULTSORT:Picard-Lefschetz theory
Algebraic geometry