# Lusternik–Schnirelman theory for closed 1-forms

### M. Farber

Tel-Aviv University, Ramat-Aviv, Israel

## Abstract

S. P. Novikov developed an analog of the Morse theory for closed 1-forms. In this paper we suggest an analog of the Lusternik–Schnirelman theory for closed 1-forms. For any cohomology class $ξ∈H_{1}(M,R)$ we define an integer $cl(ξ)$ (*the cup-length associated with $ξ$*); we prove that any closed 1-form representing $ξ$ has at least $cl(ξ)−1$ critical points. The number $cl(ξ)$ is defined using cup-products in cohomology of some flat line bundles, such that their monodromy is described by complex numbers, which are not Dirichlet units.

## Cite this article

M. Farber, Lusternik–Schnirelman theory for closed 1-forms. Comment. Math. Helv. 75 (2000), no. 1, pp. 156–170

DOI 10.1007/S000140050117