Abstract. In [DJL07] it was shown that if A is an affine hyperplane arrangement in C n , then at most one of the L 2 -Betti numbers b(2) i (C n \ A, id) is non-zero. In this note we prove an analogous statement for complements of complex affine hypersurfaces in general position at infinity. Furthermore, we recast and extend to this higher-dimensional setting results of [FLM09,LM06] about L 2 -Betti numbers of plane curve complements.
IntroductionLet M be any topological space and α : π 1 (M ) → Γ an epimorphism to a group Γ (all groups are assumed countable). Then for i ∈ N ∪ {0} we can consider theWe recall the definition and some of the most important properties of L 2 -Betti numbers in Section 2. Let X ⊂ C n (n ≥ 2) be a reduced affine hypersurface defined by a polynomial equation f = f 1 · · · f s = 0, where f i are the irreducible factors of f . Denote by X i := {f i = 0}, i = 1, · · · , s, the irreducible components of X, and letbe the hypersurface complement. Then M X has the homotopy type of a finite CW complex of dimension n. It is well-known that H 1 (M X ; Z) is a free abelian group generated by the meridian loops γ i about the non-singular part of each irreducible component X i of X. Throughout the paper we denote by φ the map π 1 (M X ) → Z given by sending each meridian γ i to 1. This is the same map as the homomorphism f * : π 1 (M X ) → π 1 (C * ) = Z induced by f . We also refer to φ as the total linking number homomorphism. We call an epimorphism α : π 1 (M X ) → Γ admissible if the total linking number homomorphism φ factors through α.The main result of this note is the following "nonresonance-type" theorem.Theorem 1.1. Let X ⊂ C n be a reduced affine hypersurface in general position at infinity, i.e., whose projective completion intersects the hyperplane at infinity transversely. If α : π 1 (M X ) → Γ is an admissible epimorphism, then the L 2 -Betti numbers of the complement M X are computed by: