Around the Thom–Sebastiani theorem, with an appendix by Weizhe Zheng

Abstract

For germs of holomorphic functions \(f: (\mathbf {C}^{m+1},0) \rightarrow (\mathbf {C},0)\), \(g: (\mathbf {C}^{n+1},0) \rightarrow (\mathbf {C},0)\) having an isolated critical point at 0 with value 0, the classical Thom–Sebastiani theorem describes the vanishing cycles group \(\Phi ^{m+n+1}(f \oplus g)\) (and its monodromy) as a tensor product \(\Phi ^m(f) \otimes \Phi ^n(g)\), where \((f \oplus g)(x,y) = f(x) + g(y), x = (x_0,{\ldots },x_m), y = (y_0,{\ldots },y_n)\). We prove algebraic variants and generalizations of this result in etale cohomology over fields of any characteristic, where the tensor product is replaced by a certain local convolution product, as suggested by Deligne. They generalize Fu (Math Res Lett 21:101–119, 2014). The main ingredient is a Kunneth formula for \(R\Psi \) in the framework of Deligne’s theory of nearby cycles over general bases. In the last section, we study the tame case, and the relations between tensor and convolution products, in both global and local situations.