AUTOMORPHISMS OF CURVES AND WEIERSTRASS SEMIGROUPS FOR HARBATER–KATZ–GABBER COVERS

Abstract. We study p-group Galois covers X → P1 with only one fully ram-ified point. These covers are important because of the Harbater–Katz–Gabber compactification of Galois actions on complete local rings. The sequence of ramification jumps is related to the Weierstrass semigroup of the global cover at the stabilized point. We determine explicitly the jumps of the ramifica-tion filtrations in terms of pole numbers. We give applications for curves with zero p–rank: we focus on maximal curves and curves that admit a big action. Moreover the Galois module structure of polydifferentials is studied and an application to the tangent space of the deformation functor of curves with automorphisms is given. 1