Wild Ramification and the Cotangent Bundle

Abstract

We define the characteristic cycle of a locally constant \'etale sheaf on a smooth variety in positive characteristic ramified along boundary as a cycle in the cotangent bundle of the variety, at least on a neighborhood of the generic point of the divisor on the boundary. The crucial ingredient in the definition is an additive structure on the boundary induced by the groupoid structure of multiple self products. We prove a compatibility with pull-back and local acyclicity in non-characteristic situations. We also give a relation with the characteristic cohomology class under a certain condition and a concrete example where the intersection with the 0-section computes the Euler-Poincar\'e characteristic.Comment: 56 pages. In v2, the local acyclicity is proved in Proposition 3.14. In v3, errors in Examples 2.18.2 and 3.18 are corrected. In v4, the assumption in Proposition 3.14 on local acyclicity is weakened. In v5, Conjectures on the integrality of the characteristic cycle and on the total dimension of nearby cycles are formulated. In v6, some corrections are made and explanations are adde