The 12-th roots of the discriminant of an elliptic curve and the torsion points

Abstract

Given an elliptic curve over a field of characteristic different from 2,3, its discriminant defines a $\mu_{12}$-torsor over the field. In this paper, we give an explicit description of this $\mu_{12}$-torsor in terms of the 3-torsion points and of the 4-torsion points on the given elliptic curve. %In addition, we show that such a description involves the Weil pairing in a certain way. As an application, we generalize a result of Coates on the 12-th root of the discriminant of an elliptic curve.