The index of an Eisenstein ideal of multiplicity one

Mazur’s fundamental work on Eisenstein ideals for prime level has a variety of arith- metic applications. In this article, we generalize his work to arbitrary square-free level. We compute the index of an Eisenstein ideal and the dimension of the m-torsion of the modular Jacobian vari- ety, where m is an Eisenstein maximal ideal. In many cases, the dimension of the m-torsion is 2, in other words, multiplicity one theorem holds.