Even Unimodular Lattices
Wolfgang Ebeling · Advanced lectures in mathematics · 1994
In this section we study modified theta functions, namely theta series with spherical coefficients, and their behavior under transformations of the modular group. The results of this section are due to E. Hecke [Hec40] and B. Schoeneberg [Sch39, Sch74]. Our presentation follows [Ogg69, Chap. VI] and [Sko87].