In this article, we investigate the plus space of level N , where 4 −1 N is a squarefree (not necessarily odd) integer. This is a generalization of Kohnen's work. We define a Hecke isomorphism ℘ k of M k+1/2 (4M) onto M + k+1/2 (8M) for any odd positive integer M. The methods of the proof of the newform theory are this isomorphism, Waldspurger's theorem, and the dimension identity.Keywords Forms of half-integral weight · Newforms · Shimura correspondence · Kohnen plus space
Mathematics Subject Classification (2000) 11F37
IntroductionThe purpose of this paper is to establish the theory of newforms for the Kohnen plus space with respect to 0 (N ), where 4 −1 N is a square-free integer. This is a continuation of Kohnen's work (cf. [1]) in the case when 4 −1 N is odd square-free.Let us describe our results. The space of cusp forms of weight k + 1/2 with respect to 0 (N ) is denoted by S k+1/2 (N ) and the Kohnen plus space S + k+1/2 (N ) is defined by S. Yamana thanks Prof. Ikeda for useful discussion, and he is supported by JSPS Research Fellowships for Young Scientists.