## Abstract

Suppose that M is an even lattice with dual M^{∗} and level N. Then the group Mp_{2}(Z) , which is the unique non-trivial double cover of SL_{2}(Z) , admits a representation ρ_{M}, called the Weil representation, on the space C[ M^{∗}/ M]. The main aim of this paper is to show how the formulae for the ρ_{M}-action of a general element of Mp_{2}(Z) can be obtained by a direct evaluation which does not depend on “external objects” such as theta functions. We decompose the Weil representation ρ_{M} into p-parts, in which each p-part can be seen as subspace of the Schwartz functions on the p-adic vector space MQp. Then we consider the Weil representation of Mp_{2}(Q_{p}) on the space of Schwartz functions on MQp, and see that restricting to Mp_{2}(Z) just gives the p-part of ρ_{M} again. The operators attained by the Weil representation are not always those appearing in the formulae from 1964, but are rather their multiples by certain roots of unity. For this, one has to find which pair of elements, lying over a matrix in SL_{2}(Q_{p}) , belong to the metaplectic double cover. Some other properties are also investigated.

Original language | American English |
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Pages (from-to) | 61-89 |

Number of pages | 29 |

Journal | Annales Mathematiques du Quebec |

Volume | 39 |

Issue number | 1 |

DOIs | |

State | Published - 1 Jun 2015 |

Externally published | Yes |

### Bibliographical note

Funding Information:The initial stage of this research has been carried out as part of my Ph.D. thesis work at the Hebrew University of Jerusalem, Israel. The final stage of this work was supported by the Minerva Fellowship (Max-Planck-Gesellschaft). I am deeply indebted to E. Lapid for his proposal to look for a p-adic proof to the factoring of the Weil representation through (a double cover of) , which initiated my work on this paper (and the corresponding part in my Ph.D. thesis). I would also like to thank my Ph.D. advisor R. Livné and H. M. Farkas for their help. I also thank J. Bruinier, N. Scheithauer and F. Strömberg for fruitful discussions while writing this paper and for referring me to []. Special thanks also to T. Yang, for referring me to []. I am also grateful to the two referees, whose remarks have greatly contributed to the improvement of the presentation of the results of this paper.

Publisher Copyright:

© 2015, Fondation Carl-Herz and Springer International Publishing Switzerland.

## Keywords

- Discriminant Forms
- Lattices
- Representations of p-adic Groups
- Weil Representations