# $q$-Expansions

We can associate a theta series to any pair of elements of a Brandt module, which give embeddings (with respect to any fixed module element) of the Brandt module in a space of weight $2$ modular forms.

## `ThetaSeries(x, y, prec): ModBrdtElt, ModBrdtElt, RngIntElt -> RngSerElt`

Returns the theta series associated to the pair $(x,y)$ of elements of a Brandt module, as an element of a power series ring.

## `qExpansionBasis(M, prec): ModBrdt, RngIntElt -> SeqEnum`

A sequence of power series elements, to precision $prec$, spanning the image of the theta functions associated to pairs in the Brandt module $M$.