$r$-Tuple Error Functions and Indefinite Theta Series of Higher-Depth

Abstract: Theta functions for definite signature lattices constitute a rich source of modular forms. A natural question is then their generalization to indefinite signature lattices. One way to ensure a convergent theta series while keeping the holomorphicity property of definite signature theta series is to restrict the sum over lattice points to a proper subset. Although such series do not have the modular properties that a definite signature theta function has, as shown by Zwegers for signature $(1,n-1)$ lattices, they can be completed to a function that has these modular properties by compromising on the holomorphicity property in a certain way. This construction has recently been generalized to signature $(2,n-2)$ lattices by Alexandrov, Banerjee, Manschot, and Pioline. A crucial ingredient in this work is the notion of double error functions which naturally lends itself to generalizations to higher dimensions. In this work we study the properties of such higher dimensional error functions which we will call $r$-tuple error functions. We then construct an indefinite theta series for signature $(r,n-r)$ lattices and show they can be completed to modular forms by using these $r$-tuple error functions.