So I am (barely) familiar with the construction of the theta function of an integral lattice L. The theta function, as I understand it, is defined as the function which takes a variable z and spits out the sum of q raised to the N(v) power, where the sum is over all vectors v in the lattice, q is equal to exp(2 pi i z), and N(v) is the norm squared. We can regard this as a formal sum for the sake of this question. Let's call it T(z).
I know there all kinds of identities about this function. For example if the lattice is even and unimodular, then T(z) is a modular form. I curious about the related function T(-z). Is there an easy way to relate it to T(z)?
What I'm really really interested in is the quotient T(z)/T(-z). I know there are people who know this stuff way better then I do, can anyone help? Is it easier in the unimodular case?