# Questions tagged [theta-series]

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### What are the known number-theoretic functions, that are related to “the number of ideals of norm $n$, that belong to the class $[c]$”?

Let $L$ be a number field, $\mathcal{O}_L$ its ring of integers, and $\mathcal{Cl(O}_L)$ its ideal class group. Let's fix an arbitrary class $[c] \in \mathcal{Cl(O}_L)$. By $r(n)=r([c], n)$, I mean ...
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### $3$-variable Jacobi style identity linked to generalised Frobenius partitions

I was fiddling around with a family of probabilistic models and came across two "identities", which appear to be linked to generalized Frobenius partitions (more on this below). I would be ...
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### Expansion of Jacobi theta function at $p$-torsion

I am aware of the formula $$\Theta(z,q)=z\exp\left( -2\sum_{k\geq 1} \frac{z^{2k}}{(2k)!}E_{2k}(q)\right)$$ for the Jacobi theta function at the origin $z=0$. The definition I am using for the theta ...
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### Modularity of certain theta series associated to hyperbolic lattice

Let $L$ be an even hyperbolic lattice, i.e. a free $\mathbb{Z}$-module with a non-degenerate inner product $\cdot$ valued in $\mathbb{Z}$ of signature $(1,n)$ such that the norm of every vector is ...
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### Are all weight 5/2 modular forms theta series?

Given a quadratic form in $5$ variables, the theta series is a weight $5/2$ modular form. For a weight $3/2$ modular form we know that there always is a linear combination of theta series that work ...
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### Log-concavity of difference of theta functions

My knowledge on theta functions is limited, but I suspect that this is a quite challenging question. The 3rd Jacobian Theta function is given by \theta_3(z,q)\,=\,\sum\limits_{n=-\...
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### Siegel's article “The volume of the fundamental domain for some infinite groups”: trouble with understanding computations

This is the article I mentioned. While the idea of what Siegel is doing in order to compute the volume of the fundamental domain described in the article (the very first one, for there are discussed ...
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### Integral quaternary forms and theta functions

The following question arises when I attempt to understand the modular parameterization of the elliptic curve $$E:y^2-y=x^3-x$$ In Mazur-Swinnerton-Dyer and Zagier's construction, a theta function ...
### Determining the Lambert series for $xq+x^2q^4+x^3q^9+…+x^nq^{n^2}+…$
I am trying to determine the polynomials $P_n(x)$ from $$xq+x^2q^4+x^3q^9+...+\ x^nq^{n^2}+...=\sum_{n\geqslant1}\frac{P_n(x)q^n}{1-xq^n};$$ that is,  \sum_{d|n}x^{\frac nd-1}P_d(x)=\begin{cases}x^...