The Eichler integral of $E_2$ and $q$-brackets of $t$-hook functions

Abstract

For functions $f: \mathcal{P}\rightarrow \mathbb{C}$ on partitions, Bloch and Okounkov defined a power series $\langle f\rangle_q$ that is the "weighted average" of $f$. As Fourier series in $q=e^{2\pi i z}$, such $q$-brackets generate the ring of quasimodular forms, and the modular forms that are powers of Dedekind's eta-function. Using work of Berndt and Han, we build modular objects from $$ f_t(\lambda):= t\sum_{h\in \mathcal{H}_t(\lambda)}\frac{1}{h^2}, $$ weighted sums over partition hook numbers that are multiples of $t$. We find that $\langle f_t \rangle_q$ is the Eichler integral of $(1-E_2(tz))/24,$ which we modify to construct a function $M_t(z)$ that enjoys weight 0 modularity properties. As a consequence, the non-modular Fourier series $$H_t^*(z):=\sum_{\lambda \in \mathcal{P}} f_t(\lambda)q^{|\lambda|-\frac{1}{24}} $$ inherits weight $-1/2$ modularity properties. These are sufficient to imply a Chowla-Selberg type result, generalizing the fact that weight $k$ algebraic modular forms evaluated at discriminant $D<0$ points $\tau$ are algebraic multiples of $\Omega_D^k,$ the $k$th power of the canonical period. If we let $\Psi(\tau):=-\pi i \left(\frac{\tau^2-3\tau+1}{12\tau}\right)-\frac{\log(\tau)}{2},$ then for $t=1$ we prove that $$ H_1^*(-1/\tau)-\frac{1}{\sqrt{-i\tau}}\cdot H_1^*(\tau)\in \overline{\mathbb{Q}}\cdot \frac{\Psi(\tau)}{\sqrt{\Omega_D}}.$$