Let $N\subset \RR^{ r}$ be a lattice, and let $\deg\colon N \to \CC$ be a piecewise-linear function that is linear on the cones of a complete rational polyhedral fan. Under certain conditions on $\deg$, the data $(N,\deg)$ determines a function $f\colon { \HHH}\to \CC$ that is a holomorphic modular form of weight $r$ for the congruence subgroup $\Gamma_{ 1} (l) $. Moreover, by considering all possible pairs $(N ,\deg)$, we obtain a natural subring ${ \TTT} (l)$ of modular forms with respect to $\Gamma_{ 1} (l) $. We construct an explicit set of generators for $\TTT (l)$, and show that ${ \TTT} (l)$ is stable under the action of the Hecke operators. Finally, we relate ${ \TTT} (l)$ to the Hirzebruch elliptic genera that are modular with respect to $\Gamma_{ 1} (l) $.
Categories: math.NT math.AG