Published online by Cambridge University Press: 01 March 2017
There are a number of fundamental results in the study of holomorphic function theory associated to the discrete group $\operatorname{PSL}(2,\mathbb{Z})$, including the following statements: the ring of holomorphic modular forms is generated by the holomorphic Eisenstein series of weights four and six, denoted by
$E_{4}$ and
$E_{6}$; the smallest-weight cusp form
$\unicode[STIX]{x1D6E5}$ has weight twelve and can be written as a polynomial in
$E_{4}$ and
$E_{6}$; and the Hauptmodul
$j$ can be written as a multiple of
$E_{4}^{3}$ divided by
$\unicode[STIX]{x1D6E5}$. The goal of the present article is to seek generalizations of these results to some other genus-zero arithmetic groups
$\unicode[STIX]{x1D6E4}_{0}(N)^{+}$ with square-free level
$N$, which are related to ‘Monstrous moonshine conjectures’. Certain aspects of our results are generated from extensive computer analysis; as a result, many of the space-consuming results are made available on a publicly accessible web site. However, we do present in this article specific results for certain low-level groups.