Kronecker's limit formula, holomorphic modular functions and q-expansions on certain moonshine groups

Jay Jorgenson, Lejla Smajlović, Holger L Then

Research output: Contribution to journalArticle (Academic Journal)peer-review

13 Citations (Scopus)
238 Downloads (Pure)


For any square-free integer $N$ such that the ``moonshine group'' $\Gamma_0(N)^+$ has genus zero, the Monstrous Moonshine Conjectures relate the Hauptmoduli of $\Gamma_0(N)^+$ to certain McKay-Thompson series associated to the representation theory of the Fischer-Griess monster group. In particular, the Hauptmoduli admits a $q$-expansion which has integer coefficients. In this article, we study the holomorphic function theory associated to higher genus groups $\Gamma_0(N)^+$. For all such arithmetic groups of genus up to and including three, we prove that the corresponding function field admits two generators whose $q$-expansions have integer coefficients, has lead coefficient equal to one, and has minimal order of pole at infinity. As corollary, we derive a polynomial relation which defines the underlying projective curve, and we deduce whether $i\infty$ is a Weierstrass point. Our method of proof is based on modular forms and includes extensive computer assistance, which, at times, applied Gauss elimination to matrices with thousands of entries, each one of which was a rational number whose numerator and denominator were thousands of digits in length.
Original languageEnglish
Pages (from-to)295-319
JournalExperimental Mathematics
Issue number3
Early online date4 Mar 2016
Publication statusPublished - 2016


  • Hauptmodul
  • Kronecker limit formula
  • Eisenstein series
  • q-expansions


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