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Published online by Cambridge University Press: 05 March 2024
We show that for every finite set of prime numbers $S$, there are at most finitely many singular moduli that are
$S$-units. The key new ingredient is that for every prime number
$p$, singular moduli are
$p$-adically disperse. We prove analogous results for the Weber modular functions, the
$\lambda$-invariants and the McKay–Thompson series associated with the elements of the monster group. Finally, we also obtain that a modular function that specializes to infinitely many algebraic units at quadratic imaginary numbers must be a weak modular unit.