On the Ramanujan-Ligozat modular unit

Abstract

Let $χ$ be the quadratic Dirichlet character of conductor $N$ associated with a given real quadratic field $k$ and let $X_χ(N)$ be the double covering of $X_0(N)$ associated with $χ$. In this note, we study a modular unit $f$ on the curve $X_χ(N)$. We prove that the transform of $f$ by the Fricke involution $\mathscr{w}_N$ is the level $N$ analogue of the Rogers-Ramanujan continued fraction introduced by Lee and Park. Furthermore, if $N$ is a positive prime discriminant, then $f$ is Ligozat's modular unit, which is known from Mazur's work on the Eisenstein ideal. We extend Ligozat's construction to any positive fundamental discriminant $N$.