On the geometry of the Humbert surface of square discriminant

For every positive integer $N$ we determine the Enriques–Kodaira type of the Humbert surface of discriminant $N^2$ which parametrises principally polarised abelian surfaces that are $(N,N)$-isogenous to a product of elliptic curves. A key step in the proof is to analyse the fixed point locus of a Fricke-like involution on the Hilbert modular surface of discriminant $N^2$ which was studied by Hermann and by Kani and Schanz. To this end, we construct certain “diagonal” Hirzebruch–Zagier divisors which are fixed by this involution. In our analysis we obtain a genus formula for these divisors, which includes the case of modular curves associated to (any) extended Cartan subgroup of ${\mathrm{G}\mathrm{L}}_2(\mathbb{Z}\mathrm{/}N\mathbb{Z})$ and which may be of independent interest.