A modularity criterion for Klein forms, with an application to modular forms of level $13$

Ick Sun Eum, Ja Kyung Koo, Dong Hwa Shin

We find some modularity criterion for a product of Klein forms of the congruence subgroup ${\Gamma }_1(N)$ and, as its application, construct a basis of the space of modular forms for $Γ_1…