On a simple model of X_0(N)

Abstract

We find plane models for all $X_0(N)$, $N\geq 2$. We observe a map from the modular curve $X_0(N)$ to the projective plane constructed using modular forms of weight $12$ for the group $\Gamma_0(N)$; the Ramanujan function $\Delta$, $\Delta(N\cdot)$ and the third power of Eisestein series of weight $4$, $E_4^3$, and prove that this map is birational equivalence for every $N\geq 2$. The equation of the model is the minimal polynomial of $\Delta(N\cdot)/\Delta$ over $\mathbb{C}(j)$.