The Atkin modular function $f(q)$ of prime level N is a modular function on $X_0^+(N)$ of minimal degree, holomorphic on the upper half plane, and whose $q$-expansion has leading coefficient $1$. It is unique up to a constant. The Atkin modular polynomial is a bivariate polynomial $\Phi_N(X,Y)$ such that $$ \Phi_N(f(q),j(q)) = 0, $$ where $j(q) = q^{-1} + 744 + 196884 q + \cdots$ is the modular $j$-invariant. Since $f(q)$ is invariant under the Atkin-Lehner operator, and the image of $j(q)$ is $j_N(q) = j(q^N)$, we also have $\Phi_N(f(q),j_N(q)) = 0$.