 Modular forms as meromorphic differentials on X(1)

A weight-$2k$ modular form descends as an invariant tensor differential. At an elliptic point of order $e$, a local invariant coordinate is $w=u^e$, so its descended coefficient has a pole of order at most $\lfloor k(e-1)/e\rfloor$. At infinity, $d\tau =dq/(2\pi iq)$ gives a pole of order at most $k$. Thus $D=k[\infty ]+\lfloor k/2\rfloor [i]+\lfloor 2k/3\rfloor [\rho ]$ bounds the poles. Conversely these bounds give holomorphic pullbacks. The Riemann-Roch theorem on the compactified modular curve proves finite-dimensionality; since $X(1)$ is the Riemann sphere, the dimension is $\max (0,1-k+\lfloor k/2\rfloor +\lfloor 2k/3\rfloor )$.