Published online by Cambridge University Press: 06 August 2021
We prove a necessary and sufficient condition for the graded algebra of automorphic forms on a symmetric domain of type IV being free. From the necessary condition, we derive a classification result. Let $M$ be an even lattice of signature
$(2,n)$ splitting two hyperbolic planes. Suppose
$\Gamma$ is a subgroup of the integral orthogonal group of
$M$ containing the discriminant kernel. It is proved that there are exactly 26 groups
$\Gamma$ such that the space of modular forms for
$\Gamma$ is a free algebra. Using the sufficient condition, we recover some well-known results.