The Hecke algebras for the orthogonal group $SO(2,3)$ and the paramodular group of degree $2$

Abstract

In this paper we consider the integral orthogonal group with respect to the quadratic form of signature $(2,3)$ given by $\left(\begin{smallmatrix} 0 & 1 \\ 1 & 0 \end{smallmatrix}\right) \perp \left(\begin{smallmatrix} 0 & 1 \\ 1 & 0 \end{smallmatrix}\right) \perp (-2N)$ for squarefree $N\in \mathbb{N}$. The associated Hecke algebra is commutative and the tensor product of its primary components, which turn out to be polynomial rings over $\mathbb{Z}$ in $2$ algebraically independent elements. The integral orthogonal group is isomorphic to the paramodular group of degree $2$ and level $N$, more precisely to its maximal discrete normal extension. The results can be reformulated in the paramodular setting by virtue of an explicit isomorphism. The Hecke algebra of the non-maximal paramodular group inside $\mathrm{Sp}(2;\mathbb{Q})$ fails to be commutative if $N> 1$.