(2+)-replication and the Baby Monster

The definitions of replicable and completely replicable functions are intimately related to the Hecke operators for the modular group. We define the notions of “$(2+)$-replicable” and “completely $(2+)$-replicable” functions by considering the Hecke operators for ${\mathrm{\Gamma }}_0(2{)}^{+}$. We prove that the McKay-Thompson series for $2\cdot \mathbb{B}$, as computed by H"ohn, are completely $(2+)$-replicable.