$(2+)$-Replication and the Baby Monster

[摘要] 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 $\Gamma_0(2)^+$. We prove that the McKay-Thompson series for $2\cdot\mathbb{B}$, as computed by Höhn, are completely $(2+)$-replicable.

[关键词] moonshine;baby monster;replication