Obstructions and kernel transport for Hecke lifts of partition q-brackets

Abstract

By the Bloch-Okounkov theorem, the $q$-bracket sends shifted symmetric functions on partitions to quasimodular forms. Following a question of van Ittersum, we study whether the Hecke action on quasimodular forms lifts through this map, in the sense of exact lifts $A_{n,2k}$ with $⟨A_{n,2k}f{⟩}_q=T_{n,2k}⟨f{⟩}_q$. Our main theorem is that Zagier's lowering operator $B=\frac{1}{2}(D-{\partial }^2)$ is injective on the genuine homogeneous subspace in every weight at least $4$, although it is not injective on the formal algebra. For exact lifts compatible with lowering, this transports Hecke actions on $q$-bracket kernels between adjacent weights, with divisibility consequences for characteristic polynomials and $p$-adic constraints on kernel eigenvalues. In fixed weight, we classify all exact lifts by an action on the kernel and a kernel-valued cocycle relative to a section. Two obstruction results show that no exact lift is an algebra homomorphism, and that even strict compatibility with multiplication by $Q_2$ already fails in weight $6$. Finally, we construct exact lifts with scalar kernel action whenever the $q$-bracket image is Hecke-stable, and exact rational computations give $q$-bracket surjectivity through weight $16$ and therefore existence of Hecke lifts through weight $16$.

23 pages, no figures