Mocanu's Jacobi-form isomorphism conjecture for the lattices D3D_3, D5D_5, and D7D_7

Let Jk,DrJ_{k,D_r} denote the space of Jacobi forms of weight kk and lattice index DrD_r. Let Mknew,0˘3b5(N)M_k^{new,\u03b5}(N) be the subspace of new elliptic modular forms of weight kk for Γ0(N)\Gamma_0(N) with Fricke eigenvalue ϵik\epsilon i^{-k}, and let Mk(N)M_k^{}(N) denote the corresponding Fricke eigenspaces. Two such spaces are Hecke equivalent when the displayed isomorphism is compatible with their Hecke-module structures.

Mocanu's conjecture. For every k2k \geq 2, there are Hecke-equivalent isomorphisms

Jk+2,D3M2knew,(2)M2k+(1),J_{k+2,D_3} \cong M_{2k}^{new,-}(2) \oplus M_{2k}^{+}(1), Jk+3,D5M2knew,+(2)M2k(1),J_{k+3,D_5} \cong M_{2k}^{new,+}(2) \oplus M_{2k}^{-}(1), Jk+4,D7M2knew,+(2)M2k+(1).J_{k+4,D_7} \cong M_{2k}^{new,+}(2) \oplus M_{2k}^{+}(1).

The conjecture generalizes the known isomorphism for index D1D_1 (equivalently, index 22), and is based on numerical examples of Euler factors of Jacobi forms. Its resolution is not established by the supplied text.

Sources & referencesView supporting material

Primary source

Shuichi Hayashida, “Isomorphism between Jacobi forms of index D_2n+1 and elliptic modular forms of level 2”, arXiv:2512.15012 (2026).

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