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

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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 ϵi−k\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 k≥2k \geq 2, there are Hecke-equivalent isomorphisms

Jk+2,D3≅M2knew,−(2)⊕M2k+(1),J_{k+2,D_3} \cong M_{2k}^{new,-}(2) \oplus M_{2k}^{+}(1), Jk+3,D5≅M2knew,+(2)⊕M2k−(1),J_{k+3,D_5} \cong M_{2k}^{new,+}(2) \oplus M_{2k}^{-}(1), Jk+4,D7≅M2knew,+(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.

References

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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