We prove a supercongruence modulo p to the fourth for the coefficients of the symmetric cube of the hypergeometric function at parameters one third, one third, one. The proof combines a modular identification on X₀ (3), an Eisenstein-type congruence for the coefficients of the Hauptmodul derivative, Lagrange–Bürmann coefficient extraction, and a Fricke–Hecke descent argument that yields uniform vanishing of all defects.
Alex Shvets (Sun,) studied this question.
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