Complete proof that the BCT instanton action SD4 = π5/6 is the evaluation of the Ramanujan discriminant modular form Δ (τ) = η (τ) 24 at the CM point τ = i, via the Chowla–Selberg formula for the Dedekind eta function. Establishes the identification of the BCT Superfluid Lattice Model as the physical realisation of the critical bosonic string compactified on the D4 root lattice. Companion technical appendix to Letter 37.
Michel Robert Cabrié (Sun,) studied this question.