Strong-coupling plaquette expansion: ⟨U_□⟩ = β/(2N²) + O(β²) for SU(N)

Strong-coupling (β → 0, i.e. g₀ → ∞) character expansion of the plaquette expectation in pure SU(N) lattice gauge theory. Expanding exp(−S_W) = exp[β Σ_□ (1/N)Re Tr U_□] order by order in β and using orthogonality ∫ dU U_ab U*_cd = δ_ac…