Theorem: L^2 Sigma_a - D = 0 (neutron-diffusion-length identity)

Theorem (diffusion-length canonical): L = sqrt(D/Sigma_a) squared and multiplied by Sigma_a gives L² Sigma_a = D, residual L² Sigma_a - D = 0 identically. Canonical sympy pin: D, Sigma_a = sp.symbols('D Sigma_a', positive=True); L =…