Theorem: sigma(omega = 0) - sigma_0 = 0 (Drude DC conductivity limit)

Theorem (Drude-DC canonical): substituting ω = 0 into σ(ω) = σ_0/(1 - i ω τ) gives σ(0) = σ_0/1 = σ_0 exactly. Canonical sympy pin: sigma_0, omega, tau = sp.symbols('sigma_0 omega tau', positive=True); sigma = sigma_0 / (1 -…