Fisher-KPP c* = 2√(rD): derived from linearised dispersion discriminant

Derivation of the classical minimum wave speed. Linearise u_t = ru(1-u) + D·u_xx at u = 0 to obtain u_t = ru + D·u_xx; seek travelling-wave form u(x, t) = exp(-λ(x - ct)) for λ > 0 (decay at infinity). Substituting yields cλ = r + Dλ²,…