SH dispersion σ(k) = ε - (k² - k_0²)²: concave max at k_0, ε_c = 0

Linear stability analysis of the Swift-Hohenberg equation around the homogeneous base state ψ = 0. Linearised growth rate: σ(k) = ε - (k² - k_0²)² extracted from ψ ∝ exp(σt + ikx). Peak at k = k_0 (symbolic: dσ/dk = 0 there, with d²σ/dk²…