2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/80404Whitham theory of modulations is developed for periodic waves described by nonlinear wave equations integrable by the inverse scattering transform method associated with $2\times2$ matrix or second order scalar spectral problems. The theory is illustrated by derivation of the Whitham equations for perturbed Korteweg-de Vries equation and nonlinear Schrödinger equation with linear damping.17 pages, no figuresExactly Solvable and Integrable SystemsPattern Formation and Solitonstext