Weakly nonlinear extension of d'Alembert's formula

We consider a weakly nonlinear solution of the Cauchy problem for the regularised Boussinesq equation, which constitutes an extension of the classical d'Alembert's formula for the linear wave equation. The solution is given by a simple and explicit formula, expressed in terms of two special functions solving the initial-value problems for two Korteweg-de Vries equations. We test the formula by considering several examples with `exactly solvable initial conditions' and their perturbations. Explicit analytical solutions are compared with the results of direct numerical simulations.