Local solutions of harmonical and bi-harmonical equations, Universal Field Equation and self-dual configurations of Yang--Mills fields in four dimensions

A general method for the construction of solutions of the d'Alambertian and double d'Alambertian (harmonic and bi-harmonic) equations with local dependence of arbitrary functions upon two independent arguments is proposed. The connection between solutions of this kind and self-dual configurations of gauge fields having no singularities is established.