A method was obtained for numerical solution of the problem concerning the resolvent of the determining integral relaxation and creep equations for the case of nonlinearity of the viscoelastic properties of synthetic fibres. Numerical rotation of the integral relaxation and creep kernels and the corresponding normalized functions in the form of the normalized arctangent of the logarithm of the reduced time allows drawing a conclusion concerning the validity of the choice of a given function as the basis for the model of the viscoelastic properties. The optimality criteria for selection of the mathematical model of the viscoelasticity of synthetic fibres are reported.