  The unitarity relations for isovector nucleon form factors in the two-pion
approximation for the intermediate states are analytically extended in a model
independent way from the real half-axis t > 4 $\mu^2$ ($\mu$ is the pion mass)
to the whole complex t-plane. As a result, the explicit integral
representations for the nucleon form factors are constructed in terms of the
pion form factor and imaginary parts of the t-channel p-wave piN-scattering
amplitudes at t < 0. New method based on the explicit integral representations
is developed for numerical evaluation of the two-pion contribution to the
isovector nucleon form factors. The results of different methods are compared
and their efficiency is discussed.
