2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/191763The relativistic amplitudes of pion photo- and electroproduction are calculated by dispersion relations at constant t. Several sum rules and low-energy theorems for the threshold amplitudes are investigated within this technique. The continuation of the amplitudes to sub-threshold kinematics is shown to provide a unique framework to derive the low-energy constants of chiral perturbation theory by global properties of the excitation spectrum.18 pages, 10 figures, Proc. Yang Fest, Taipei, June 2007High Energy Physics - Phenomenologytext