Exact Sum Rules as Consequences of Low-Energy Theorems and Unsubtracted Dispersion Relations

Abstract
Working with the equal-time commutation relations of vector current densities including the Schwinger term, the low-energy theorems for the Compton scattering of isovector photons by nucleons have been obtained. Assuming unsubtracted dispersion relations, sum rules which have been previously obtained by several authors have been reproduced and the relations between the previous derivations and the present work are discussed.