The Olsson sum rule and the rho Regge pole
  We consider the Olsson sum rule, i.e., the forward dispersion relation for pion-pion scattering with exchange of isospin unity at threshold. We show that, if using the S0, S2 and P wave expressions of Colangelo, Gasser and Leutwyler, then either the sum rule is not satisfied or, if adjusting the residue of the rho exchange Regge amplitude to have the sum rule satisfied (as recently proposed by Caprini, Colangelo and Leutwyler) then the subsequent high energy amplitude is in disagreement with experimental pi-pi cross sections. 
F. J. Yndurain