Unitarity Constraints on effective interaction in $\pi N$ scattering
  Positivity constraints are derived on pion-nucleon scattering amplitudes and their even-order derivatives inside the Mandelstam Triangle with the help of dispersion relations. Fairly interesting constraints are obtained on some of the low energy constants, by a combination of the chiral perturbation theory for heavy baryons and existing fitting results from available pion-nucleon phase shifts at intermediate energies. 
Mingxing Luo, Yong Wang and Guohuai Zhu