Positivity constraints on LECs of chiPT lagrangian at cO(p⁶) level

Positivity constraints on the LECs of $\cO(p^6)$ $\chi$PT lagrangian are discussed. We demonstrate that the constraints are automatically satisfied inside the Mandelstam triangle for $\pi\pi$ scatterings, when $N_C$ is large. Numerical tests are made in the $N_C=3$ case, and it is found that these constraints are also well respected.