An Effective Chiral Meson Lagrangian at $O(P^6)$ from the NJL Model
  In this work we present a strong chiral meson Lagrangian up to and including $O(p^6)$ in the momentum expansion. It is derived from the Nambu--Jona-Lasinio (NJL) model using the heat-kernel method. Identities related to the properties of covariant derivatives of the chiral matrix $U$ as well as field transformations have been used to obtain a minimal set of linearly independent terms. 
A.A.Bel'kov, A.V.Lanyov, A.Schaale, S.Scherer