The finite size spectrum of the 2-dimensional O(3)O(3) nonlinear σ-model

Nonlinear integral equations are proposed for the description of the full finite size spectrum of the 2-dimensional O(3)O(3) nonlinear σ-model in a periodic box. Numerical results for the energy eigenvalues are compared to the rotator spectrum and perturbation theory for small volumes and with the recently proposed generalized Lüscher formulas at large volumes.