Journal of Statistical Physics, Vol. 40, Nos. 1/2, 1985. p. 303-328.
A fermionic perturbation theory is developed for the statistical mechanics of the nonlinear Schr6dinger model. The theory is based on an interacting-fermion picture of the Bethe wave function. The inner product of the Bethe wave function is explicitly evaluated, and a simple graphical representation of it is given. The basic equations obtained for the free energy agree with those of Yang and Yang. In particular, the present theory gives a clear-cut meaning to the e function of Yang and Yang: It represents a fermion energy at finite temperatures.