Using a non-perturbative method based on the selfconsistent Quasi-particle
Random-Phase Approximation (QRPA) we describe the properties of the pion in the
linear σ-model. It is found that the pion is massless in the chiral
limit, both at zero- and finite temperature, in accordance with Goldstone's
theorem.Comment: To appear in Nucl.Phys. A, 16 pages, 2 Postscript figure