1. S. Weinberg, Physica A 96, 327 (1979)
2. J. Gasser, H. Leutwyler, Ann. Phys. (New York) 158, 142 (1984)
3. J. Gasser, H. Leutwyler, Nucl. Phys. B 250, 465 (1985)
4. A. Pich, Lectures at Les Houches Summer School in Theoretical Physics, Session 68: Probing the Standard Model of Particle Interactions, Les Houches, France, 28 Jul-5 Sep 1997, [hep-ph/9806303]
5. G. Ecker, Lectures given at Advanced School on Quantum Chromodynamics (QCD 2000), Benasque, Huesca, Spain, 3-6 Jul 2000, [hep-ph/0011026]