1. Polyakov’s early work is summarized in A. M. Polyakov, Nucl. Phys. B121, 429 (1977).
2. See, for example, R. Feynman and A. Hibbs, Quantum Mechanics and Path Integrals (McGraw-Hill, New York, 1965).
3. For a review of lumps, see my 1975 Erice lectures, “Classical Lumps and Their Quantum Descendants”, in New Phenomena in Sub-nuclear Physics, ed. by A. Zichichi (Plenum Press, New York, 1977).
4. This is, of course, nothing but the standard prescription for handling collective coordinates in soliton problems. See J. L. Gervais and B. Sakita, Phys. Rev. D11, 2943 (1975).
5. The treatment here follows that of C. Callan and S. Coleman, Phys. Rev. D16, 1762 (1977). The idea of handling unstable states this way goes back to Langer’s analysis of the droplet model in statistical mechanics.