1. Misner, C. W. and Wheeler, J. A. (1957). Gravitation, Electromagnetism, Unquantized Charge, and Mass as Propeerties of Curved Empty Space,Ann. Phys.,2, 525 603. Reprinted in Wheeler,Geometrodynamics, Academic Press, New York, 1961.
2. InGeometrodynamics, pp. 231, Wheeder cites H. Weyl:Raum Zeit Materie, 4th ed., Sect.34 (Berlin, 192), A. Finstein and N. Rosen (1935).Phys. Rev. 48, 73, and J. L. Synge (1947) as having previously mentioned the idea of multiply connected spaces in Genral Relativity.
3. For the experimental evidence against the existence of magnetic monopoles see R. L. Fleischeret al., Phys. Rev.,177, 2029 (1969) and other references cited therein.
4. For an elementary exposition of homology theory, see for example, note 1or any textbook on homology, theory such as I. M. Singer and J. A. Thorpe,Lecture Notes on Elementary Topology and Geometry, Scott, Foreman and Company, Gleville, Illinois, 1967.
5. We assume our space-time to be compact but not boundaryless. In fact for simplicity we assume throughout that our manifold is of the form [0,1]×S whereS is a boundaryless, compact, 3-dimensional manifold (a closed three-universe in physicist terminology). See R. P. Geroch,J. Math. Phys.,8, 782 (1967) for reasons for this assumption.