1. P.A.M. Dirac, Proc. R. Soc. A 133, 60 (1931)
2. T.T. Wu, C.N. Yang, Properties of Matter under Unusual Conditions, ed. by H. Mark, S. Fernbach (Interscience, New York, 1968), p. 349
3. G. ’t Hooft, Nucl. Phys. 79, 276 (1974)
4. A.M. Polyakov, JETP Lett. 20, 194 (1974)
5. G.E. Volovik, The Universe in a Helium Droplet. International Series of Monographs on Physics No. 117 (Oxford University Press, Oxford, 2003)