1. A. Yu. Morozov, “String theory: what is it?,” Sov. Phys. Usp. 35, 671–714 (1992).
2. L. H. Kauffman, The Interface of Knots and Physics (World Scientific, Singapore, 2001), p. 788.
3. L. D. Landau and E. M. Lifshitz, Course of Theoretical Physics, Vol. 3: Quantum Mechanics: Non-Relativistic Theory (Oxford, Pergamon, 1977; Nauka, Moscow, 1989), p. 688.
4. M. E. Peskin and D. V. Schroeder, Introduction to Quantum Field Theory (Perseus, Reading, MA, 1995), p. 842.
5. A. A. Belavin, A. M. Polyakov, A. S. Schwartz, and Yu. S. Tyupkin, “Pseudoparticle solutions of the Yang-Mills equations,” Phys. Lett. B 59, 85–87 (1975).