1. L. D. Landau and E. M. Lifshitz, Course of Theoretical Physics, Vol. 2: The Classical Theory of Fields (Nauka, Moscow, 1988; Pergamon, Oxford, 1975).
2. M. Duff, L. Okun, and G. Veneziano, physics/0110060.
3. V. Rubakov, Usp. Fiz. Nauk 171, 913 (2001) [Phys. Usp. 44, 871 (2001)].
4. G. Volovik, Pis’ma Zh. Éksp. Teor. Fiz. 76, 89 (2002) [JETP Lett. 76, 77 (2002)].