1. Goncharov, Yu. P., Lett. Math. Phys. 19, 73 (1990); Bytsenko, A. A. and Goncharov, Yu. P., Mod. Phys. Lett. A6, 669 (1991).
2. Shubin, M. A., Pseudodifferential Operators and Spectral Theory, Nauka, Moscow, 1978.
3. Ikeda, A. and Taniguchi, Y., Osaka J. Math. 15, 515 (1978).
4. Stein, E. M. and Weiss, G., Introduction to Fourier Analysis on Euclidean Spaces, Princeton University Press, Princeton, NJ, 1971.
5. Venkov, A. B., Russian Math. Surveys 34(3), 79 (1979).