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2. V. I. Fushchich:Lett. Nuovo Cimento,11, 508 (1974).
3. V. I. Fushchich: DAN USSR,230, 570 (1976) (in Russian).
4. A. G. Nikitin, Yu. N. Segeda andV. I. Fushchich:Theor. Mat. Fiz.,29, 82 (1976) (in Russian).
5. V. I. Fushchich:Lett. Nuovo Cimento,6, 133 (1973).