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2. 2) N. Sanchez, Phys. Rev. 24D (1981) 2100
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4. H.J. de Vega and N. Sanchez, Nucl. Phys. 299B, 818 (1987)
5. P. Goddard, J. Goldstone, C. Rebbi and C.B. Thorn — Nucl. Phys. 56B (1973) 109.