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2. R. Hagedorn, in “Quark Matter ’84”, Helsinki Symposium, ed. K. Kajantie, published in “Lecture Notes in Physics” 221 (Springer, Heidelberg, 1985).
3. E.L. Feinberg, Physics Reports 5 (1972) 237.
4. H. Yukawa, Proc.Phys.Math.Soc.Japan 17 (1935) 48.
5. W. Heitler, Proc.Cambr.Phil.Soc. 37 (1941) 291.