1. Kokkedee, J.: The quark model. New York: Benjamin 1969.
2. Morpurgo, G.: Lectures on the quark model, 1968 International School of Physics “Ettore Majorana” Erice. New York and London: Academic Press 1969.
3. Morpurgo, G.: Ref. [1], see in particular chapter 8.
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5. Royen, R. P. van, Weißkopf, V.: Nuovo Cimento50 A, 617 (1967).