1. P. Freyd,Abelian categories, Harper and Row, New York (1964).
2. H. Hule,Polynome über universalen Algebren, Monatsh. Math. 73, 329–340 (1969).
3. H. Hule,Polynomial normal forms and the embedding of polynomial algebras, Colloq. Math. Soc. Janos Bolyai (1975).
4. H. Lausch and W. Nöbauer,Algebra of polynomials, North-Holland, Amsterdam (1973)
5. B. Mitchell,Theory of categories, Academic Press (1965).