Smith, Jonathan D. H. (1976), "Chapter 5: Plain algebras and equational completeness", Mal'cev varieties, Lecture Notes in Mathematics, vol. 554, Springer-Verlag, pp. 93–112, doi:10.1007/BFb0095452, MR0432511
Smith, Jonathan D. H. (1976), "Chapter 5: Plain algebras and equational completeness", Mal'cev varieties, Lecture Notes in Mathematics, vol. 554, Springer-Verlag, pp. 93–112, doi:10.1007/BFb0095452, MR0432511