Coba jelasin Lihat pula dari Sifat universal dong

Borceux, F. Handbook of Categorical Algebra: vol 1 Basic category theory (1994) Cambridge University Press, (Encyclopedia of Mathematics and its Applications)

N. Bourbaki, Livre II : Algèbre (1970), Hermann, .

Milies, César Polcino; Sehgal, Sudarshan K.. An introduction to group rings. Algebras and applications, Volume 1. Springer, 2002.

Jacobson. Basic Algebra II. Dover. 2009.