<The Detail of This Chapter.pdf>
(R, +) is a Abel group;
(R, *) is a semi-group;
(+, *) satisfy distributive law;
then (R, +, *) is a ring.
<if * holds commutativity, R is a commutative ring; if (R, *) is a unity semi-group, R is a unity ring.>
from the distributive law, we can get the relationship of + and *.