F: s+s → s can also be defined as binary operation on S, which is called binary operation for short. It is also called S-F closure.
X and+here are binary operators, and the specific operations can be multiplication and division ... The specific problems are being analyzed.
For example 1
F: n+n→ n. It means that addition is a binary operation on the set of natural numbers n, and n is close to F.
F: n× n→ n. It means that multiplication is a binary operation on the set of natural numbers n, and n is close to F.