Definition:Generator

Generator of a Semigroup
Let $$\varnothing \subset X \subseteq S$$, where $$\left({S, \circ}\right)$$ is a semigroup.

Then there exists $$\left({T, \circ}\right)$$, the smallest subsemigroup of $$\left({S, \circ}\right)$$ which contains $$X$$.

In this case, $$X$$ is the generator (or set of generators) of $$\left({T, \circ}\right)$$, or that $$X$$ generates $$\left({T, \circ}\right)$$.

$$\left({T, \circ}\right)$$ is the subsemigroup generated by $$X$$.

This is written $$T = \left\langle {X} \right\rangle$$.

This subsemigroup is proven to exist by Generator of a Semigroup.

Generator of a Group
If $$\left({G, \circ}\right)$$ is a group, then $$H = \left\langle {X}\right\rangle$$ is the subgroup of $$\left({G, \circ}\right)$$ generated by $$X$$.

If $$X$$ is a singleton, i.e. $$X = \left\{{x}\right\}$$, then we can (and usually do) write $$T = \left\langle {x}\right\rangle$$ for $$T = \left\langle {\left\{{x}\right\}}\right\rangle$$.

This subgroup is proven to exist by Generator of a Group.

Generator of a Ring
Let $$\left({R, +, \circ}\right)$$ be a ring.

Let $$S \subseteq R$$.

The subring generated by $$S$$ is the smallest subring of $$R$$ containing $$S$$.

Notation
We can also write $$\left\langle {X \cup Y} \right\rangle$$ as $$\left\langle {X, Y} \right\rangle$$.