User:Dfeuer/CTR5

Theorem
Let $\left({S, \circ}\right)$ be a semigroup.

Let $\mathcal R$ be a transitive relation on $S$ which is compatible with $\circ$.

Let $x, i \in S$.

Suppose that $i$ is idempotent for $\circ$.

Let $n \in \N_{>0}$ be a positive integer.

Then the following statements hold:


 * $i \mathrel{\mathcal R} x \implies i \mathrel{\mathcal R} x^n$
 * $x \mathrel{\mathcal R} i \implies x^n \mathrel{\mathcal R} i$

Proof
By User:Dfeuer/Operating Repeatedly on Transitive Relationship Compatible with Associative Operation:


 * $i \mathrel{\mathcal R} x \implies i^n \mathrel{\mathcal R} x^n$
 * $x \mathrel{\mathcal R} i \implies x^n \mathrel{\mathcal R} i^n$

By the definition of an idempotent element, $i^n = i$, so the theorem holds.