Definition:Principal Ideal of Ring

Definition
Let $\struct {R, +, \circ}$ be a ring with unity.

Let $a \in R$.

We define:
 * $\ideal a = \ds \set {\sum_{i \mathop = 1}^n r_i \circ a \circ s_i: n \in \N, r_i, s_i \in R}$

The ideal $\ideal a$ is called the principal ideal of $R$ generated by $a$.

Notation
From Principal Ideal of Commutative Ring the notions of principal left ideal, principal right ideal and principal ideal coincide.

So often, in some sources, a principal ideal of a commutative ring with unity is denoted as $aR$.

This is done most often in the case where it is important to identify the ring that the principal ideal belongs to.

The notation $aR$ is often used when the ring $R$ in question is the integers $\Z$ or the $p$-adic integers $\Z_p$. So it is common for $n\Z$ to denote the principal ideal of $\Z$ generated by $n$ and $p^k\Z_p$ to denote the principal ideal of $\Z_p$ generated by $p^k$.

Also see

 * Principal Ideal is Ideal: $\ideal a$ is a principal ideal if $\gen a$ is the ideal generated by $a$.