Definition:Characteristic of Ring/Definition 2

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Let $\left({R, +, \circ}\right)$ be a ring with unity whose zero is $0_R$ and whose unity is $1_R$.

Let $g: \Z \to R$ be the initial homomorphism, with $g \left({n}\right) = n \cdot 1_R$.

Let $\left({p}\right)$ be the principal ideal of $\left({\Z, +, \times}\right)$ generated by $p$.

The characteristic $\operatorname{Char} \left({R}\right)$ is the positive integer $p \in \Z_{\ge 0}$ such that $\left({p}\right)$ is the kernel of $g$.

Also see

By Ideal of Ring of Integers has Unique Positive Generator, there exists a unique $p \in \Z_{\ge 0}$ such that $\ker g$ is the principal ideal $\left({p}\right)$.