Unit of Ring of Mappings iff Image is Subset of Ring Units

Theorem
Let $\struct {R, +, \circ}$ be a ring with unity $1_R$.

Let $U_R$ be the set of units in $R$.

Let $S$ be a set.

Let $\struct {R^S, +', \circ'}$ be the induced structure on the set of mappings $R^S$.

Let $f: S \to U_R$ be a mapping from $S$ to $U_R$.

Then $f$ is a unit of $\struct {R^S, +', \circ'}$ and the inverse of $f$ is the mapping:
 * $f^{-1} : S \to U_R : \forall x \in S : \map {f^{-1}} {x} = \map f x^{-1}$

Proof
From Structure Induced by Ring Operations is Ring, $\struct {R^S, +', \circ'}$ is a ring and so $\struct {R^S, +', \circ'}$ has units.

From Structure Induced by Ring with Unity Operations is Ring with Unity, $\struct {R^S, +', \circ'}$ has a unity $f_{1_R}$ defined by:
 * $\forall x \in S: \map {f_{1_R}} x = 1_R$

By assumption:
 * $\forall x \in S: \exists \map f x^{-1} : \map f x \circ \map f x^{-1} = \map f x^{-1} \circ \map f x = 1_R$

Let $f^{-1} : S \to U_R$ be defined by:
 * $\forall x \in S : \map {f^{-1}} {x} = \map f x^{-1}$

Consider the mapping $f \circ’ f^{-1}$:
 * $\forall x \in S : \map {f \circ’ f^{-1}} x = \map f x \circ’ \map {f^{-1}} x = \map f x = \map f x^{-1} = 1_R$

Hence $f \circ’ f^{-1} = f_{1_R}$.

Similarly, $f^{-1} \circ’ f = f_{1_R}$.

Hence $f$ is a unit of $\struct {R^S, +', \circ'}$ and the inverse of $f$ is the mapping $f^{-1}$.