# Combination Theorem for Continuous Mappings/Topological Division Ring/Inverse Rule

## Theorem

Let $\struct {S, \tau_{_S} }$ be a topological space.

Let $\struct {R, +, *, \tau_{_R} }$ be a topological division ring.

Let $g: \struct {S, \tau_{_S} } \to \struct {R, \tau_{_R} }$ be a continuous mapping.

Let $U = S \setminus \set {x : \map g x = 0}$

Let $g^{-1}: U \to R$ be the mapping defined by:

$\forall x \in U : \map {g^{-1} } x = \map g x^{-1}$

Let $\tau_{_U}$ be the subspace topology on $U$.

Then:

$g^{-1}: \struct {U, \tau_{_U} } \to \struct {R, \tau_{_R} }$ is continuous.

## Proof

Let $R^* = R \setminus \set 0$.

Let $\tau^*$ be the subspace topology on $R^*$.

By definition of a topological division ring:

$\phi: \struct {R^*, \tau^*} \to \struct {R, \tau_{_R} }$ such that $\forall x \in R^*: \map \phi x = x^{-1}$ is a continuous mapping

Let $g^*: \struct {U, \tau_{_U} } \to \struct {R^*, \tau^*}$ be the restriction of $g$ to $U \subseteq R$.

From Restriction of Continuous Mapping is Continuous, $g^*$ is a continuous mapping.

From Composite of Continuous Mappings is Continuous, the composition $\phi \circ g^* : \struct {U, \tau_{_U} } \to \struct {R, \tau_{_R} }$ is continuous.

Now:

 $\, \displaystyle \forall x \in U : \,$ $\displaystyle \map {\paren {g^{-1} } } x$ $=$ $\displaystyle \map g x^{-1}$ Definition of $g^{-1}$ $\displaystyle$ $=$ $\displaystyle \map \phi {\map g x}$ Definition of $\phi$ $\displaystyle$ $=$ $\displaystyle \map \phi {\map {g^*} x}$ Since $\map g x \neq 0$ $\displaystyle$ $=$ $\displaystyle \map {\paren {\phi \circ g^*} } x$ Definition of Composition of Mappings

From Equality of Mappings:

$g^{-1} = \phi \circ g^*$

The result follows.

$\blacksquare$