# Criterion for Ring with Unity to be Topological Ring

## Theorem

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

Let $\tau$ be a topology over $R$.

Suppose that $+$ and $\circ$ are $\tau$-continuous mappings.

Then $\struct{R, +, \circ, \tau}$ is a topological ring.

## Proof

As we presume $\circ$ to be continuous, we need only prove that $\struct{R, +, \tau}$ is a topological group.

As we presume $+$ to be continuous, we need only show that negation is continuous.

As $\struct{R, \circ}$ is a semigroup and $\circ$ is continuous then $\struct{R, \circ, \tau}$ is a topological semigroup.

From Identity Mapping is Homeomorphism, the identity mapping $I_R : \struct{R, \tau} \to \struct{R, \tau}$ is continuous.

From Multiple Rule for Continuous Mappings into Topological Semigroup, the mapping $\paren{- 1_R} \circ I_R : R \to R$ defined by:

- $\forall b \in R : \map {\paren{\paren{- 1_R} \circ I_R } } b = \paren{- 1_R} \circ b$

is continuous.

From Product with Ring Negative, for each $b \in R : - b = \paren{- 1_R} \circ b$.

Hence negation is continuous.

$\blacksquare$