Identity Mapping is Continuous

Theorem
Let $T = \left({S, \tau}\right)$ be a topological space.

The identity mapping $I_S: S \to S$ defined as:
 * $\forall x \in S: I_S \left({x}\right) = x$

is a continuous mapping.

Proof
Let $U \in \tau$.

We have that the Identity Mapping is a Bijection, so $I_S^{-1}$ is well-defined and:
 * $\forall x \in U: I_S^{-1} \left({x}\right) = x$

Thus $I_S^{-1} \left({U}\right) = U \in \tau$.

Hence, by definition of continuous mapping, $I_S$ is continuous.