Space of Somewhere Differentiable Continuous Functions on Closed Interval is Meager in Space of Continuous Functions on Closed Interval

Theorem
Let $I = \closedint a b$.

Let $\map \CC I$ be the set of continuous functions on $I$.

Let $\map \DD I$ be the set of continuous functions on $I$ that are differentiable at a point.

Let $d$ be the metric induced by the supremum norm.

Then $\map \DD I$ is meager in $\struct {\map \CC I, d}$.

Proof
Let:


 * $\ds A_{n, \, m} = \set {f \in \map \CC I: \exists x \in I: \forall t \in I: 0 < \size {t - x} < \frac 1 m \implies \size {\frac {\map f t - \map f x} {t - x} } \le n}$

and:


 * $\ds A = \bigcup_{\tuple {n, \, m} \mathop \in \N^2} A_{n, \, m}$

Lemma 2
From Lemma 2:


 * $A$ is the countable union of nowhere dense sets.

So, by definition of a meager space:


 * $A$ is meager in $\struct {\map \CC I, d}$.

By Subset of Meager Set is Meager Set and Lemma 1:


 * $\map \DD I$ is meager in $\struct {\map \CC I, d}$.