# Subspace of Smooth Real Functions

## Corollary to Subspace of Real Functions of Differentiability Class

Let $\mathbb J = \set {x \in \R: a < x < b}$ be an open interval of the real number line $\R$.

Let $\map {\CC^{\paren \infty} } {\mathbb J}$ denote the set of all continuous real functions on $\mathbb J$ which are differentiable on $\mathbb J$ at all orders.

Then $\struct {\map {\CC^{\paren \infty} } {\mathbb J}, +, \times}_\R$ is a subspace of the $\R$-vector space $\struct {\R^{\mathbb J}, +, \times}_\R$.

## Proof

Note that by the definition of smooth real function:

$\displaystyle \map {\CC^{\paren \infty} } {\mathbb J} = \bigcap_{m = 0}^\infty \map {\CC^{\paren m} } {\mathbb J}$
$\struct {\map {\CC^{\paren m} } {\mathbb J}, +, \times}_\R$ is a subspace of $\struct {\R^{\mathbb J}, +, \times}_\R$ for all $m$.

Then, from Linear Subspaces Closed under Intersection:

$\displaystyle \struct {\map {\CC^{\paren \infty} } {\mathbb J}, +, \times}_\R$ is a subspace of $\struct {\R^{\mathbb J}, +, \times}_\R$.

$\blacksquare$