# Definition:Piecewise Continuously Differentiable Function

## Definition

Let $f$ be a real function defined on a closed interval $\closedint a b$.

### Definition 1

$f$ is piecewise continuously differentiable if and only if:

$(1): \quad f$ is continuous
$(2): \quad$ there exists a finite subdivision $\set {x_0, \ldots, x_n}$ of $\closedint a b$, $x_0 = a$ and $x_n = b$, such that:
$(2.1): \quad f$ is continuously differentiable on $\openint {x_{i - 1}} {x_i}$ for every $i \in \set {1, \ldots, n}$
$(2.2): \quad$ the one-sided limits $\displaystyle \lim_{x \mathop \to {x_{i - 1}}^+} \map {f'} x$ and $\displaystyle \lim_{x \mathop \to {x_i}^-} \map {f'} x$ exist for every $i \in \set {1, \ldots, n}$.

### Definition 2

$f$ is piecewise continuously differentiable if and only if:

there exists a finite subdivision $\set {x_0, \ldots, x_n}$ of $\closedint a b$, $x_0 = a$ and $x_n = b$, such that:
$f$ is continuously differentiable on $\closedint {x_{i − 1} } {x_i}$, where the derivative at $x_{i − 1}$ understood as right-handed and the derivative at $x_i$ understood as left-handed, for every $i \in \set {1, \ldots, n}$.

## Also defined as

Other definitions of Piecewise Continuously Differentiable Function exist. Some examples are shown in the list below. The list is condensed; see the discussion page for a detailed description.

- $(2)$ is replaced by: $f$ is continuously differentiable on the intervals $\openint {x_{i − 1} } {x_i}$.

- $(1)$ is replaced by: $f$ is piecewise continuous, and $(2)$ is replaced by:

- The codomain of $f$ is $\C$ instead of $\R$.

## Also known as

A piecewise continuously differentiable function is referred to in some sources as a piecewise smooth function.

However, as a smooth function is defined on $\mathsf{Pr} \infty \mathsf{fWiki}$ as being of differentiability class $\infty$, this can cause confusion, so is not recommended.