Equivalence of Definitions of Piecewise Continuously Differentiable Function

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

Definition 2 implies Definition 1
Assume that $f$ satisfies the requirement of Definition 2.

We need to prove that $f$ satisfies the requirements of Definition 1.

$f$ is continuous by Piecewise Continuously Differentiable Function/Definition 2 is Continuous.

This proves (1) in Definition 1.

$f$ is continuously differentiable on $\closedint {x_{i-1}} {x_i}$, the derivatives at $x_{i-1}$ and $x_i$ understood as one-sided derivatives, for every $i \in \set {1, \ldots, n}$, by definition.

Therefore, $f$ is continuously differentiable at every point of every $\openint {x_{i-1}} {x_i}$ since $\openint {x_{i-1}} {x_i}$ is a subset of $\closedint {x_{i-1}} {x_i}$.

This proves (2.1) in Definition 1.

$f$ has a continuous right-hand derivative at $x_{i-1}$ for every $i \in \set {1, \ldots, n}$.

This means that $f'_+ \paren {x_{i-1}}$ = $\displaystyle \lim_{x \mathop \to {x_{i-1}}^+} f' \paren {x}$.

This implies that $\displaystyle \lim_{x \mathop \to {x_{i-1}}^+} f' \paren {x}$ exists for every $i \in \set {1, \ldots, n}$.

Also, $f$ has a continuous left-hand derivative at $x_i$ for every $i \in \set {1, \ldots, n}$.

This means that $f'_- \paren {x_i}$ = $\displaystyle \lim_{x \mathop \to {x_i}^-} f' \paren {x}$.

This implies that $\displaystyle \lim_{x \mathop \to {x_i}^-} f' \paren {x}$ exists for every $i \in \set {1, \ldots, n}$.

These two conclusions prove that requirement (2.2) in Definition 1 is statisfied.

This finishes the proof that $f$ satisfies the requirements of Definition 1.

Definition 1 implies Definition 2
Assume that $f$ satisfies the requirements of Definition 1.

We need to prove that $f$ satisfies the requirement of Definition 2.

$f$ is continuously differentiable on $\openint {x_{i-1}} {x_i}$ for every $i \in \set {1, \ldots, n}$ by definition.

Also, the one-sided limits $\displaystyle \lim_{x \mathop \to {x_{i-1}}^+} f' \paren {x}$ and $\displaystyle \lim_{x \mathop \to {x_i}^-} f' \paren {x}$ exist for every $i \in \set {1, \ldots, n}$.

$f$ is continuously differentiable on $\closedint {x_{i-1}} {x_i}$, the derivatives at $x_{i-1}$ and $x_i$ understood as one-sided derivatives, for every $i \in \set {1, \ldots, n}$, by Extendability Theorem for Derivatives Continuous on Open Intervals.

This is exactly the requirement of Definition 2.