Power Series Expansion for Hyperbolic Cosine Function

Theorem
The hyperbolic cosine function has the power series expansion:

valid for all $x \in \R$.

Proof
From Derivative of Hyperbolic Cosine Function:
 * $\dfrac \rd {\rd x} \cosh x = \sinh x$

From Derivative of Hyperbolic Sine Function:
 * $\dfrac \rd {\rd x} \sinh x = \cosh x$

Hence:
 * $\dfrac {\rd^2} {\rd x^2} \cosh x = \cosh x$

and so for all $m \in \N$:

where $k \in \Z$.

This leads to the Maclaurin series expansion:

From Series of Power over Factorial Converges, it follows that this series is convergent for all $x$.

Also see

 * Power Series Expansion for Hyperbolic Sine Function