# Power Series Expansion for Hyperbolic Cosine Function

## Theorem

 $\displaystyle \cosh x$ $=$ $\displaystyle \sum_{n \mathop = 0}^\infty \frac {x^{2 n} } {\left({2 n}\right)!}$ $\displaystyle$ $=$ $\displaystyle 1 + \frac {x^2} {2!} + \frac {x^4} {4!} + \frac {x^6} {6!} + \cdots$

valid for all $x \in \R$.

## Proof

$\dfrac \d {\d x} \cosh x = \sinh x$
$\dfrac \d {\d x} \sinh x = \cosh x$

Hence:

$\dfrac {\d^2} {\d x^2} \cosh x = \cosh x$

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

 $\displaystyle m = 2 k: \ \$ $\displaystyle \dfrac {\d^m} {\d x^m} \cosh x$ $=$ $\displaystyle \cosh x$ $\displaystyle m = 2 k + 1: \ \$ $\displaystyle \dfrac {\d^m} {\d x^m} \cosh x$ $=$ $\displaystyle \sinh x$

where $k \in \Z$.

This leads to the Maclaurin series expansion:

 $\displaystyle \cosh x$ $=$ $\displaystyle \sum_{r \mathop = 0}^\infty \left({\frac {x^{2 k} } {\left({2 k}\right)!} \cosh \left({0}\right) + \frac {x^{2 k + 1} } {\left({2 k + 1}\right)!} \sinh \left({0}\right)}\right)$ $\displaystyle$ $=$ $\displaystyle \sum_{r \mathop = 0}^\infty \frac {x^{2 k} } {\left({2 k}\right)!}$ $\sinh \left({0}\right) = 0$, $\cosh \left({0}\right) = 1$

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

$\blacksquare$