Power Series Expansion for Hyperbolic Cosine Function
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Theorem
The hyperbolic cosine function has the power series expansion:
\(\ds \cosh x\) | \(=\) | \(\ds \sum_{n \mathop = 0}^\infty \frac {x^{2 n} } {\paren {2 n}!}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds 1 + \frac {x^2} {2!} + \frac {x^4} {4!} + \frac {x^6} {6!} + \cdots\) |
valid for all $x \in \R$.
Proof
From Derivative of Hyperbolic Cosine:
- $\dfrac \d {\d x} \cosh x = \sinh x$
From Derivative of Hyperbolic Sine:
- $\dfrac \d {\d x} \sinh x = \cosh x$
Hence:
- $\dfrac {\d^2} {\d x^2} \cosh x = \cosh x$
and so for all $m \in \N$:
\(\ds m = 2 k: \ \ \) | \(\ds \dfrac {\d^m} {\d x^m} \cosh x\) | \(=\) | \(\ds \cosh x\) | |||||||||||
\(\ds m = 2 k + 1: \ \ \) | \(\ds \dfrac {\d^m} {\d x^m} \cosh x\) | \(=\) | \(\ds \sinh x\) |
where $k \in \Z$.
This leads to the Maclaurin series expansion:
\(\ds \cosh x\) | \(=\) | \(\ds \sum_{r \mathop = 0}^\infty \paren {\frac {x^{2 k} } {\paren {2 k}!} \map \cosh 0 + \frac {x^{2 k + 1} } {\paren {2 k + 1}!} \map \sinh 0}\) | ||||||||||||
\(\ds \) | \(=\) | \(\ds \sum_{r \mathop = 0}^\infty \frac {x^{2 k} } {\paren {2 k}!}\) | $\map \sinh 0 = 0$, $\map \cosh 0 = 1$ |
From Series of Power over Factorial Converges, it follows that this series is convergent for all $x$.
$\blacksquare$
Also see
- Power Series Expansion for Hyperbolic Sine Function
- Power Series Expansion for Hyperbolic Tangent Function
- Power Series Expansion for Hyperbolic Cotangent Function
- Power Series Expansion for Hyperbolic Secant Function
- Power Series Expansion for Hyperbolic Cosecant Function
Sources
- 1968: Murray R. Spiegel: Mathematical Handbook of Formulas and Tables ... (previous) ... (next): $\S 20$: Series for Hyperbolic Functions: $20.34$
- 1976: K. Weltner and W.J. Weber: Mathematics for Engineers and Scientists ... (previous) ... (next): $8$. Taylor Series and Power Series: Appendix: Table $8.2$: Power Series of Important Functions
- 1998: David Nelson: The Penguin Dictionary of Mathematics (2nd ed.) ... (previous) ... (next): hyperbolic functions
- 2008: David Nelson: The Penguin Dictionary of Mathematics (4th ed.) ... (previous) ... (next): hyperbolic functions