Fourier Cosine Coefficients for Even Function over Symmetric Range

Theorem
Let $f \left({x}\right)$ be an even real function defined on the interval $\left[{-l \,.\,.\, l}\right]$.

Let the Fourier series of $f \left({x}\right)$ be expressed as:


 * $\displaystyle f \left({x}\right) \sim \frac {a_0} 2 + \sum_{n \mathop = 1}^\infty \left({a_n \cos n \pi x + b_n \sin n \pi x}\right)$

Then for all $n \in \Z_{\ge 0}$:
 * $a_n = \displaystyle \frac 2 l \int_0^l f \left({x}\right) \cos \frac {n \pi x} l \, \mathrm d x$

Proof
As suggested, let the Fourier series of $f \left({x}\right)$ be expressed as:


 * $\displaystyle f \left({x}\right) \sim \frac {a_0} 2 + \sum_{n \mathop = 1}^\infty \left({a_n \cos n \pi x + b_n \sin n \pi x}\right)$

By definition of Fourier series:


 * $a_n = \displaystyle \frac 1 l \int_{-l}^{-l + 2 l} f \left({x}\right) \cos \frac {n \pi x} l \, \mathrm d x$

From Cosine Function is Even:
 * $\cos a = \cos \left({-a}\right)$

for all $a$.

By Even Function Times Even Function is Even, $f \left({x}\right) \cos \frac {n \pi x} l$ is even.

Thus:

Also see

 * Fourier Sine Coefficients for Even Function over Symmetric Range


 * Fourier Cosine Coefficients for Odd Function over Symmetric Range
 * Fourier Sine Coefficients for Odd Function over Symmetric Range