Fourier Cosine Coefficients for Even Function over Symmetric Range

Theorem
Let $\map f x$ be an even real function defined on the interval $\openint {-\lambda} \lambda$.

Let the Fourier series of $\map f x$ be expressed as:


 * $\map f x \sim \dfrac {a_0} 2 + \displaystyle \sum_{n \mathop = 1}^\infty \paren {a_n \cos \frac {n \pi x} \lambda + b_n \sin \frac {n \pi x} \lambda}$

Then for all $n \in \Z_{\ge 0}$:
 * $a_n = \dfrac 2 \lambda \displaystyle \int_0^\lambda \map f x \cos \frac {n \pi x} \lambda \rd x$

Proof
As suggested, let the Fourier series of $\map f x$ be expressed as:


 * $\map f x \sim \dfrac {a_0} 2 + \displaystyle \sum_{n \mathop = 1}^\infty \paren {a_n \cos \frac {n \pi x} \lambda + b_n \sin \frac {n \pi x} \lambda}$

By definition of Fourier series:


 * $a_n = \dfrac 1 \lambda \displaystyle \int_{-\lambda}^{-\lambda + 2 \lambda} \map f x \cos \frac {n \pi x} \lambda \rd x$

From Cosine Function is Even:
 * $\cos a = \map \cos {-a}$

for all $a$.

By Even Function Times Even Function is Even, $\map f x \cos \dfrac {n \pi x} \lambda$ 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