Fourier Sine 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:


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

Then for all $n \in \Z_{> 0}$:
 * $b_n = 0$

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


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

By definition of Fourier series:

From Sine Function is Odd:
 * $\sin a = -\map \sin {-a}$

for all $a$.

By Odd Function Times Even Function is Odd, $\map f x \sin \dfrac {n \pi x} l$ is odd.

The result follows from the corollary to Definite Integral of Odd Function:
 * $\displaystyle \frac 1 \lambda \int_{-\lambda}^\lambda \map f x \sin \frac {n \pi x} \lambda \rd x = 0$

Also see

 * Fourier Cosine Coefficients for Even Function over Symmetric Range


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