Fourier Sine Coefficients for Odd Function over Symmetric Range

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

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


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

Then for all $n \in \Z_{> 0}$:
 * $b_n = \ds \frac 2 \lambda \int_0^\lambda \map f x \sin \frac {n \pi x} \lambda \rd x$

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


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

By definition of Fourier series:


 * $b_n = \ds \frac 1 \lambda \int_{-\lambda}^{-\lambda + 2 \lambda} \map f x \sin \frac {n \pi x} \lambda \rd x$

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

for all $a$.

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

Thus:

Also see

 * Fourier Cosine Coefficients for Odd Function over Symmetric Range


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