Fourier Series for Even Function over Symmetric Range
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Theorem
Let $\map f x$ be an even real function defined on the interval $\openint {-\lambda} \lambda$.
Then the Fourier series of $\map f x$ can be expressed as:
- $\map f x \sim \dfrac {a_0} 2 + \ds \sum_{n \mathop = 1}^\infty a_n \cos \frac {n \pi x} \lambda$
where for all $n \in \Z_{\ge 0}$:
- $a_n = \dfrac 2 \lambda \ds \int_0^\lambda \map f x \cos \frac {n \pi x} \lambda \rd x$
Proof
By definition of the Fourier series for $f$:
- $\map f x \sim \dfrac {a_0} 2 + \ds \sum_{n \mathop = 1}^\infty \paren {a_n \cos \frac {n \pi x} \lambda + b_n \sin \frac {n \pi x} \lambda}$
From Fourier Cosine Coefficients for Even Function over Symmetric Range:
- $a_n = \ds \dfrac 2 \lambda \int_0^\lambda \map f x \cos \frac {n \pi x} \lambda \rd x$
for all $n \in \Z_{\ge 0}$.
From Fourier Sine Coefficients for Even Function over Symmetric Range
- $b_n = 0$
for all $n \in \Z_{\ge 0}$:
$\blacksquare$
Also see
Sources
- 1961: I.N. Sneddon: Fourier Series ... (previous) ... (next): Chapter One: $\S 4$. Even and Odd Functions: $(6)$