# Half-Range Fourier Cosine Series over Negative Range

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## Theorem

Let $\map f x$ be a real function defined on the closed real interval $\closedint 0 l$.

Let $f$ be expressed using the half-range Fourier cosine series over $\closedint 0 l$:

- $\displaystyle \map f x \sim \frac {a_0} 2 + \sum_{n \mathop = 1}^\infty a_n \cos \frac {n \pi x} l$

where:

- $a_n = \displaystyle \frac 2 l \int_0^l \map f x \cos \frac {n \pi x} l \rd x$

for all $n \in \Z_{\ge 0}$.

Then over the closed real interval $\closedint {-l} 0$, $\map C x$ takes the values:

- $\map S x = \map f {-x}$

That is, the real function expressed by the half-range Fourier cosine series over $\closedint 0 l$ is an even function over $\closedint {-l} l$.

## Proof

From Fourier Series for Even Function over Symmetric Range, $\map C x$ is the Fourier series of an even real function over the interval $\closedint 0 l$.

We have that $\map C x \sim \map f x$ over $\closedint 0 l$.

Thus over $\closedint {-l} 0$ it follows that $\map S x = \map f {-x}$.

$\blacksquare$

## Sources

- 1961: I.N. Sneddon:
*Fourier Series*... (previous) ... (next): Chapter One: $\S 6$. Half-Range Cosine Series