# 4. Half Range Fourier Series

If a function is defined over half the range, say `0` to
*L*, instead of the full range from `-L` to `L`,
it may be expanded in a series of sine terms only or of cosine
terms only. The series produced is then called a **half range
Fourier series**.

Conversely, the Fourier Series of an even or odd function can be analysed using the half range definition.

## Even Function and Half Range Cosine Series

An **even** function can be expanded using half its range
from

- `0` to
*L*or - `-L` to `0` or
- `L` to `2L`

That is, the range of integration is *L*. The Fourier
series of the **half range even function** is given by:

`f(t)=a_0/2+sum_(n=1)^oo\ a_n\ cos (n pi t)/L`

for `n = 1, 2, 3, ...` , where

`a_0=2/Lint_0^Lf(t)dt`

`a_n=2/Lint_0^Lf(t)\ cos (n pi t)/Ldt`

and `b_n= 0`.

### Illustration

In the figure below, `f(t) = t` is sketched from `t = 0` to `t = π`.

An even function means that it must be symmetrical about the `f(t)` axis and this is shown in the following figure by the broken line between `t = -π` and `t = 0`.

It is then assumed that the "triangular wave form" produced is periodic with period `2pi` outside of this range as shown by the dotted lines.

### Example

We are given that

`f(t)={: {(-t,if, -pi<=t<0),(t,if, 0<=t < pi):}`

and `f(t)` is periodic with period `2π`.

a) Sketch the function for 3 cycles.

b) Find the Fourier trigonometric series for `f(t)`, using half-range series.

## Odd Function and Half Range Sine Series

An odd function can be expanded using half its range from `0` to
*L*, i.e. the range of integration has value *L*. The Fourier
series of the odd function is:

Since *a*_{o} = 0 and *a*_{n} = 0, we have:

`f(t)=sum_(n=1)^oo\ b_n\ sin (n pi t)/L` for

n= 1, 2, 3,...

`b_n=2/Lint_0^Lf(t)\ sin (n pi t)/Ldt`

In the figure below, *f*(*t*) = *t* is sketched
from *t* = 0 to *t* = *π*, as before.

An **odd** function means that it is symmetrical about the
origin and this is shown by the red broken lines between *t* = −*π* and `t = 0`.

It is then assumed that the waveform produced is periodic of
period 2*π* outside of this range as shown by the dotted lines.

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