# Definition:Fourier Series

## Contents

## Definition

### Formulation 1

Let $\alpha \in \R$ be a real number.

Let $\lambda \in \R_{>0}$ be a strictly positive real number.

Let $f: \R \to \R$ be a function such that $\displaystyle \int_\alpha^{\alpha + 2 \lambda} \map f x \rd x$ converges absolutely.

Let:

\(\displaystyle a_n\) | \(=\) | \(\displaystyle \dfrac 1 \lambda \int_\alpha^{\alpha + 2 \lambda} \map f x \cos \frac {n \pi x} \lambda \rd x\) | |||||||||||

\(\displaystyle b_n\) | \(=\) | \(\displaystyle \dfrac 1 \lambda \int_\alpha^{\alpha + 2 \lambda} \map f x \sin \frac {n \pi x} \lambda \rd x\) |

Then:

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

is the **Fourier Series** for $f$.

### Formulation 2

Let $a, b \in \R$ be real numbers.

Let $f: \R \to \R$ be a function such that $\displaystyle \int_a^b \map f x \rd x$ converges absolutely.

Let:

\(\displaystyle A_m\) | \(=\) | \(\displaystyle \dfrac 2 {b - a} \int_a^b \map f x \cos \frac {2 m \pi \paren {x - a} } {b - a} \rd x\) | |||||||||||

\(\displaystyle B_m\) | \(=\) | \(\displaystyle \dfrac 2 {b - a} \int_a^b \map f x \sin \frac {2 m \pi \paren {x - a} } {b - a} \rd x\) |

Then:

- $\displaystyle \frac {A_0} 2 + \sum_{m \mathop = 1}^\infty \paren {A_m \cos \frac {2 m \pi \paren {x - a} } {b - a} + B_m \sin \frac {2 m \pi \paren {x - a} } {b - a} }$

is the **Fourier Series** for $f$.

### Fourier Coefficient

The constants:

- $a_0, a_1, a_2, \ldots, a_n, \ldots; b_1, b_2, \ldots, b_n, \ldots$

are the **Fourier coefficients** of $f$.

## Fourier Series on Range of $2 \pi$

Let $\alpha \in \R$ be a real number.

Let $f: \R \to \R$ be a function such that $\displaystyle \int_\alpha^{\alpha + 2 \pi} \map f x \rd x$ converges absolutely.

Let:

\(\displaystyle a_n\) | \(=\) | \(\displaystyle \dfrac 1 \pi \int_\alpha^{\alpha + 2 \pi} \map f x \cos n x \rd x\) | |||||||||||

\(\displaystyle b_n\) | \(=\) | \(\displaystyle \dfrac 1 \pi \int_\alpha^{\alpha + 2 \pi} \map f x \sin n x \rd x\) |

Then:

- $\dfrac {a_0} 2 + \displaystyle \sum_{n \mathop = 1}^\infty \paren {a_n \cos n x + b_n \sin n x}$

is called the **Fourier Series** for $f$.

## Also known as

The **Fourier series**, as defined on the general open interval $\openint \alpha {\alpha + 2 \lambda}$, can often be seen referred to as the **whole-range Fourier series**.

This is in order to distinguish it from the $2$ varieties of **half-range Fourier series**, which are applied to the open interval $\openint 0 \lambda$.

## Also defined as

The form given here is more general than that usually given.

The usual form is one of the cases where $\alpha = 0$ or $\alpha = -\lambda$, thus giving a range of integration of either $\openint 0 {2 \lambda}$ or $\openint {-\lambda} \lambda$.

The actual range may often be chosen for convenience of analysis.

## Also see

- Coefficients of Cosine Terms in Convergent Trigonometric Series
- Coefficients of Sine Terms in Convergent Trigonometric Series

- Results about
**Fourier series**can be found here.

## Source of Name

This entry was named for Joseph Fourier.

## Historical Note

Despite the fact that the Fourier series bears the name of Joseph Fourier, they were first studied by Leonhard Paul Euler.

Fourier himself made considerable use of this series during the course of his analysis of the behaviour of heat.

The first person to feel the need for a careful study of its convergence was Augustin Louis Cauchy.

In $1829$, Johann Peter Gustav Lejeune Dirichlet gave the first satisfactory proof about the sums of Fourier series for certain types of function.

The criteria set by Dirichlet were extended and generalized by Riemann in his $1854$ paper *Ueber die Darstellbarkeit einer Function durch eine trigonometrische Reihe*.