# Periodic function

A **periodic function** is a function that repeats its values at regular intervals, for example, the trigonometric functions, which repeat at intervals of 2π radians. Periodic functions are used throughout science to describe oscillations, waves, and other phenomena that exhibit periodicity. Any function that is not periodic is called **aperiodic**.

A function `f` is said to be **periodic** if, for some **nonzero** constant `P`, it is the case that

for all values of `x` in the domain. A nonzero constant P for which this is the case is called a **period** of the function. If there exists a least positive^{[1]} constant `P` with this property, it is called the **fundamental period** (also **primitive period**, **basic period**, or **prime period**.) Often, "the" period of a function is used to mean its fundamental period. A function with period `P` will repeat on intervals of length `P`, and these intervals are sometimes also referred to as **periods** of the function.

Geometrically, a periodic function can be defined as a function whose graph exhibits translational symmetry, i.e. a function `f` is periodic with period `P` if the graph of `f` is invariant under translation in the `x`-direction by a distance of `P`. This definition of periodicity can be extended to other geometric shapes and patterns, as well as be generalized to higher dimensions, such as periodic tessellations of the plane. A sequence can also be viewed as a function defined on the natural numbers, and for a periodic sequence these notions are defined accordingly.

Everyday examples are seen when the variable is *time*; for instance the hands of a clock or the phases of the moon show periodic behaviour. **Periodic motion** is motion in which the position(s) of the system are expressible as periodic functions, all with the *same* period.

For a function on the real numbers or on the integers, that means that the entire graph can be formed from copies of one particular portion, repeated at regular intervals.

The trigonometric functions sine and cosine are common periodic functions, with period 2π (see the figure on the right). The subject of Fourier series investigates the idea that an 'arbitrary' periodic function is a sum of trigonometric functions with matching periods.

According to the definition above, some exotic functions, for example the Dirichlet function, are also periodic; in the case of Dirichlet function, any nonzero rational number is a period.

Since the cosine and sine functions are both periodic with period 2π, the complex exponential is made up of cosine and sine waves. This means that Euler's formula (above) has the property such that if *L* is the period of the function, then

A function whose domain is the complex numbers can have two incommensurate periods without being constant. The elliptic functions are such functions. ("Incommensurate" in this context means not real multiples of each other.)

Any function that consists only of periodic functions with the same period is also periodic (with period equal or smaller), including:

One common subset of periodic functions is that of **antiperiodic functions**. This is a function *f* such that *f*(*x* + *P*) = −*f*(*x*) for all *x*. (Thus, a *P*-antiperiodic function is a 2*P*-periodic function.) For example, the sine and cosine functions are π-antiperiodic and 2π-periodic. While a *P*-antiperiodic function is a 2*P*-periodic function, the converse is not necessarily true.

A further generalization appears in the context of Bloch's theorems and Floquet theory, which govern the solution of various periodic differential equations. In this context, the solution (in one dimension) is typically a function of the form:

where *k* is a real or complex number (the *Bloch wavevector* or *Floquet exponent*). Functions of this form are sometimes called **Bloch-periodic** in this context. A periodic function is the special case *k* = 0, and an antiperiodic function is the special case *k* = π/*P*.

In signal processing you encounter the problem, that Fourier series represent periodic functions and that Fourier series satisfy convolution theorems (i.e. convolution of Fourier series corresponds to multiplication of represented periodic function and vice versa), but periodic functions cannot be convolved with the usual definition, since the involved integrals diverge. A possible way out is to define a periodic function on a bounded but periodic domain. To this end you can use the notion of a quotient space:

Consider a real waveform consisting of superimposed frequencies, expressed in a set as ratios to a fundamental frequency, f: F = 1⁄f [f_{1} f_{2} f_{3} … f_{N}] where all non-zero elements ≥1 and at least one of the elements of the set is 1. To find the period, T, first find the least common denominator of all the elements in the set. Period can be found as T = LCD⁄f. Consider that for a simple sinusoid, T = 1⁄f. Therefore, the LCD can be seen as a periodicity multiplier.

If no least common denominator exists, for instance if one of the above elements were irrational, then the wave would not be periodic.^{[2]}