In mathematics, a periodic function is a function that repeats its values, after adding some definite period to the variable. 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. More explicitly, a function f is periodic with period t if
- f(x + t) = f(x)
for all values of x in the domain of f.
A simple example is the function f that gives the "fractional part" of its argument:
- f( 0.5 ) = f( 1.5 ) = f( 2.5 ) = ... = 0.5.
If a function f is periodic with period t then for all x in the domain of f and all integers n,
- f( x + nt ) = f ( x ).
In the above example, the value of t is 1, since f( x ) = f( x + 1 ) = f( x + 2 ) ...
Sine and cosine are periodic functions, with period 2π. The subject of Fourier series investigates the idea that an 'arbitrary' periodic function is a sum of trigometric functions with matching periods.
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.)
- for all x in E, f(x+T) = f(x).
Note that unless + is assumed commutative this definition depends on writing T on the right.
Some naturally-occurring sequences are periodic, for example (eventually) the decimal expansion of any rational number (see recurring decimal). We can therefore speak of the period or period length of a sequence. This is (if one insists) just a special case of the general definition.