Periodic sequence
In mathematics, a periodic sequence (sometimes called a cycle) is a sequence for which the same terms are repeated over and over:
The number p of repeated terms is called the period (period).
Definition
A periodic sequence is a sequence a1, a2, a3, ... satisfying
for all values of n. If we regard a sequence as a function whose domain is the set of natural numbers, then a periodic sequence is simply a special type of periodic function.
Examples
The sequence of digits in the decimal expansion of 1/7 is periodic with period six:
More generally, the sequence of digits in the decimal expansion of any rational number is eventually periodic (see below).
The sequence of powers of −1 is periodic with period two:
More generally, the sequence of powers of any root of unity is periodic. The same holds true for the powers of any element of finite order in a group.
A periodic point for a function ƒ: X → X is a point p whose orbit
is a periodic sequence. Periodic points are important in the theory of dynamical systems.
Every function from a finite set to itself has a periodic point; cycle detection is the algorithmic problem of finding such a point.