- ak ≡ 1 (mod n).
The order of a modulo n is usually written ordn(a), or On(a).
To determine the multiplicative order of 4 modulo 7, we compute 42 = 16 ≡ 2 (mod 7) and 43 = 64 ≡ 1 (mod 7), so ord7(4) = 3.
Even without knowledge that we are working in the multiplicative group of integers modulo n, we can show that a actually has an order by noting that the powers of a can only take a finite number of different values modulo n, so according to the pigeonhole principle there must be two powers, say s and t and without loss of generality s>t, such that as ≡ at (mod n). Since a and n are coprime, this implies that a has an inverse element a-1 and we can multiply both sides of the congruence with a-t, yielding as-t ≡ 1 (mod n).
The concept of multiplicative order is a special case of the order of group elements. The multiplicative order of a number a modulo n is the order of a in the multiplicative group whose elements are the residues modulo n of the numbers coprime to n, and whose group operation is multiplication modulo n. This is the group of units of the ring Zn; it has φ(n) elements, φ being Euler's totient function, and is denoted as U(n) or U(Zn).
As a consequence of Lagrange's theorem, ordn(a) always divides φ(n). If ordn a is actually equal to φ(n) and therefore as large as possible, then a is called a primitive root modulo n. This means that the group U(n) is cyclic and the residue class of a generates it.