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Primitive root modulo n

Primitive roots modulo n are a concept in number theory. If n is an integer, the numbers coprime to n, taken modulo n, form a group with multiplication as operation; it is written as (Z/nZ)×. This group is cyclic if and only if n is equal to 2 or 4 or pk or 2 pk for an odd prime number p and k ≥ 1. A generator of this cyclic group is called a primitive root modulo n.

Take for example n = 14. The elements of (Z/14Z)× are the congruence classes of 1, 3, 5, 9, 11 and 13. Then 3 is a primitive root modulo 14, as we have 32 = 9, 33 = 13, 34 = 11, 35 = 5 and 36 = 1 (modulo 14). The only other primitive root modulo 14 is 5.

Here is a table containing the smallest primitive root for various values of n (see A046145 (http://www.research.att.com/cgi-bin/access.cgi/as/njas/sequences/eisA.cgi?Anum=A046145)):

n primitive root mod n
21
32
43
52
65
73
8-
92
103
112
12-
132
143

No simple general formula to compute primitive roots modulo n is known. There are however methods to locate a primitive root that are faster than simply trying out all candidates: first compute φ(n), the order of (Z/nZ)×. Then determine the different prime factors of φ(n), say p1,...,pk. Now, for every element m of (Z/nZ)×, compute

<math>m^{\phi(n)/p_i}\mod n \qquad\mbox{ for } i=1,\ldots,k</math>
using the fast exponentiating by squaring. As soon as you find a number m for which these k results are all different from 1, you stop: m is a primitive root.

The number of primitive roots modulo n is equal to φ(φ(n)) since, in general, a cyclic group with r elements has φ(r) generators.

There exist positive constants C, ε and p0 such that, for every prime pp0, there exists a primitive root modulo p that is less than C p1/4+ε. If the generalized Riemann hypothesis is true, then for every prime number p, there exists a primitive root modulo p that is less than 70 (ln(p))2.



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