What is a primitive root of a prime number?

Question

Is there any good site reference I can refer to?

Answer 1

If p is a prime number and if a is any number not divisible by p, then a is a primitive root of p if and only if the smallest positive integer t for which a^t = 1 (mod p) is t = p - 1.

Here is an example. For the prime 5, 4^2 = 1 (mod 5), so 4 is not a primitive root of 5. However, 3^1 = 3 (mod 5),
3^2 = 4 (mod 5), 3^3 = 2 (mod 5), and 3^4 = 1 (mod 5); therefore, 3 is a primitive root of 5.