# Reduced residue system

Any subset R of the set of integers is called a reduced residue system modulo n if

1. gcd(r, n) = 1 for each r contained in R;
2. R contains φ(n) elements;
3. no two elements of R are congruent modulo n.[1][2]

Here $\varphi$ denotes Euler's totient function.

A reduced residue system modulo n can be formed from a complete residue system modulo n by removing all integers not relatively prime to n. For example, a complete residue system modulo 12 is {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11}. 1, 5, 7 and 11 are the only integers in this set which are relatively prime to 12, and so the corresponding reduced residue system modulo 12 is {1,5,7,11}. Note that the cardinality of this set is $\varphi(12) = 4$. Some other reduced residue systems modulo 12 are

• {13,17,19,23}
• {−11,−7,−5,−1}
• {−7,−13,13,31}
• {35,43,53,61}

## Facts

• If {r1, r2, ... , rφ(n)} is a reduced residue system with n > 2, then $\sum r_i \equiv 0 \pmod n$.
• If n is prime, then every number in a reduced residue system mod n (except for 1) is a generator for the multiplicative group of integers mod n.