Cardinality of Reduced Residue System
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Theorem
Let $n \ge 2$.
Let $\Z'_n$ be the reduced residue system modulo $n$.
Then:
- $\card {\Z'_n} = \map \phi n$
where $\map \phi n$ is the Euler phi function.
Proof
Recall the definition of $\Z'_n$:
- $\Z'_n = \set {\eqclass k n \in \Z_n: k \perp n}$
and the definition of $\map \phi n$:
- $\map \phi n = \card {\set {k: 1 \le k \le n, k \perp n} }$
The result follows from Integer is Congruent to Integer less than Modulus.
$\blacksquare$
Sources
- 1971: George E. Andrews: Number Theory ... (previous) ... (next): $\text {4-2}$ Residue Systems: Theorem $\text {4-5}$