What Is a Proper Divisor?

A proper divisor of a positive integer $n$ is a divisor of $n$ other than $n$ itself.

Examples:

• Divisors of 6: $1, 2, 3, 6$
• Proper divisors of 6: $1, 2, 3$

Notes:

• Every number $n > 1$ has at least one proper divisor (1).
• Prime numbers have exactly one proper divisor (1).