Divisors of a natural number

Remarks

The set $D(n)$ is finite, because each $d \in D(n)$ satisfies $1 \le d \le n$. It always contains $1$ and $n$. For example, $D(12) = \{1, 2, 3, 4, 6, 12\}$. The map $d \mapsto n/d$ is a bijection from $D(n)$ to itself. Sums over $D(n)$ define the divisor function and the Dirichlet convolution.