## 6 June 2017

### Nine

$9$ is a composite number.

$9$ is a perfect square: $9=3^2$.

An eneagon is a polygon of $9$ sides.

A number is divisible by $9$ if the sum of its digits is a number divisible by $9$: $4 \, 158$ is divisible by $9$ because $4+1+5+8=18$ and $18$ is divisible by $9$.