Question

Which numbers are divisible by 9

Answer 1

A number is divisible by 9, if the sum is a multiple of 9 or if the sum of its digits is divisible by 9.

Answer 2

Any real number is divisible by 9. If you want integers divisible by 9, perfectly, then they have to follow this simple rule:
$9n \geq 9$

where:
$n \ \textgreater \ 1$