Odd number
An odd number cannot be divided by two.
Explanation
The Greeks called odd numbers gnomons. A gnomon is a hook on a sundial.
11 • • • • • • 9 • • • • • • 7 • • • • • • 5 • • • • • • 3 • • • • • • 1 • • • • • •
In the hooks are an odd number of bullets. Every odd number is the difference of two squares, because 2n + 1 = (n + 1)2 − n2 and that gives
1 = 12 - 02
3 = 22 - 12
5 = 32 - 22
7 = 42 - 32
9 = 52 - 42
11 = 62 - 52