1
Now for a little puzzle in triangular numbers.
2
Every whole number is either triangular , or the sum of two triangular numbers or the sum of three triangular numbers.
3
The analogous series for triangular numbers is 1, 3, 6, 1, 6, 3, 1, 9, 9.
4
But indeed unity is triangular ; for every triangular number ( Triangular numbers are those of which equilateral triangles can be formed in this way: