Polygonal Number

Any number made by the dots in the shape of a regular polygon.

There are:
• Triangular numbers {1, 3, 6, 10, 15, 21, ...} shown here
• Square numbers {1, 4, 9, 16, 25, ...}
• Pentagonal numbers {1, 5, 12, 22, 35, ...} shown below
• etc.