The base case of the definition.

A recursive definition (or a recursive algorithm) needs two parts:

- If the problem is easy, solve it immediately.
- If the problem can't be solved immediately,
divide it into smaller problems, then:
- Solve the smaller problems by applying this procedure to each of them.

In terms of pyramidal numbers, this is:

Pyramid(1) = 1 Pyramid(N) = Pyramid(N-1) + Triangle(N)

Oddly, the base case for pyramidal numbers is the peak of the pyramid.