The base case is when a rock fragment is small enough. No more work needs to be done.

Here are the two parts to recursion:

- If the problem is easy, solve it immediately.
- An easy problem is a
**base case**.

- An easy problem is a
- 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.

And here is how this applies to triangle numbers:

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

The problem "`Triangle(N)`

" is divided into two
problems: "add N to something"
and `Triangle(N-1)`

".