As long as everything in the definition for
defined someplace, everything is fine.
Here (for reference) is
Pyramid(1) = 1 Pyramid(N) = Pyramid(N-1) + Triangle(N)
And here (for review) is
Triangle( 1 ) = 1 Triangle( N ) = N + Triangle( N-1 )
Given these two definitions (and, if you are picky, the definitions of
addition and subtraction),
Pyramid() is completely defined.