SICP Exercise 1.10.

Ackermann's function

(define (A x y)
  (cond ((= y 0) 0)
        ((= x 0) (* 2 y))
        ((= y 1) 2)
        (else (A (- x 1)
                 (A x (- y 1))))))

> (A 1 10)
1024
> (A 2 4)
65536
> (A 3 3)
65536