Exercise 1.11 in SICP. First attempt is linear recursive.

exercise-1-11.scm

;; Define a function f such that:
;; f(n) = n if n<3 and f(n) = f(n - 1) + 2f(n - 2) + 3f(n - 3) if n> 3
;; This is the recursive implementation that doesn't scale well
(define (f n)
  (if (< n 3) 
    n 
    (+ (f (- n 1)) 
       (* 2 (f (- n 2))) 
       (* 3 (f (- n 3))))
  )
) 

;; Example call that will take some time to complete
(f 18)