exercise_1_27.scm

(define (expmod base exp m)
  (cond ((= exp 0) 1)
	((even? exp)
	 (remainder (square (expmod base (/ exp 2) m))
		    m))
	(else (remainder (* base (expmod base (- exp 1) m))
			 m))))

(define (fermat-test n)
  (define (try-it a)
    (= (expmod a n n) a))
  (try-it (+ 1 (random (- n 1)))))

(define (fast-prime? n times)
  (cond ((= times 0) true)
	((fermat-test n) (fast-prime? n (- times 1)))
	(else false)))

;; (fermat-test 561)
;; (fermat-test 1105)
;; (fermat-test 1729)
;; (fermat-test 2465)
;; (fermat-test 2821)
;; (fermat-test 6601)