some solutions for MIT Press's "structure and interpretation of computer programs", I guess
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(define (square x)
(* x x))
(define (smallest-divisor n)
(find-divisor n 2))
(define (find-divisor n test-divisor)
(cond ((> (square test-divisor) n) n)
((divides? test-divisor n) test-divisor)
(else (find-divisor n (+ test-divisor 1)))))
(define (divides? a b)
(= (remainder b a) 0))
(define (prime? n)
(= n (smallest-divisor n)))
(define (exptmod base expn m)
(cond ((= expn 0) 1)
((even? expn)
(square (exptmod base (/ expn 2) m))
(* base (exptmod base (- expn 1) m))
(define (fermat-test n)
(define (try-it a)
(= (exptmod a n n) a))
(try-it (+ 1 (random (- n 1)))))
(define (fast-prime? n times)
(cond ((= times 0) #t)
((fermat-test n) (fast-prime? n (- times 1)))
(else #f)))