Project Euler - Problem 21

問題

• 原文

  Evaluate the sum of all the amicable numbers under 10000.

• 日本語訳

  10000未満の友愛数の合計を求めよ。

解答

ほとんど書き下しただけです。注意点は完全数を計算に含めないことくらいでしょうか。

(use srfi-1)

(define (divisors n)
  (define (divisor? m) (zero? (remainder n m)))
  (let loop ((i 2) (early '()) (later '()))
    (if (> (* i i) n) (cons 1 (append-reverse early later))
        (cond
         ((= (* i i) n) (loop (+ i 1) (cons i early) later))
         ((divisor? i) (loop (+ i 1) (cons i early) (cons (/ n i) later)))
         (else (loop (+ i 1) early later))))))

(define (d n) (apply + (divisors n)))

(define (amicable-number? n)
  (define m (d n))
  (and (not (= n m)) (= n (d m))))

(define (solve)
  (apply + (filter amicable-number? (iota (- 10000 1) 1))))

(define (main argv)
  (display (solve))
  (newline))