40－32÷2＝？小学生「4!」理系「よくわかってんじゃん」文系「やっぱわかんないか～ｗ」

kaijou.scm

;; a - b ÷ c = d! を満たす自然数の組 (a,b,c,d) ≦1億 を探す
(define MX 1e8)

(let ld ((d 3) (d- 2)) ; d- = (d-1)!
  (when (<= (* d d-) MX)
    (let lc ((c 2)) ;; c>1
      (when (< c d-) ;; b>0
        (receive (b r) (quotient&remainder
                        (* c d (- d- c))
                        (- c 1))
          (when (zero? r)
            (let1 a (+ b (* c d))
              (when (<= a MX)
                (format #t "‾d - ‾d ÷ ‾d = ‾d!¥n" a b c d))))
          (lc (+ c 1)))))
    (ld (+ d 1) (* d- d))))

kaijou2.scm

;; a - b ÷ c = d! を満たす自然数の組 (a,b,c,d) ≦1億 と
;; a + b ÷ c = d! を満たす自然数の組 (a,b,c,d) ≦1億 を探す
(define MX 1e8)

(let ld ((d 3) (d- 2)) ; d- = (d-1)!
  (when (<= (* d d-) MX)
    (let lc ((c 2)) ;; c>1
      ;; (when (< c d-) ;; b>0
      (when (<= c (/ MX d)) ;; a = b+cd <= MX; cd <= MX-b
        (receive (q r) (quotient&remainder
                        (* d (- d- c))
                        (- c 1))
          (when (and (not (zero? q)) (zero? r))
            (let* ([b (* c q)]
                   [a (+ b (* c d))])
              (when (<= a MX)
                (if (< b 0)
                    (format #t "‾d + ‾d ÷ ‾d = ‾d!¥n" a (- b) c d)
                    (format #t "‾d - ‾d ÷ ‾d = ‾d!¥n" a b c d)))))
          (lc (+ c 1)))))
    (ld (+ d 1) (* d- d))))