(defun recurring-cycle (d)
  "Calculate the length of the recurring cycle for 1/d."
  (labels ((remainders (d r rs)
             (if (eql r 0)
                 0               
                 (let ((s (mod r d)))
                   (if (member s rs)
                       (1+ (position s rs))
                       (remainders d (* 10 s) (cons s rs)))))))
    (remainders d 1 nil)))

(defun p26 ()
  "Find the number below 1000 with the longest recurring cycle in its decimal representation."
  (let* ((results (loop :for n :from 1 :to 999 
                        collect (cons n (recurring-cycle n))))
         (max-pair (reduce #'(lambda (a b)
                               (if (> (cdr a) (cdr b))
                                   a
                                   b))
                           results)))
    (car max-pair)))

; SLIME 2.28
CL-USER> (p26)
983
CL-USER> 

— Me@2025-01-20 03:57:23 PM

.

.

2025.01.20 Monday (c) All rights reserved by ACHK