A happy numeric coincidence

Re:A happy numeric coincidence by kazinator on Wednesday January 02, 2019 @05:57PM (#18811162)

TXR lisp:

  This is the TXR Lisp interactive listener of TXR 203.
  Quit with :quit or Ctrl-D on empty line. Ctrl-X ? for cheatsheet.
  1> (each ((x (range 2 1000))) ;; 1 is uninteresting
       (each ((y (range 1 100)))
         (let ((z (expt x y)))
           (when (= x (sum (digits z)))
             (put-line `@x @y`)))))
  2 1
  3 1
  4 1
  5 1
  [...]                                                                
  963 69
  964 75
  964 78
  991 71
  999 75

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Re:A happy numeric coincidence by kazinator on Wednesday January 02, 2019 @06:16PM (#18811289)

Using successive multiplication instead of exponent operation:

  6> (each ((x (range 2 1000)))
       (each ((y (range 1 100)) ;; needed just for the put-line
              (z [giterate true (op * x) x]))
         (when (= x (sum (digits z)))
           (put-line `@x @y`))))

(Makes no difference; run time is vastly dominated by the contribution of the (sum (digits z)) business.)

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