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A346526
Positive integers k that are the product of two integers greater than 1 and ending with the same digit as k.
1
25, 36, 75, 96, 100, 121, 125, 156, 175, 200, 216, 225, 231, 256, 275, 276, 300, 325, 336, 341, 375, 396, 400, 416, 425, 441, 451, 456, 475, 500, 516, 525, 561, 575, 576, 600, 625, 636, 651, 671, 675, 676, 696, 700, 725, 736, 756, 775, 781, 800, 816, 825, 861, 875
OFFSET
1,1
COMMENTS
Union of 100*A000027, A053742, A324297 and A346507.
FORMULA
Lim_{n->infinity} a(n)/a(n-1) = 1.
EXAMPLE
25 = 5*5, 36 = 6*6, 75 = 5*15, 96 = 6*16, 100 = 10*10, 121 = 11*11, 125 = 5*25, 156 = 6*26, 175 = 5*35, 200 = 10*20, 216 = 6*36, 225 = 15*15, 231 = 11*21, ...
PROG
(PARI) isok(k) = my(u=k%10); sumdiv(k, d, (d>1) && (d<k) && ((d%10)==u) && ((k/d % 10) == u)) > 0; \\ Michel Marcus, Jul 23 2021
(Lisp)
(setf candidates (list 25)) (setf result nil)
(defun factor (num small-num) (equalp 0 (mod num small-num)))
(defun same-end-digit (num1 num2 num3) (and (equalp (mod num1 10) (mod num2 10)) (equalp (mod num2 10) (mod num3 10))))
(defun good-factor-p (num) (loop for i from 5 to (sqrt num) do ( if (factor num i) ( if (same-end-digit num i (/ num i) ) (return T) ))))
(loop for i from 26 to 9000 do ( if (or (equalp 0 (mod i 10)) (equalp 1 (mod i 10)) (equalp 5 (mod i 10)) (equalp 6 (mod i 10))) (push i candidates)))
(dolist (element candidates) (if (good-factor-p element) (push element result)))
(format t (write-to-string result)) \\ FUNG Cheok Yin, Aug 12 2021
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Stefano Spezia, Jul 22 2021
STATUS
approved