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A051280 Numbers n such that n = k/d(k) has exactly 3 solutions, where d(k) = number of divisors of k. 12
3, 25, 40, 49, 54, 121, 125, 135, 140, 169, 189, 216, 220, 250, 260, 289, 297, 340, 351, 361, 375, 380, 400, 459, 460, 500, 513, 529, 580, 620, 621, 675, 729, 740, 770, 783, 820, 837, 841, 860, 875, 882, 910, 940, 961, 999, 1060, 1107, 1152, 1161, 1180, 1188, 1190 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Many terms are of the form a(k) * p^m/(m+1), where p is coprime to the three solutions for k. The sequence of "primitive" terms (i.e. not expressible this way) begins 3, 40, 54, 125, 135, 216, 250.... Are there any such numbers that admit a fourth solution? - Charlie Neder, Feb 13 2019
LINKS
EXAMPLE
There are exactly 3 numbers k, 9, 18 and 24, with k/d(k) = 3.
MATHEMATICA
(Select[Table[k / Length @ Divisors[k], {k, 1, 200000}], IntegerQ] // Sort // Split // Select[#, Length[#] == 3 &] &)[[All, 1]][[1 ;; 53]] (* Jean-François Alcover, Apr 22 2011 *)
CROSSREFS
Sequence in context: A266702 A264937 A354725 * A145609 A259923 A120285
KEYWORD
nonn,easy,nice
AUTHOR
STATUS
approved

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Last modified March 18 22:56 EDT 2024. Contains 370952 sequences. (Running on oeis4.)