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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 T. D. Noe, Table of n, a(n) for n = 1..1000 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 Cf. A033950, A036763, A051278, A051279, A051346. Sequence in context: A042899 A266702 A264937 * A145609 A259923 A120285 Adjacent sequences:  A051277 A051278 A051279 * A051281 A051282 A051283 KEYWORD nonn,easy,nice AUTHOR STATUS approved

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Last modified September 20 19:48 EDT 2020. Contains 337265 sequences. (Running on oeis4.)