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 A051279 Numbers n such that n = k/d(k) has exactly 2 solutions, where d(k) = number of divisors of k. 13
 1, 2, 5, 7, 8, 11, 13, 16, 17, 19, 23, 24, 28, 29, 31, 37, 41, 43, 44, 47, 48, 52, 53, 56, 59, 61, 67, 68, 71, 73, 76, 79, 80, 81, 83, 84, 88, 89, 92, 97, 101, 103, 104, 107, 109, 113, 116, 120, 124, 127, 131, 132, 136, 137, 139, 148, 149, 151, 152, 154, 156 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Because d(k) <= 2*sqrt(k), it suffices to check k from 1 to 4*n^2. - Nathaniel Johnston, May 04 2011 A051521(a(n)) = 2. - Reinhard Zumkeller, Dec 28 2011 LINKS Nathaniel Johnston and T. D. Noe, Table of n, a(n) for n = 1..1000 (first 150 terms from Nathaniel Johnston) EXAMPLE There are exactly 2 numbers k, 40 and 60, with k/d(k)=5. MAPLE with(numtheory): A051279 := proc(n) local ct, k: ct:=0: for k from 1 to 4*n^2 do if(n=k/tau(k))then ct:=ct+1: fi: od: if(ct=2)then return n: else return NULL: fi: end: seq(A051279(n), n=1..40); # Nathaniel Johnston, May 04 2011 MATHEMATICA A051279 = Reap[Do[ct = 0; For[k = 1, k <= 4*n^2, k++, If[n == k/DivisorSigma[0, k], ct++]]; If[ct == 2, Print[n]; Sow[n]], {n, 1, 160}]][[2, 1]](* Jean-François Alcover, Apr 16 2012, after Nathaniel Johnston *) PROG (Haskell) a051279 n = a051279_list !! (n-1) a051279_list = filter ((== 2) . a051521) [1..] -- Reinhard Zumkeller, Dec 28 2011 CROSSREFS Cf. A033950, A036763, A051278, A051280, A051346. Sequence in context: A047268 A039580 A189296 * A288464 A111199 A276545 Adjacent sequences:  A051276 A051277 A051278 * A051280 A051281 A051282 KEYWORD nonn,easy,nice AUTHOR STATUS approved

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Last modified September 20 20:02 EDT 2020. Contains 337265 sequences. (Running on oeis4.)