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A051278 Numbers n such that n = k/d(k) has a unique solution, where d(k) = number of divisors of k. 13
4, 6, 9, 10, 12, 14, 15, 20, 21, 22, 26, 32, 33, 34, 35, 36, 38, 39, 42, 46, 50, 51, 55, 57, 58, 60, 62, 65, 66, 69, 70, 74, 75, 77, 78, 82, 85, 86, 87, 90, 91, 93, 94, 95, 96, 98, 100, 102, 106, 108, 110, 111, 114, 115, 118, 119, 122, 123, 126, 128, 129, 130 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
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)) = 1. - Reinhard Zumkeller, Dec 28 2011
LINKS
EXAMPLE
36 is the unique number k with k/d(k)=4.
MAPLE
with(numtheory): A051278 := 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=1)then return n: else return NULL: fi: end: seq(A051278(n), n=1..40);
MATHEMATICA
cnt[n_] := Count[Table[n == k/DivisorSigma[0, k], {k, 1, 4*n^2}], True]; Select[Range[130], cnt[#] == 1 &] (* Jean-François Alcover, Oct 22 2012 *)
PROG
(Haskell)
a051278 n = a051278_list !! (n-1)
a051278_list = filter ((== 1) . a051521) [1..]
-- Reinhard Zumkeller, Dec 28 2011
CROSSREFS
Sequence in context: A319240 A331051 A325270 * A328028 A366318 A339424
KEYWORD
nonn,easy,nice
AUTHOR
STATUS
approved

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Last modified March 19 03:33 EDT 2024. Contains 370952 sequences. (Running on oeis4.)