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
T. D. Noe, Table of n, a(n) for n=1..1000
EXAMPLE
36 is the unique number k with k/d(k)=4.
MAPLE
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
KEYWORD
nonn,easy,nice
AUTHOR
STATUS
approved