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A051279
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n=k/d(k) has exactly 2 solutions, where d(k) = number of divisors of k.
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8
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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; internal format)
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OFFSET
| 1,2
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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
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LINKS
| Nathaniel Johnston, Table of n, a(n) for n = 1..150
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EXAMPLE
| There are exactly 2 numbers k, 40 and 60, with k/d(k)=5.
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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
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PROG
| (Haskell)
a051279 n = a051279_list !! (n-1)
a051279_list = filter ((== 2) . a051521) [1..]
-- Reinhard Zumkeller, Dec 28 2011
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CROSSREFS
| Cf. A033950, A036763, A051278, A051280, A051346.
Sequence in context: A047268 A039580 A189296 * A111199 A138671 A032724
Adjacent sequences: A051276 A051277 A051278 * A051280 A051281 A051282
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KEYWORD
| nonn,easy,nice
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), R. K. Guy, David W. Wilson (davidwwilson(AT)comcast.net)
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