A143896 Numbers that are among the d smallest numbers with d divisors.

1, 2, 3, 4, 6, 8, 9, 10, 12, 14, 16, 18, 20, 24, 25, 28, 30, 32, 36, 40, 42, 44, 48, 54, 56, 60, 64, 66, 70, 72, 80, 81, 84, 90, 96, 100, 108, 112, 120, 126, 132, 140, 144, 150, 156, 160, 162, 168, 176, 180, 192, 196, 208, 210, 216, 225, 240, 252, 256, 264, 270, 272, 280

Offset

1,2

Links

Ivan Neretin, Table of n, a(n) for n = 1..8000

Example

15 does not qualify because it has 4 divisors and the 4 smallest numbers with 4 divisors are 6, 8, 10 and 14.

Mathematica

ds[n_] := ds[n] = DivisorSigma[0, n]; fQ[n_] := Block[{d = ds@n}, MemberQ[ Select[ Range@n, Take[ DivisorSigma[0, # ] == d &], d], n]]; Select[ Range@ 299, fQ@# &] (* Robert G. Wilson v, Sep 09 2008 *)

Crossrefs

Cf. A007416.

Sequence in context: A196277 A368385 A280196 * A160719 A379554 A263079

Adjacent sequences: A143893 A143894 A143895 * A143897 A143898 A143899

Keyword

nonn

Author

J. Lowell, Sep 04 2008

Extensions

More terms from Robert G. Wilson v, Sep 09 2008

Status

approved