A131845 a(1)=a(2)=1; a(n) = number of terms of the sequence (from among terms a(1) through a(n-1)) that divide (a(n-1)*a(n-2)).

1, 1, 2, 3, 4, 5, 5, 4, 7, 6, 6, 8, 9, 10, 10, 9, 12, 11, 10, 9, 14, 11, 7, 6, 10, 16, 13, 8, 8, 9, 17, 8, 10, 17, 12, 13, 13, 5, 8, 18, 22, 19, 7, 6, 12, 23, 14, 9, 19, 10, 14, 20, 26, 24, 24, 27, 27, 10, 26, 21, 20, 32, 24, 23, 23, 5, 9, 13, 13, 7, 11, 9, 13, 16, 20, 27, 38, 22, 13, 17

Offset

1,3

Links

Robert Israel, Table of n, a(n) for n = 1..10000

Example

a(11)*a(12) = 6*8 = 48. The terms, from among the first 12 terms of the sequence, which divide 48 are: a(1)=1, a(2)=1, a(3)=2, a(4)=3, a(5)=4, a(8)=4, a(10)=6, a(11)=6 and a(12)=8. There are 9 such terms, so a(13) = 9.

Maple

a[1]:=1: a[2]:=1: for n from 3 to 70 do a[n]:=0: for i to n-1 do if type(a[n-1]*a[n-2]/a[i], integer)=true then a[n]:=a[n]+1 else end if end do end do: seq(a[n], n=1..70); # Emeric Deutsch, Oct 16 2007

Mathematica

a = {1, 1}; Do[AppendTo[a, Length[Select[Range[Length[a]], Mod[a[[ -1]]*a[[ -2]], a[[ # ]]] == 0 &]]], {80}]; a (* Stefan Steinerberger, Oct 07 2007 *)

Crossrefs

Sequence in context: A100771 A234470 A113771 * A062186 A085763 A273264

Adjacent sequences: A131842 A131843 A131844 * A131846 A131847 A131848

Keyword

nonn

Author

Leroy Quet, Oct 04 2007

Extensions

More terms from Stefan Steinerberger, Oct 07 2007

Status

approved