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A130120 a(0)=1. a(n) = number of earlier terms of the sequence that divide n(n+1)/2. 1

%I

%S 1,1,2,3,3,4,4,5,7,5,4,5,5,3,10,14,6,5,5,10,17,6,2,12,21,8,5,12,6,12,

%T 12,8,19,6,11,23,11,3,7,28,16,9,9,6,26,16,3,23,28,11,15,16,5,9,22,26,

%U 28,8,2,26,26,2,13,39,30,21,10,7,19,23,13,26,26,2,16,32,17,18,10,25,45,10,2

%N a(0)=1. a(n) = number of earlier terms of the sequence that divide n(n+1)/2.

%e a(7)=5 because among the first seven terms of the sequence, namely 1,1,2,3,3,4,4, only 1,1,2,4 and 4 divide 7*8/2=28.

%p a[0]:=1: for n from 1 to 100 do a[n]:=0: for j from 0 to n-1 do if type(n*(n+1)/2/a[j], integer)=true then a[n]:=a[n]+1 else fi: od: od: seq(a[n],n=0..100); # _Emeric Deutsch_, May 22 2007

%t a = {1}; For[n = 1, n < 80, n++, AppendTo[a, Length[Select[(n*(n + 1)/2)/a, IntegerQ[ # ] & ]]]]; a (* _Stefan Steinerberger_, Jun 01 2007 *)

%Y Cf. A130121.

%K nonn

%O 0,3

%A _Leroy Quet_, May 10 2007

%E More terms from _Emeric Deutsch_ and _Stefan Steinerberger_, May 22 2007

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Last modified October 22 10:39 EDT 2021. Contains 348160 sequences. (Running on oeis4.)