A123953 - OEIS

%I #14 Oct 11 2019 11:38:16

%S 1,1,2,2,2,5,6,3,3,5,8,4,4,13,9,3,9,11,12,9,4,12,21,6,4,22,14,6,12,14,

%T 15,14,5,17,25,5,10,37,23,6,14,22,23,20,7,21,44,14,13,33,25,13,24,35,

%U 23,16,17,39,58,13,14,61,35,20,20,26,43,33,19,27,48,25,26,72,37,17,32,35

%N a(0)=1. a(n) = number of terms among terms a(0) through a(n-1) of the sequence which are coprime to n(n+1)/2.

%e Among terms a(0),a(1),...a(8) there are 5 terms coprime to 9*10/2 = 45. (These terms are a(0)=1, a(1)=1, a(2)=2, a(3)=2 and a(4)=2.) So a(9) = 5.

%t f[l_List] := Block[{n = Length[l]},Append[l, Length @ Select[l, GCD[n*(n + 1)/2, # ] == 1 &]]];Nest[f, {1}, 80] (* _Ray Chandler_, Jun 29 2008 *)

%t a[0] = 1; a[n_] := a[n] = Count[ GCD[ Table[ a[i], {i, 0, n - 1}], n(n + 1)/2], 1]; Table[ a[n], {n, 0, 77}] (* _Robert G. Wilson v_ *)

%Y Cf. A119989.

%K nonn

%O 0,3

%A _Leroy Quet_, Nov 22 2006

%E Extended by _Ray Chandler_ and _Robert G. Wilson v_, Nov 23 2006