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a(1)=1; for n>1, a(n) = number of earlier terms a(k), 1<=k<=n-1, such that (k+a(k)) is coprime to n.
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%I #9 Oct 10 2019 11:37:04

%S 1,0,2,1,2,3,5,4,5,2,10,3,12,5,5,8,16,5,17,6,9,9,21,7,18,13,15,12,28,

%T 6,28,16,16,17,22,11,36,18,20,18,40,12,41,22,18,23,45,14,40,22,26,26,

%U 52,16,41,22,27,28,56,13,60,29,26,33,49,20,66,34,34,23,70,24,70,39,33,36,56

%N a(1)=1; for n>1, a(n) = number of earlier terms a(k), 1<=k<=n-1, such that (k+a(k)) is coprime to n.

%e (a(3)+3) is coprime to 6; (a(4)+4) is coprime to 6; and (a(5)+5) is coprime to 6. These 3 cases are the only cases where (a(k)+k) is coprime to 6, for 1<=k<=5. So a(6)=3.

%t f[l_List] := Block[{n = Length[l] + 1},Append[l, Count[Table[GCD[n, k + l[[k]]], {k, n - 1}], 1]]];Nest[f, {1}, 76] (* _Ray Chandler_, Jan 22 2007 *)

%Y Cf. A127460, A127463.

%K nonn

%O 1,3

%A _Leroy Quet_, Jan 15 2007

%E Extended by _Ray Chandler_, Jan 22 2007