A125844 - OEIS

%I #11 Oct 31 2019 11:43:41

%S 1,5,3,2,7,11,9,4,13,17,19,8,15,23,6,21,10,25,29,27,14,31,12,35,33,16,

%T 37,41,39,22,43,47,18,49,20,45,53,24,51,26,55,59,57,32,61,28,65,63,34,

%U 67,71,69,38,73,30,77,79,40,75,83,36,81,44,85,89,46,87,50,91,42,95,93

%N a(n) = the smallest positive integer which does not occur earlier in the sequence and which is coprime to the n-th composite integer.

%C This sequence is a permutation of the positive integers.

%e The positive integers which do not occur among the first 6 terms of the sequence are 4,6,8,9,10,12,... Of these integers, 9 is the smallest that is coprime to 14, the 7th composite. So a(7) = 9.

%t f[l_List] := Block[{n = Length[l] + 1, c = 1, k = n},While[k > 0,c++;While[PrimeQ[c], c++ ];k--;];k = 1;While[MemberQ[l, k] || GCD[k, c] > 1, k++ ];Append[l, k]];Nest[f, {}, 75] (* _Ray Chandler_, Feb 07 2007 *)

%Y Cf. A002808.

%K nonn

%O 1,2

%A _Leroy Quet_, Feb 03 2007

%E Extended by _Ray Chandler_, Feb 07 2007