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%I #5 Apr 03 2017 14:41:59
%S 9,4,15,6,21,8,25,10,27,12,33,14,35,16,39,18,45,20,49,22,51,24,55,26,
%T 57,28,63,30,65,32,69,34,75,36,77,38,81,40,85,42,87,44,91,46,93,48,95,
%U 50,99
%N Alternately odd and even composite numbers complementing the parity of the index.
%t Module[{nn=100,ecs,ocs},ocs=Select[Range[1,nn,2],CompositeQ];ecs = Take[ Range[ 4,nn,2],Length[ocs]];Riffle[ocs,ecs]] (* _Harvey P. Dale_, Apr 03 2017 *)
%Y Cf. A075065.
%Y Cf. A005381.
%K easy,nonn
%O 1,1
%A _Amarnath Murthy_, Sep 08 2002