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a(1) = 1 and then alternately even and odd composite numbers matching the parity of the index.
1

%I #5 Dec 05 2013 19:55:36

%S 1,4,9,6,15,8,21,10,25,12,27,14,33,16,35,18,39,20,45,22,49,24,51,26,

%T 55,28,57,30,63,32,65,34,69,36,75,38,77,40,81,42,85,44,87,46,91,48,93,

%U 50,95,52,99,54,105,56,111,58,115,60,117,62,119,64,121,66

%N a(1) = 1 and then alternately even and odd composite numbers matching the parity of the index.

%t Module[{nn=150,comps,odds,evens,len},comps=Complement[Range[nn], Prime[Range[ PrimePi[nn]]]]; odds=Select[ comps, OddQ];evens=Select[comps,EvenQ]; len=Min[ Length[odds],Length[evens]]; Riffle[Take[odds,len],Take[evens,len]]] (* _Harvey P. Dale_, Feb 19 2013 *)

%Y Cf. A075066.

%K easy,nonn

%O 1,2

%A _Amarnath Murthy_, Sep 08 2002

%E More terms from _Harvey P. Dale_, Feb 19 2013