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%I #6 Feb 14 2018 08:23:40
%S 3,5,7,10,13,15,18,20,23,26,29,33,35,37,39,43,47,48,51,54,55,59,61,64,
%T 69,72,73,75,76,78,85,87,91,93,97,98,102,104,106,108,112,114,118,120,
%U 122,124,129,134,136,138,141,143,145,149,152,155,156,158,160
%N Rank of n-th twice-prime when all the primes and twice-primes are jointly ranked.
%H Clark Kimberling, <a href="/A298862/b298862.txt">Table of n, a(n) for n = 1..1000</a>
%e A001751 = ordered sequence of primes and twice-primes: 2,3,4,5,6,7,10,... in which twice-primes occupy ranks 3,5,7,...
%t z = 1000; u = Prime[Range[z]]; w = Take[Union[u, 2 u], z];
%t p[n_] := If[MemberQ[u, w[[n]]], 0, 1];
%t Take[w, z]; (* A001751 *)
%t t = Table[p[n], {n, 1, z}];
%t Flatten[Position[t, 0]]; (* A298861 *)
%t Flatten[Position[t, 1]]; (* A298862 *)
%Y Cf. A000040, A001751, A298861 (complement).
%K nonn,easy
%O 1,1
%A _Clark Kimberling_, Feb 13 2018
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