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A074038 If n is the k-th single (i.e., non-twin) prime then a(n) = k, otherwise a(n) = 0.

%I A074038 #19 Dec 03 2021 10:45:05
%S A074038 0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,2,0,0,0,0,0,0,0,0,0,0,0,
%T A074038 0,0,3,0,0,0,0,0,0,0,0,0,4,0,0,0,0,0,5,0,0,0,0,0,0,0,0,0,0,0,0,0,6,0,
%U A074038 0,0,0,0,0,0,0,0,0,0,7,0,0,0,8,0,0,0,0,0,9,0,0,0,0,0,0,0,10,0,0
%N A074038 If n is the k-th single (i.e., non-twin) prime then a(n) = k, otherwise a(n) = 0.
%H A074038 Antti Karttunen, <a href="/A074038/b074038.txt">Table of n, a(n) for n = 1..65537</a>
%F A074038 A007510(a(n)) = n.
%t A074038 upto = 100;
%t A074038 A074038list[upto_] := Module[{k = 0, v = Table[0, {upto}], n},
%t A074038     For[n = 1, n <= upto, n++, If[PrimeQ[n] && (2 == n ||
%t A074038     ((!PrimeQ[n-2]) && (!PrimeQ[n+2]))), k++; v[[n]] = k,
%t A074038     v[[n]] = 0]]; v];
%t A074038 v074038 = A074038list[upto];
%t A074038 a[n_] := v074038[[n]];
%t A074038 Table[a[n], {n, 1, upto}] (* _Jean-François Alcover_, Dec 03 2021, after PARI code *)
%o A074038 (PARI)
%o A074038 up_to = 65537;
%o A074038 A074038list(up_to) = { my(k=0,v=vector(up_to)); for(n=1,up_to,if(isprime(n)&&(2==n||((!isprime(n-2))&&(!isprime(n+2)))),k++;v[n]=k, v[n] = 0)); (v); };
%o A074038 v074038 = A074038list(up_to);
%o A074038 A074038(n) = v074038[n];
%Y A074038 Cf. A007510, A049084, A074039.
%K A074038 nonn
%O A074038 1,23
%A A074038 _Reinhard Zumkeller_, Aug 13 2002