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%I #19 Feb 26 2015 17:25:43
%S 0,0,1,1,1,1,2,2,2,2,2,2,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,
%T 3,3,4,4,4,4,4,4,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,6,6,
%U 6,6,6,6,7,7,7,7,7,7,8,8,8,8,8,8,8,8,8
%N Number of primes p < n such that 4*p+1 is also prime.
%H Enrique Pérez Herrero, <a href="/A182434/b182434.txt">Table of n, a(n) for n = 1..10000</a>
%F a(10^n) = A182265(n).
%F a(n) = sum(i=2..n, floor(phi(4*i^2+i)/(4*i^2-4*i))). - _Enrique Pérez Herrero_, May 02 2012.
%t Accumulate[Table[Boole[PrimeQ[n]&&PrimeQ[4n+1]],{n,1,200}]]
%t Accumulate[If[AllTrue[{#,4#+1},PrimeQ],1,0]&/@Range[90]] (* The program uses the AllTrue function from Mathematica version 10 *) (* _Harvey P. Dale_, Feb 26 2015 *)
%Y Cf. A023212, A182265.
%Y Cf. A005384, A156660, A156874, A092816.
%K nonn
%O 1,7
%A _Enrique Pérez Herrero_, Apr 28 2012
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