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A069879 Number of pairs {i,j} with i different from j; 1<=i<=n; 1<= j <=n such that i+j is a prime number.

%I A069879 #7 Sep 29 2017 17:29:06
%S A069879 0,2,4,8,10,14,18,22,28,36,42,50,56,62,70,80,88,96,104,112,122,134,
%T A069879 144,156,168,180,194,208,220,234,248,262,276,292,308,326,344,362,380,
%U A069879 400,418,438,456,474,494,514,532,550,570,590,612,636,658,682,708,734
%N A069879 Number of pairs {i,j} with i different from j; 1<=i<=n; 1<= j <=n such that i+j is a prime number.
%F A069879 a(n) = 2 * A071917(n). - _Alois P. Heinz_, Sep 29 2017
%p A069879 with(numtheory):
%p A069879 b:= proc(n) option remember; `if`(n=0, 0,
%p A069879       b(n-1)+pi(2*n-1)-pi(n))
%p A069879     end:
%p A069879 a:= n-> 2*b(n):
%p A069879 seq(a(n), n=1..70);  # _Alois P. Heinz_, Sep 29 2017
%t A069879 Table[2*Count[Subsets[Range[n],{2}],_?(PrimeQ[Total[#]]&)],{n,50}] (* _Harvey P. Dale_, Jan 23 2015 *)
%Y A069879 Partial sums of 2*A060715(n).
%K A069879 nonn
%O A069879 1,2
%A A069879 _Santi Spadaro_, May 04 2002