%I #2 Oct 01 2013 21:35:23
%S 2,5,-30,-411,-4698,-45824,-428661
%N Sum of characteristic function of twin primes < 10^n.
%F Let t(x) = 1 if m is a twin prime member, 0 if isolated prime or -1 if not prime. then a(n) = sum(x=1,10^n,t(x)).
%o (PARI) for(j=1,10,print1(sum(x=1,10^j,t(x))",")) t(n) = if(abs(istwin(n))==1||n==5,1,if(isprime(n),-1,0)) istwin(n) = local(p1, p2); if(n==5,return(2));if(isprime(n),p1=n-2;p2=n+2; if(isprime(p1),return(1));if(isprime(p2),return(-1));return(0))
%K sign
%O 1,1
%A _Cino Hilliard_, Jun 10 2007
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