Search: id:A129951
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%I A129951
%S A129951 2,5,30,411,4698,45824,428661
%V A129951 2,5,-30,-411,-4698,-45824,-428661
%N A129951 Sum of characteristic function of twin primes < 10^n.
%F A129951 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 A129951 (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))
%Y A129951 Sequence in context: A019027 A019031 A140786 this_sequence A127298 A000133 
               A059086
%Y A129951 Adjacent sequences: A129948 A129949 A129950 this_sequence A129952 A129953 
               A129954
%K A129951 sign
%O A129951 1,1
%A A129951 Cino Hilliard (hillcino368(AT)hotmail.com), Jun 10 2007

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