1,1

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)).

(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))

Sequence in context: A019027 A019031 A140786 this_sequence A127298 A000133 A059086

Adjacent sequences: A129948 A129949 A129950 this_sequence A129952 A129953 A129954

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Cino Hilliard (hillcino368(AT)hotmail.com), Jun 10 2007