A129951 Sum of characteristic function of twin primes < 10^n.

2, 5, -30, -411, -4698, -45824, -428661

Offset

1,1

Links

Table of n, a(n) for n=1..7.

Formula

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)).

Prog

(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))

Crossrefs

Sequence in context: A366455 A140786 A369726 * A127298 A275255 A219273

Adjacent sequences: A129948 A129949 A129950 * A129952 A129953 A129954

Keyword

sign

Author

Cino Hilliard, Jun 10 2007

Status

approved