%I A129950
%S A129950 0,1,1,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,
%T A129950 0,1,0,0,0,1,0,1,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,0,1,0,0,0,
%U A129950 1,0,1,0,0,0,0,0,1,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0
%V A129950 0,-1,1,0,1,0,1,0,0,0,1,0,1,0,0,0,1,0,1,0,0,0,-1,0,0,0,0,0,1,0,1,0,0,0,
               0,0,-1,0,0,0,1,
%W A129950 0,1,0,0,0,-1,0,0,0,0,0,-1,0,0,0,0,0,1,0,1,0,0,0,0,0,1,0,0,0,1,0,1,0,0,
               0,0,0,-1,0,0,0,
%X A129950 -1,0,0,0,0,0,-1,0,0,0,0,0,0,0,-1,0,0,0
%N A129950 Characteristic function of twin primes: 1 if twin prime member, 0 if 
               isolated prime or -1 if not prime.
%o A129950 (PARI) for(j=1,100,print1(t(j)",")) t(x) = if(abs(istwin(x))==1||x==5,
               1,if(isprime(x),-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 A129950 Sequence in context: A010060 A118247 A122257 this_sequence A010051 A131929 
               A100821
%Y A129950 Adjacent sequences: A129947 A129948 A129949 this_sequence A129951 A129952 
               A129953
%K A129950 sign
%O A129950 1,1
%A A129950 Cino Hilliard (hillcino368(AT)hotmail.com), Jun 10 2007