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%I
%S 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,
%T 0,0,0,-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,
%U -1,0,0,0,1,0,1,0,0,0,0,0,-1,0,0,0,-1,0,0,0,0,0,-1,0,0,0,0,0,0,0,-1,0,0,0
%N Indicator function of twin primes: 1 if twin prime member, 0 if not prime, -1 else (isolated prime or 2).
%C Absolute values are the same as A010051.
%o (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))
%o (PARI) a(n)=if(isprime(n),(-1)^(!isprime(n-2) & !isprime(n+2),0)
%K sign
%O 1,1
%A Cino Hilliard (hillcino368(AT)hotmail.com), Jun 10 2007
%E Definition and a(67) corrected by M. F. Hasler, Jan 20 2012
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