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A129950
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Indicator function of twin primes: 1 if twin prime member, 0 if not prime, -1 else (isolated prime or 2).
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1
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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, 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, -1, 0, 0, 0, 0, 0, -1, 0, 0, 0, 0, 0, 0, 0, -1, 0, 0, 0
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Absolute values are the same as A010051.
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PROG
| (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))
(PARI) a(n)=if(isprime(n), (-1)^(!isprime(n-2) & !isprime(n+2), 0)
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CROSSREFS
| Sequence in context: A010060 A118247 A122257 * A010051 A131929 A100821
Adjacent sequences: A129947 A129948 A129949 * A129951 A129952 A129953
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KEYWORD
| sign
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AUTHOR
| Cino Hilliard (hillcino368(AT)hotmail.com), Jun 10 2007
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EXTENSIONS
| Definition and a(67) corrected by M. F. Hasler, Jan 20 2012
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