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A129950
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Indicator function of twin primes: 1 if n is a twin prime member, 0 if not prime, -1 else (isolated prime or 2).
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4
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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
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OFFSET
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1,1
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COMMENTS
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Absolute values are the same as A010051.
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LINKS
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FORMULA
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MATHEMATICA
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Array[If[PrimeQ@ #, If[Or[PrimeQ[# - 2], PrimeQ[# + 2]], 1, -1], 0] &, 100] (* Michael De Vlieger, Jan 03 2019 *)
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PROG
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(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); \\ Typos corrected by Antti Karttunen, Jan 03 2019
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CROSSREFS
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KEYWORD
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sign
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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