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 A129950 Indicator function of twin primes: 1 if n is a twin prime member, 0 if not prime, -1 else (isolated prime or 2). 4
 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; text; internal format)
 OFFSET 1,1 COMMENTS Absolute values are the same as A010051. LINKS Antti Karttunen, Table of n, a(n) for n = 1..20000 Antti Karttunen, Data supplement: n, a(n) computed for n = 1..100000 FORMULA a(n) = 2*A164292(n) - A010051(n). - Antti Karttunen, Jan 03 2019 MATHEMATICA Array[If[PrimeQ@ #, If[Or[PrimeQ[# - 2], PrimeQ[# + 2]], 1, -1], 0] &, 100] (* Michael De Vlieger, Jan 03 2019 *) 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); \\ Typos corrected by Antti Karttunen, Jan 03 2019 CROSSREFS Cf. A001097, A001359, A006512, A010051, A077800, A164292. Sequence in context: A122257 A332219 A227625 * A010051 A252233 A283991 Adjacent sequences:  A129947 A129948 A129949 * A129951 A129952 A129953 KEYWORD sign AUTHOR Cino Hilliard, Jun 10 2007 EXTENSIONS Definition and a(67) corrected by M. F. Hasler, Jan 20 2012 STATUS approved

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Last modified September 28 01:27 EDT 2021. Contains 347698 sequences. (Running on oeis4.)