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A129950 Indicator function of twin primes: 1 if twin prime member, 0 if not prime, -1 else (isolated prime or 2). 2
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

Table of n, a(n) for n=1..100.

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)

CROSSREFS

Sequence in context: A010060 A118247 A122257 * A010051 A131929 A100821

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 January 22 08:38 EST 2018. Contains 298042 sequences.