This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 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 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

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.