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 A106667 a(n) = 1 if prime(n) + 2 is a prime, a(n) = -1 if prime(n) + 2 is a semiprime, otherwise 0. 1
 -1, 1, 1, -1, 1, -1, 1, -1, -1, 1, -1, -1, 1, 0, -1, -1, 1, 0, -1, 1, 0, 0, -1, -1, 0, 1, 0, 1, -1, -1, -1, -1, 1, -1, 1, 0, -1, 0, -1, 0, 1, -1, 1, 0, 1, -1, -1, 0, 1, 0, -1, 1, 0, -1, -1, -1, 1, 0, 0, 1, 0, -1, -1, 1, 0, -1, 0, -1, 1, 0, -1, -1, 0, 0, -1, 0, -1, 0, -1, -1, 1, 0, 1, 0, 0, -1, -1, 0, 1, 0, -1, -1, -1, -1 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Robert Israel, Table of n, a(n) for n = 1..10000 EXAMPLE a(1) = -1 because prime(1) = 2 and 2 + 2 = 4 is a semiprime; a(2) = 1 because prime(2) = 3 and 3 + 2 = 5 is a prime; a(14) = 0 because prime(14) = 43 and 43 + 2 = 45 is neither prime nor semiprime. MAPLE p:= 1: for n from 1 to 100 do   p:= nextprime(p);   if isprime(p+2) then A[n]:=1   elif numtheory:-bigomega(p+2)=2 then A[n]:=-1   else A[n]:= 0   fi od: seq(A[n], n=1..100); # Robert Israel, Aug 29 2018 CROSSREFS Cf. A001359, A063637. Sequence in context: A014503 A014230 A014451 * A133011 A296079 A340363 Adjacent sequences:  A106664 A106665 A106666 * A106668 A106669 A106670 KEYWORD easy,sign,less AUTHOR Giovanni Teofilatto, May 13 2005 EXTENSIONS Corrected, and definition clarified, by Robert Israel, Aug 29 2018 STATUS approved

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Last modified June 15 17:41 EDT 2021. Contains 345049 sequences. (Running on oeis4.)