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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 A167850

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 February 23 05:58 EST 2019. Contains 320411 sequences. (Running on oeis4.)