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 A176279 Numbers k such that 2*prime(k)-1 and 2*prime(k)+1 are both prime or both composite. 0

%I

%S 1,2,6,7,14,15,17,18,19,20,21,26,27,28,29,31,33,35,36,38,39,42,44,45,

%T 48,49,53,55,56,57,59,61,64,65,66,69,70,71,74,76,77,78,79,80,82,84,87,

%U 88,89,90,91,92,93,96,98,99,100,102,103,104,105,107,109,112,113,115,117

%N Numbers k such that 2*prime(k)-1 and 2*prime(k)+1 are both prime or both composite.

%e 1 is a term because 2*prime(1) - 1 = 3 (prime) and 2*prime(1) + 1 = 5 (prime);

%e 2 is a term because 2*prime(2) - 1 = 5 (prime) and 2*prime(2) + 1 = 7 (prime),

%e 7 is a term because 2*prime(7) - 1 = 33 (composite) and 2*prime(7) + 1 = 35 (composite).

%p for n from 1 to 200 do p := ithprime(n) ; if isprime(2*p-1) = isprime(2*p+1) then printf("%d,",n) ; end if; end do: # _R. J. Mathar_, Apr 20 2010

%Y Cf. A100484.

%K nonn

%O 1,2

%A _Juri-Stepan Gerasimov_, Apr 14 2010

%E Corrected (6 inserted, 74 inserted) by _R. J. Mathar_, Apr 20 2010

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Last modified June 1 09:53 EDT 2020. Contains 334762 sequences. (Running on oeis4.)