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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
1, 2, 6, 7, 14, 15, 17, 18, 19, 20, 21, 26, 27, 28, 29, 31, 33, 35, 36, 38, 39, 42, 44, 45, 48, 49, 53, 55, 56, 57, 59, 61, 64, 65, 66, 69, 70, 71, 74, 76, 77, 78, 79, 80, 82, 84, 87, 88, 89, 90, 91, 92, 93, 96, 98, 99, 100, 102, 103, 104, 105, 107, 109, 112, 113, 115, 117
OFFSET
1,2
EXAMPLE
1 is a term because 2*prime(1) - 1 = 3 (prime) and 2*prime(1) + 1 = 5 (prime);
2 is a term because 2*prime(2) - 1 = 5 (prime) and 2*prime(2) + 1 = 7 (prime),
7 is a term because 2*prime(7) - 1 = 33 (composite) and 2*prime(7) + 1 = 35 (composite).
MAPLE
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
CROSSREFS
Cf. A100484.
Sequence in context: A306924 A087376 A199974 * A376296 A265739 A281167
KEYWORD
nonn
AUTHOR
EXTENSIONS
Corrected (6 inserted, 74 inserted) by R. J. Mathar, Apr 20 2010
STATUS
approved