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

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

Links

Table of n, a(n) for n=1..67.

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 * A265739 A281167 A210660

Adjacent sequences: A176276 A176277 A176278 * A176280 A176281 A176282

Keyword

nonn

Author

Juri-Stepan Gerasimov, Apr 14 2010

Extensions

Corrected (6 inserted, 74 inserted) by R. J. Mathar, Apr 20 2010

Status

approved