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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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified April 9 04:05 EDT 2020. Contains 333343 sequences. (Running on oeis4.)