A067812 Nonprime n such that 2n+1 is prime.

1, 6, 8, 9, 14, 15, 18, 20, 21, 26, 30, 33, 35, 36, 39, 44, 48, 50, 51, 54, 56, 63, 65, 68, 69, 74, 75, 78, 81, 86, 90, 95, 96, 98, 99, 105, 111, 114, 116, 119, 120, 125, 128, 134, 135, 138, 140, 141, 146, 153, 155, 156, 158, 165, 168, 174, 176, 183, 186, 189, 194

Offset: 1

Author

Joseph L. Pe, Feb 07 2002

Keywords

• easy
• nonn

Examples

			21*2+1 = 43, a prime, so 21 belongs to the sequence.
		

Crossrefs

Cf. A005384.

Programs

Maple

with(numtheory): for n from 2 to 500 do if tau(n) > 2 and isprime(2*n+1) then printf(`%d,`,n) fi: od:

Mathematica

Select[Range[200], PrimeQ[2#+1]&&!PrimeQ[ # ]&]

Extensions

Edited by Dean Hickerson and James Sellers, Feb 12 2002