A067812 - OEIS

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

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

OFFSET

1,2

LINKS

Table of n, a(n) for n=1..61.

EXAMPLE

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

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[ # ]&]

CROSSREFS