OFFSET
1,2
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
KEYWORD
easy,nonn
AUTHOR
Joseph L. Pe, Feb 07 2002
EXTENSIONS
Edited by Dean Hickerson and James A. Sellers, Feb 12 2002
STATUS
approved