OFFSET
1,2
COMMENTS
Apart from the first term, numbers n with at least one of 2n-1 and 2n+1 composite. - Charles R Greathouse IV, Mar 25 2010
It is possible to create a sieve for twin primes because these numbers are of the form i+j+2*i*j and 1+i+j+2*i*j for i,j >= 1. Except 1, complement of A040040 where 2*a(n)-1 and 2*a(n)+1 are twin primes. - Davide Rotondo, Jan 19 2021
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
FORMULA
a(n) = A167692(n)/2.
MATHEMATICA
Select[Range[0, 100], Mod[#, 2] == 0 && !PrimeQ[#] && (! PrimeQ[# - 1] || !PrimeQ[# + 1]) & ]/2 (* G. C. Greubel, Jul 07 2016 *)
CROSSREFS
KEYWORD
nonn,less
AUTHOR
Juri-Stepan Gerasimov, Nov 17 2009
STATUS
approved