A168044 Half of the even nonisolated nonprimes A167692.

0, 4, 5, 7, 8, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 22, 23, 24, 25, 26, 27, 28, 29, 31, 32, 33, 34, 35, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 52, 53, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 70, 71, 72, 73, 74, 76, 77, 78, 79, 80, 81, 82, 83

Offset: 1

Author

Juri-Stepan Gerasimov, Nov 17 2009

Keywords

• nonn
• less

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

Crossrefs

Cf. A167692.

Except 1, complement of A040040.

Programs

Mathematica

Select[Range[0,100], Mod[#, 2] == 0 && !PrimeQ[#] && (! PrimeQ[# - 1] || !PrimeQ[# + 1]) & ]/2 (* G. C. Greubel, Jul 07 2016 *)

Formula

a(n) = A167692(n)/2.