A175735 n-th non-Chen semiprime minus n-th Chen semiprime.

-2, 5, 16, 23, 25, 40, 49, 44, 52, 59, 56, 57, 70, 67, 77, 85, 81, 89, 79, 81, 91, 94, 101, 92, 91, 100, 102, 103, 125, 129, 137, 160, 169, 166, 167, 160, 157, 156, 154, 145, 160, 160, 162, 168, 172, 180, 181, 203, 202, 209, 204, 209, 213, 212, 208, 215, 228, 227, 237, 236

Offset: 1

Author

Juri-Stepan Gerasimov, Aug 25 2010

Keywords

• sign
• less

Comments

Chen semiprimes: semiprimes m such that m+4 is either a prime or a semiprime.

Programs

Maple

isA175634 := proc(n) isA001358(n) and (isprime(n+4) or isA001358(n+4)) ; end proc:
A175634 := proc(n) option remember; if n = 1 then 6; else for a from procname(n-1)+1 do if isA175634(a) then return a; end if; end do: end if; end proc:
isA175680 := proc(n) isA001358(n) and not isA175634(n) ; end proc:
A175680 := proc(n) option remember; if n = 1 then 4; else for a from procname(n-1)+1 do if isA175680(a) then return a; end if; end do: end if; end proc:
A175735 := proc(n) A175680(n)-A175634(n) ; end proc:
seq(A175735(n),n=1..120) ; # R. J. Mathar, Aug 25 2010

Formula

a(n) = A175680(n) - A175634(n).

Extensions

Keyword:sign,less set; corrected (87 replaced by 91, 210 replaced by 204) - R. J. Mathar, Aug 25 2010. Also corrected by D. S. McNeil, Aug 25 2010