cp's OEIS Frontend

A221865 The nonprimes k such that k + 2 is either a prime or a semiprime.

%I A221865 #28 May 05 2013 13:44:33
%S A221865 0,1,4,8,9,12,15,20,21,24,27,32,33,35,36,39,44,45,49,51,55,56,57,60,
%T A221865 63,65,69,72,75,77,80,81,84,85,87,91,92,93,95,99,104,105,111,116,117,
%U A221865 119,120,121,125,129,132,135,140,141,143,144,147,153,155,156,159,161,164,165,171,175,176,177,183
%N A221865 The nonprimes k such that k + 2 is either a prime or a semiprime.
%C A221865 Chen primes A109611(n) such that A109611(n)-/+ a(n) are both prime: 2, 29, 53, 113, 139,...
%C A221865 Unrelated: Numbers n such that n + 2^bigomega(n) is either a prime or a semiprime: 1, 2, 3, 5, 6, 7, 9, 10, 11, 13, 15, 17, 18, 19, 21, 22, 23, 25, 27,...
%C A221865 A179384 is a subsequence. - _R. J. Mathar_, Apr 26 2013
%p A221865 A221865 := proc(n)
%p A221865     option remember;
%p A221865     if n =1 then
%p A221865         0;
%p A221865     else
%p A221865         for a from procname(n-1)+1 do
%p A221865             if not isprime(a) then
%p A221865             if isprime(a+2) or numtheory[bigomega](a+2) = 2 then
%p A221865                 return a;
%p A221865             end if;
%p A221865             end if;
%p A221865         end do:
%p A221865     end if;
%p A221865 end proc: # _R. J. Mathar_, Apr 26 2013
%t A221865 Select[Range[0,200],!PrimeQ[#]&&PrimeOmega[#+2]<3&] (* _Harvey P. Dale_, May 05 2013 *)
%Y A221865 Cf. A001358, A109611, A175634.
%K A221865 nonn
%O A221865 1,3
%A A221865 _Gerasimov Sergey_, Apr 18 2013
%E A221865 Corrected by _R. J. Mathar_, Apr 26 2013