cp's OEIS Frontend

A199859 Numbers k such that 6k-5 is a composite number of the form (6x-5)*(6y-5) when x or y is not equal to 1 except for k=1.

%I A199859 #18 May 11 2019 18:36:10
%S A199859 1,9,16,23,29,30,37,42,44,51,55,58,61,65,68,72,79,80,81,86,93,94,99,
%T A199859 100,105,107,114,118,120,121,128,130,133,135,137,142,146,149,155,156,
%U A199859 159,161,163,170,172,175,177,180,184,185,191,192,194,198,205,211,212
%N A199859 Numbers k such that 6k-5 is a composite number of the form (6x-5)*(6y-5) when x or y is not equal to 1 except for k=1.
%C A199859 Numbers whose associate in A091300 has at least one nontrivial factorization into two factors of A016921.
%p A199859 isA016921 := proc(n)
%p A199859     (n mod 6)=1 ;
%p A199859 end proc:
%p A199859 isA091300 := proc(n)
%p A199859     (not isprime(n)) and isA016921(n) ;
%p A199859 end proc:
%p A199859 isA199859 := proc(n)
%p A199859     if n = 1 then
%p A199859         return true;
%p A199859     elif isA091300(6*n-5) then
%p A199859         for d in numtheory[divisors](6*n-5) minus {1,6*n-5} do
%p A199859             if isA016921(d) and isA016921((6*n-5)/d) then
%p A199859                 return true;
%p A199859             end if;
%p A199859         end do:
%p A199859         return false;
%p A199859     else
%p A199859         return false;
%p A199859     end if;
%p A199859 end proc:
%p A199859 for n from 1 to 210 do
%p A199859     if isA199859(n) then
%p A199859         printf("%d,",n) ;
%p A199859     end if ;
%p A199859 end do; # _R. J. Mathar_, Nov 25 2011
%Y A199859 Cf. A091300.
%K A199859 nonn,easy
%O A199859 0,2
%A A199859 _Eleonora Echeverri-Toro_, Nov 11 2011