%I
%S 1,7,9,15,21,31,37,39,61,75,91,115,117,121,129,141,147,151,169,171,
%T 187,195,201,217,241,249,285,301,319,339,357,399,417,421,435,457,477,
%U 481,487,499,507,529,567,577,579,595,607,621,627,631,645,691,711,715,717
%N Numbers n with property that (2/(n+1))*(Concatenated triangular numbers T (n), T(n+1)) is prime.
%C All terms are odd. No term == 3 mod 10.
%e n=1: T(1)=1, T(2)=3, (2/2)*13=13 (prime),
%e n=7: T(7)=28, T(8)=36, (2/8)*2836=709 (prime),
%e n=9: T(9)=45, T(8)=55, (2/10)*4555=911 (prime).
%Y A000217 Triangular numbers.
%K base,nonn
%O 1,2
%A _Zak Seidov_, Mar 29 2009
