%I
%S 20,31,35,39,44,99,135,139,155,164,200,211,271,275,284,340,359,360,
%T 404,416,424,444,484,496,511,564,596,611,640,676,724,800,836,859,860,
%U 871,876,884,919,940,944,951,971,976,995,1000,1004,1064,1116,1131,1144,1159
%N Concatenated triangular numbers T (n), T(n + 1) and T(n + 2) are prime.
%e n = 20 : T (20) = 210, T (21) = 231, T (22) = 253, 210231253 is prime;
%e n = 31 : T (31) = 496, T (32) = 528, T (33) = 561, 496528561 is prime.
%t p3cQ[n_]:=Module[{c1=(n(n+1))/2,c2=((n+1)(n+2))/2,c3=((n+2)(n+3))/2}, PrimeQ[FromDigits[Flatten[IntegerDigits/@{c1,c2,c3}]]]]; Select[Range[ 1250], p3cQ] (* _Harvey P. Dale_, Sep 13 2011 *)
%Y A158750 Concatenated triangular numbers that are prime. A000217 Triangular numbers,
%K nonn,base
%O 1,1
%A _Zak Seidov_, Mar 28 2009
