This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A226788 #12 Jun 11 2015 10:39:44
%S A226788 45,78,4950,5253,295296,369370,415416,499500,502503,594595,652653,
%T A226788 760761,22542255,49995000,50025003,88278828,1033010331,1487714878,
%U A226788 4999950000,5000250003,490150490151,499999500000,500002500003,509949509950,33471093347110,49999995000000,50000025000003
%N A226788 Triangular numbers obtained as the concatenation of n and n+1.
%e A226788 If n=295, n//n+1 = 295296 = 768*769/2, a triangular number.
%t A226788 TriangularQ[n_] := IntegerQ[Sqrt[1 + 8*n]]; t = {}; Do[s = FromDigits[Join[IntegerDigits[n], IntegerDigits[n+1]]]; If[TriangularQ[s], AppendTo[t, s]], {n, 100000}]; t (* _T. D. Noe_, Jun 18 2013 *)
%t A226788 Select[FromDigits[Join[Flatten[IntegerDigits[#]]]]&/@Partition[ Range[ 5000010],2,1], OddQ[Sqrt[8#+1]]&] (* _Harvey P. Dale_, Jun 11 2015 *)
%o A226788 (PARI)
%o A226788 concatint(a,b)=eval(concat(Str(a),Str(b)))
%o A226788 istriang(x)=issquare(8*x+1)
%o A226788 {for(n=1,10^7,a=concatint(n,n+1);if(istriang(a),print(a)))}
%Y A226788 Cf. A003098, A068899, A226742, A226772, A226789.
%K A226788 nonn,base
%O A226788 1,1
%A A226788 _Antonio Roldán_, Jun 18 2013