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 A095864 #8 Jul 27 2012 18:50:36
%S A095864 6,91,990,9870,99681,998991,9997156,99991011,999961560,9999878910,
%T A095864 99999957291,999999911791,9999997965180,99999998250180,
%U A095864 999999997764120,9999999895728690,99999999552805215,999999998765257141
%N A095864 Largest n-digit triangular number.
%C A095864 Triangular number with index given by A095863.
%F A095864 a(n) = A068093(n+1) - A068092(n+1).
%t A095864 Join[{6},(#(#+1))/2&/@Table[Floor[(Sqrt[8*10^n+1]-1)/2],{n,2,30}]] (* _Harvey P. Dale_, Jul 27 2012 *)
%t A095864 triInverse[n_] := Floor[(Sqrt[1 + 8*n] - 1)/2]; tri[n_] := n*(n+1)/2; Table[tri[triInverse[10^n - 1]], {n, 18}] (* _T. D. Noe_, Jul 27 2012 *)
%Y A095864 Cf. A068092, A068093, A068094, A095863, A095865, A095866.
%K A095864 base,easy,nonn
%O A095864 1,1
%A A095864 _Ray Chandler_, Jun 28 2004