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 A383942 #24 Aug 27 2025 09:12:18
%S A383942 78,8778,887778,88877778,8888777778,888887777778,88888877777778,
%T A383942 8888888777777778,888888887777777778,88888888877777777778,
%U A383942 8888888888777777777778,888888888887777777777778,88888888888877777777777778,8888888888888777777777777778
%N A383942 a(n) = (8*10^(2n) - 10^(n+1) + 2) / 9.
%C A383942 This is one of four infinite families of triangular numbers consisting of two different digits. The other three families are A319170, A037156 (n>1), and A309597 (n>2).
%H A383942 Paolo Xausa, <a href="/A383942/b383942.txt">Table of n, a(n) for n = 1..495</a>
%H A383942 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (111,-1110,1000).
%F A383942 a(n) = A000217(A073551(n+1)).
%F A383942 G.f.: 6*x*(13 + 20*x)/((1 - x)*(1 - 10*x)*(1 - 100*x)). - _Stefano Spezia_, Aug 19 2025
%t A383942 A383942[n_] := (8*10^(2*n) - 10^(n+1) + 2)/9; Array[A383942, 15] (* or *)
%t A383942 LinearRecurrence[{111, -1110, 1000}, {78, 8778, 887778}, 15] (* _Paolo Xausa_, Aug 27 2025 *)
%o A383942 (Python) def A383942(n): return (8*10**(2*n)-10**(n+1)+2)//9
%Y A383942 Cf. A000217, A037156, A062691, A073551, A256340, A309597, A319170.
%Y A383942 Subsequence of: A119216, A119230, A119236.
%K A383942 nonn,base,easy,changed
%O A383942 1,1
%A A383942 _David Radcliffe_, Aug 18 2025