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 A119238 #17 Feb 24 2024 01:15:55
%S A119238 1,5,19,25,199,625,1999,2102,5582,9376,9909,10100,16833,19999,41012,
%T A119238 55741,74491,83885,88102,90625,151774,199999,201002,558257,626522,
%U A119238 740312,798097,810062,890625,1001000,1030563,1668333,1999999,2646751
%N A119238 Numbers k such that the decimal representation of k is contained as substring in that of the k-th triangular number.
%H A119238 Giovanni Resta, <a href="/A119238/b119238.txt">Table of n, a(n) for n = 1..101</a> (terms < 10^12)
%t A119238 Select[Range@100000, ({}!=StringPosition[ToString[ #(#+1)/2], ToString@# ]) &]
%t A119238 Select[Range[3*10^6],SequenceCount[IntegerDigits[(#(#+1))/2], IntegerDigits[ #]]>0&] (* The program uses the SequenceCount function from Mathematica version 10 *) (* _Harvey P. Dale_, Dec 26 2015 *)
%Y A119238 Cf. A046831.
%K A119238 base,nonn
%O A119238 1,2
%A A119238 _Giovanni Resta_, May 10 2006