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 A187126 #18 May 19 2021 21:01:12
%S A187126 0,1,3,5,6,10,11,15,16,20,21,25,26,28,30,31,35,36,40,41,45,46,50,51,
%T A187126 55,56,60,61,65,66,70,71,75,76,78,80,81,85,86,90,91,95,96,100,101,105,
%U A187126 106,110,111,115,116,120,121,125,126,128,130,131,135,136,140
%N A187126 Triangular numbers k*(k+1)/2 mod 1000, sorted and uniqued.
%C A187126 Possible last three digits of triangular numbers k*(k+1)/2 (leading zeros omitted).
%C A187126 All triangular numbers less than 1000 belong to this sequence. The sequence is finite with 424 terms: a(424)=996 is the last term.
%H A187126 Shyam Sunder Gupta, <a href="/A187126/b187126.txt">Table of n, a(n) for n = 1..424</a>
%e A187126 The last three digits of k*(k+1)/2 can be 000, 001, 003, 021, 140, etc., but not 002, 004, 012, 139 etc.
%t A187126 Union[Table[Mod[n*(n + 1)/2, 1000], {n, 1, 1000}]]
%Y A187126 Cf. A000217, A122986.
%K A187126 base,fini,full,nonn
%O A187126 1,3
%A A187126 _Shyam Sunder Gupta_, Aug 30 2013