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 A187127 #15 May 19 2021 21:39:36
%S A187127 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 A187127 53,55,56,60,61,65,66,70,71,75,76,78,80,81,85,86,90,91,95,96
%N A187127 Triangular numbers k*(k+1)/2 mod 100, sorted and uniqued.
%C A187127 Possible last two digits of triangular numbers k*(k+1)/2 (leading zeros omitted).
%C A187127 All triangular numbers less than 100 belong to this sequence. The sequence is finite with 44 terms: a(44)=96 is the last term.
%e A187127 The last two digits of k*(k+1)/2 can be 00, 01, 03, 20, 91, etc., but not 02, 04, 12, 97, etc.
%t A187127 Union[Table[Mod[n*(n + 1)/2, 100], {n, 1, 100}]]
%Y A187127 Cf. A000217, A010461.
%K A187127 base,fini,full,nonn
%O A187127 1,3
%A A187127 _Shyam Sunder Gupta_, Aug 30 2013