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%I #18 May 19 2021 21:01:12
%S 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 55,56,60,61,65,66,70,71,75,76,78,80,81,85,86,90,91,95,96,100,101,105,
%U 106,110,111,115,116,120,121,125,126,128,130,131,135,136,140
%N Triangular numbers k*(k+1)/2 mod 1000, sorted and uniqued.
%C Possible last three digits of triangular numbers k*(k+1)/2 (leading zeros omitted).
%C 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 Shyam Sunder Gupta, <a href="/A187126/b187126.txt">Table of n, a(n) for n = 1..424</a>
%e 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 Union[Table[Mod[n*(n + 1)/2, 1000], {n, 1, 1000}]]
%Y Cf. A000217, A122986.
%K base,fini,full,nonn
%O 1,3
%A _Shyam Sunder Gupta_, Aug 30 2013
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