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 A112355 #2 Mar 30 2012 17:36:43
%S A112355 3,10,15,21,28,36,45,55,66,78,91,105,120,136,153,171,190,210,231,253,
%T A112355 276,300,325,351,378,406,435,465,496,528,561,595,630,666,703,741,780,
%U A112355 820,861,903,946,990,1035,1081,1128,1176,1225,1275,1326
%N A112355 Triangular numbers that are the sum of three positive triangular numbers.
%C A112355 A112353 is a subsequence: it requires the three positive triangular numbers to be distinct. The positive terms of A076140 are a subsequence.
%e A112355 21 is a term because 21 = 3 + 3 + 15 and these four numbers are positive triangular numbers (A000217(6) = A000217(2) + A000217(2) + A000217(5)). {Also 21 = 1 + 10 + 10 = A000217(1) + A000217(4) + A000217(4).}.
%Y A112355 Cf. A000217 (triangular numbers), A089982 (triangular numbers that are the sum of two positive triangular numbers), A112353, A076140 (the three triangular numbers summed are identical).
%K A112355 nonn
%O A112355 1,1
%A A112355 _Rick L. Shepherd_, Sep 06 2005