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 A302355 #6 Apr 06 2018 17:28:46
%S A302355 1,6,72,1390,37515,1307691,56000728,2847503268,167737660533,
%T A302355 11236731677941,843757483026150,70200772129462767,6410711453857626149,
%U A302355 637516967943664853331,68581800216461580653064,7935677122691714769565104,982824624566131043920711329,129722104862557293606783635718
%N A302355 a(n) = coefficient of x^n in the n-th iteration (n-fold self-composition) of the g.f. of triangular numbers (A000217).
%H A302355 N. J. A. Sloane, <a href="/transforms.txt">Transforms</a>
%H A302355 <a href="/index/Pol#polygonal_numbers">Index to sequences related to polygonal numbers</a>
%e A302355 The initial coefficients of successive iterations of g.f. A(x) = x/(1 - x)^3 are as follows:
%e A302355 n = 1: 0, (1), 3, 6, 10, 15, ... g.f. A(x)
%e A302355 n = 2: 0, 1, (6), 30, 137, 588, ... g.f. A(A(x))
%e A302355 n = 3: 0, 1, 9, (72), 543, 3933, ... g.f. A(A(A(x)))
%e A302355 n = 4: 0, 1, 12, 132, (1390), 14208, ... g.f. A(A(A(A(x))))
%e A302355 n = 5: 0, 1, 15, 210, 2840, (37515), ... g.f. A(A(A(A(A(x)))))
%t A302355 Table[SeriesCoefficient[Nest[Function[x, x/(1 - x)^3], x, n], {x, 0, n}], {n, 18}]
%Y A302355 Cf. A000217, A030280, A119821, A302356.
%K A302355 nonn
%O A302355 1,2
%A A302355 _Ilya Gutkovskiy_, Apr 06 2018