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 A125691 #19 Feb 01 2021 00:14:58
%S A125691 1,1,4,6,17,31,76,152,349,729,1624,3462,7605,16367,35720,77216,168009,
%T A125691 363937,790748,1714550,3722857,8075759,17529780,38034200,82547781,
%U A125691 179120601,388729744,843543766,1830612397,3972513951,8620807376,18707736832,40597645009,88100048129
%N A125691 Expansion of 1/(1 - x - 3x^2 + x^3).
%C A125691 Antidiagonal sums of number triangle A125690.
%H A125691 Indranil Ghosh, <a href="/A125691/b125691.txt">Table of n, a(n) for n = 0..200</a>
%H A125691 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,3,-1).
%t A125691 CoefficientList[Series[1/(1 - x - 3x^2 + x^3), {x, 0, 28}], x] (* _Indranil Ghosh_, Mar 10 2017 *)
%t A125691 LinearRecurrence[{1,3,-1},{1,1,4},40] (* _Harvey P. Dale_, Jul 13 2019 *)
%o A125691 (PARI) Vec(1/(1 - x - 3*x^2 + x^3) + O(x^29)) \\ _Indranil Ghosh_, Mar 10 2017
%Y A125691 Cf. A125690.
%K A125691 easy,nonn
%O A125691 0,3
%A A125691 _Paul Barry_, Nov 30 2006