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 A103143 #15 Aug 07 2022 07:39:10
%S A103143 0,0,1,1,2,6,11,23,52,108,229,493,1046,2226,4751,10115,21544,45912,
%T A103143 97801,208345,443882,945630,2014547,4291823,9143260,19478724,41497453,
%U A103143 88405957,188339582,401237898,854795351,1821051995,3879561040,8264999088,17607716113
%N A103143 a(n) = a(n-1) + a(n-2) + 3*a(n-3), with a(0) = 1, a(1) = 0, a(2) = 1.
%C A103143 Diagonal sums of the Pascal-(1,3,1) triangle A081578.
%H A103143 R. J. Mathar, <a href="https://arxiv.org/abs/1406.7788">Tilings of rectangular regions by rectangular tiles: Counts derived from transfer matrices</a>, arXiv:1406.7788 [math.CO], 2014.
%H A103143 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (1,1,3).
%F A103143 G.f.: 1/(1-x-x^2-3x^3).
%t A103143 LinearRecurrence[{1, 1, 3}, {0, 0, 1}, 50] (* _Vladimir Joseph Stephan Orlovsky_, May 25 2011 *)
%Y A103143 Cf. A077947, A081578.
%K A103143 easy,nonn
%O A103143 0,5
%A A103143 _Paul Barry_, Jan 24 2005
%E A103143 Name clarified by _Michel Marcus_, Aug 07 2022