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 A102000 #18 Dec 11 2022 11:47:31
%S A102000 1,2,4,8,32,80,208,560,1552,4144,11152,30128,81424,219440,592016,
%T A102000 1597616,4310800,11629616,31377808,84661168,228421648,616292144,
%U A102000 1662802576,4486362800,12104509712,32658782768,88115674000,237742180784
%N A102000 a(0),...,a(3) = 1, 2, 4, 8; thereafter a(n) = a(n-1) + 2*a(n-2) + 4*a(n-3) + 8*a(n-4), n>3.
%C A102000 Based on taking the n-th power of the matrix M = [1 1 1 1 / 2 0 0 0 / 0 2 0 0 / 0 0 2 0] that generates the D_4 lattice.
%C A102000 a(n)/a(n-1) tends to 2.698068913... an eigenvalue of M and a root of the characteristic polynomial x^4 - x^3 - 2*x^2 - 4*x - 8.
%H A102000 J. H. Conway and N. J. A. Sloane, <a href="http://neilsloane.com/doc/splag.html">Sphere Packings, Lattices and Groups</a>, Springer-Verlag, 3rd edition, 1999. (See Chap. 4.)
%H A102000 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (1,2,4,8).
%F A102000 G.f.: (-1-x+4*x^3)/(-1+x+2*x^2+4*x^3+8*x^4). [_R. J. Mathar_, Feb 13 2010]
%t A102000 LinearRecurrence[{1, 2, 4, 8}, {1, 2, 4, 8}, 28] (* _Hugo Pfoertner_, Dec 11 2022 *)
%K A102000 nonn,easy
%O A102000 0,2
%A A102000 _Gary W. Adamson_, Dec 23 2004
%E A102000 More terms from _R. J. Mathar_, Feb 13 2010
%E A102000 Edited by _N. J. A. Sloane_, Dec 11 2022