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 A109731 #9 Jul 31 2015 18:00:48
%S A109731 1,-4,-6,15,23,-56,-86,209,321,-780,-1198,2911,4471,-10864,-16686,
%T A109731 40545,62273,-151316,-232406,564719,867351,-2107560,-3236998,7865521,
%U A109731 12080641,-29354524,-45085566,109552575,168261623,-408855776
%N A109731 a(n) = - 4*a(n-2) - a(n-4), a(0) = 1, a(1) = -4, a(2) = -6, a(3) = 15.
%C A109731 Sequence A002530 and A002531 are also generated by the floretion given in the program code.
%H A109731 Robert Munafo, <a href="http://www.mrob.com/pub/math/seq-floretion.html">Sequences Related to Floretions</a>
%H A109731 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (0, -4, 0, -1).
%F A109731 a(2n) = ((-1)^n)*A054491(n), a(2n+1) = ((-1)^n+1)*A001353(n+1). G.f. (1-4*x-2*x^2-x^3)/(x^4+4*x^2+1)
%p A109731 Floretion Algebra Multiplication Program, FAMP Code: 1lestesseq[A*B] with A = + .25'i + .25i' + 'ij' + .25'jk' + .25'kj' and B = + j' + k' + 'ii'.
%t A109731 LinearRecurrence[{0,-4,0,-1},{1,-4,-6,15},40] (* _Harvey P. Dale_, Mar 05 2013 *)
%Y A109731 Cf. A054491, A001353, A002530.
%K A109731 easy,sign
%O A109731 0,2
%A A109731 _Creighton Dement_, Aug 09 2005