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 A089181 #44 Jan 01 2024 11:26:52
%S A089181 1,2,-3,-20,-19,102,337,-40,-2439,-4598,7877,47940,40741,-254098,
%T A089181 -793383,191920,5937521,10531602,-20499443,-114720100,-85944099,
%U A089181 631152502,1863913697,-690240120,-14427876119,-24024071398,52946990037,274062479860,177496029461
%N A089181 (1,3) entry of powers of the orthogonal design shown in A090592.
%H A089181 Harvey P. Dale, <a href="/A089181/b089181.txt">Table of n, a(n) for n = 1..1000</a>
%H A089181 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (2, -7).
%F A089181 a(n) = 2*a(n-1) - 7*a(n-2); a(1)=1, a(2)=2. - _T. D. Noe_, Dec 11 2006
%F A089181 G.f.: x/(1 - 2*x + 7*x^2). - _Philippe Deléham_, Mar 04 2012
%t A089181 LinearRecurrence[{2,-7},{1,2},40] (* _Harvey P. Dale_, Nov 04 2011 *)
%o A089181 (Sage) [lucas_number1(n,2,7) for n in range(1, 18)] # _Zerinvary Lajos_, Apr 23 2009
%o A089181 (PARI) x='x+O('x^30); Vec(x/(1-2*x+7*x^2)) \\ _G. C. Greubel_, Oct 22 2018
%o A089181 (Magma) I:=[1,2]; [n le 2 select I[n] else 2*Self(n-1) - 7*Self(n-2): n in [1..30]]; // _G. C. Greubel_, Oct 22 2018
%o A089181 (GAP) a:=[1,2];; for n in [3..30] do a[n]:=2*a[n-1]-7*a[n-2]; od; a; # _Muniru A Asiru_, Oct 23 2018
%K A089181 sign
%O A089181 1,2
%A A089181 _Simone Severini_, Dec 08 2003
%E A089181 Corrected by _T. D. Noe_, Dec 11 2006
%E A089181 Extended by _T. D. Noe_, May 23 2011