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 A084240 #21 Dec 13 2021 16:36:19
%S A084240 1,0,-4,20,-84,340,-1364,5460,-21844,87380,-349524,1398100,-5592404,
%T A084240 22369620,-89478484,357913940,-1431655764,5726623060,-22906492244,
%U A084240 91625968980,-366503875924,1466015503700,-5864062014804,23456248059220,-93824992236884,375299968947540
%N A084240 a(n) = -5*a(n-1) - 4*a(n-2), a(0)=1, a(1)=0.
%C A084240 A Jacobsthal related sequence.
%C A084240 Inverse binomial transform of A084246.
%H A084240 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (-5,-4).
%F A084240 G.f.: (1+5*x)/((1+x)*(1+4*x)).
%F A084240 E.g.f.: (4*exp(-x) - exp(-4*x))/3.
%F A084240 a(n) = A084241(n) + (-1)^n
%F A084240 a(n) = (-1)^(n+1)*(A002450(n)-1) = (-1)^(n+1)*(A001045(2*n)-1).
%F A084240 a(n) = (4*(-1)^n - (-4)^n)/3.
%F A084240 a(n) = (-1)^(n-1) * A080674(n-1). - _Falk Hüffner_, Jan 11 2021
%t A084240 LinearRecurrence[{-5,-4},{1,0},30] (* _Harvey P. Dale_, Dec 13 2021 *)
%Y A084240 Cf. A001045, A002450, A080674, A084241.
%K A084240 easy,sign
%O A084240 0,3
%A A084240 _Paul Barry_, May 23 2003