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 A083943 #9 Jul 28 2020 12:58:45
%S A083943 0,1,-1,-1,-5,-9,-21,-41,-85,-169,-341,-681,-1365,-2729,-5461,-10921,
%T A083943 -21845,-43689,-87381,-174761,-349525,-699049,-1398101,-2796201,
%U A083943 -5592405,-11184809,-22369621,-44739241,-89478485,-178956969,-357913941,-715827881,-1431655765
%N A083943 A generalized Jacobsthal sequence.
%C A083943 a(2n)=-A001045(2n), a(2n+1)=2-A001045(2n+1).
%H A083943 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (2,1,-2).
%F A083943 a(n)=1-2^n/3-2(-1)^n/3; a(n)=a(n-1)+2a(n-2)-2, a(0)=0, a(1)=1; G.f.: x(1-3x)/((1+x)(1-x)(1-2x)); E.g.f.: exp(x)-(exp(2x)+2exp(-x))/3.
%t A083943 LinearRecurrence[{2,1,-2},{0,1,-1},50] (* _Harvey P. Dale_, Jul 28 2020 *)
%Y A083943 Cf. A083944.
%K A083943 easy,sign
%O A083943 0,5
%A A083943 _Paul Barry_, May 09 2003