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 A083321 #30 Sep 08 2022 08:45:10
%S A083321 1,0,-4,18,-64,210,-664,2058,-6304,19170,-58024,175098,-527344,
%T A083321 1586130,-4766584,14316138,-42981184,129009090,-387158344,1161737178,
%U A083321 -3485735824,10458256050,-31376865304,94134790218,-282412759264,847255055010,-2541798719464,7625463267258
%N A083321 a(n) = (-1)^n + (-2)^n - (-3)^n.
%H A083321 Vincenzo Librandi, <a href="/A083321/b083321.txt">Table of n, a(n) for n = 0..1000</a>
%H A083321 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-6,-11,-6).
%F A083321 G.f.: (1+6*x+7*x^2)/((1+x)*(1+2*x)*(1+3*x)).
%F A083321 E.g.f.: exp(-x)+exp(-2*x)-exp(-3*x).
%F A083321 a(n) = (-1)^(n-1)*(3^n - 2^n - 1) for n >= 0. - _M. F. Hasler_, Apr 19 2020
%t A083321 a[n_]:=(-1)^n*(1+2^n-3^n); (* _Vladimir Joseph Stephan Orlovsky_, Dec 05 2008, corrected by _M. F. Hasler_, Apr 20 2020 *)
%t A083321 LinearRecurrence[{-6,-11,-6},{1,0,-4},30] (* _Harvey P. Dale_, Mar 05 2022 *)
%o A083321 (Magma) [(-1)^n+(-2)^n-(-3)^n: n in [0..30]]; // _Vincenzo Librandi_, Sep 05 2011
%o A083321 (PARI) apply( A083321(n)=(-1)^n*(1+2^n-3^n), [0..33]) \\ _M. F. Hasler_, Apr 19 2020
%Y A083321 Cf. A001550 (1^n + 2^n + 3^n).
%K A083321 easy,sign
%O A083321 0,3
%A A083321 _Paul Barry_, Apr 25 2003