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 A331890 #20 Sep 08 2022 08:46:25
%S A331890 3,-1,-1,8,-9,-1,26,-43,15,80,-181,131,210,-703,755,368,-2529,3671,
%T A331890 -406,-8323,16071,-8560,-24157,64859,-57822,-55351,242891,-303184,
%U A331890 -50409,839375,-1395334,455141,2618943,-5864752,4156091,6946547,-22832142,24197777
%N A331890 a(n) = -a(n-1) - a(n-2) + 2*a(n-3) with a(0)=3, a(1)=-1, a(2)=-1.
%C A331890 a(n) is the reflected sequence (cf. A074058) of the generalized tribonacci sequence b(n) with b(0) = 3 and b(n) = A186575(n-1) for n > 0.
%H A331890 Mario Catalani, <a href="http://arxiv.org/abs/math/0210201">Polymatrix and Generalized Polynacci Numbers</a>, arXiv:math/0210201 [math.CO], 2002.
%H A331890 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,-1,2).
%F A331890 G.f.: (3 + 2*x + x^2)/(1 + x + x^2 - 2*x^3).
%F A331890 a(n) = 3*A077975(n)+2*A077975(n-1)+A077975(n-2). - _R. J. Mathar_, Feb 28 2020
%t A331890 LinearRecurrence[{-1,-1,2},{3,-1,-1},38] (* _Stefano Spezia_, Jan 31 2020 *)
%o A331890 (Magma) a:=[3,-1,-1]; [n le 3 select a[n] else -Self(n-1)-Self(n-2)+2*Self(n-3):n in [1..30]]; // _Marius A. Burtea_, Feb 02 2020
%Y A331890 Cf. A074058, A186575.
%K A331890 easy,sign
%O A331890 0,1
%A A331890 _Wojciech Florek_, Jan 30 2020
%E A331890 Definition clarified by _N. J. A. Sloane_, Apr 23 2020