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 A077856 #17 Sep 08 2022 08:45:07
%S A077856 1,3,6,9,10,6,-6,-27,-53,-72,-63,0,136,336,537,603,334,-471,-1878,
%T A077856 -3618,-4886,-4275,-45,9072,22465,35904,40272,22176,-31823,-126093,
%U A077856 -242538,-327159,-285686,-1674,609498,1506357,2404891,2693928,1476609,-2145600,-8461736,-16254480,-21901623
%N A077856 Expansion of (1-x)^(-1)/(1-2*x+x^2+x^3).
%H A077856 Vincenzo Librandi, <a href="/A077856/b077856.txt">Table of n, a(n) for n = 0..1000</a>
%H A077856 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (3,-3,0,1).
%F A077856 a(n) = Sum_{k=1..floor(n/3+1)} (-1)^k*binomial(n-k+3, 2*k). - _Vladeta Jovovic_, Feb 10 2003
%F A077856 a(n) = 3*a(n-1) - 3*a(n-2) + a(n-4). - _Chai Wah Wu_, May 25 2016
%t A077856 LinearRecurrence[{3, -3, 0, 1}, {1, 3, 6, 9}, 50] (* _Vincenzo Librandi_, May 27 2016 *)
%o A077856 (PARI) Vec((1-x)^(-1)/(1-2*x+x^2+x^3)+O(x^99)) \\ _Charles R Greathouse IV_, Sep 27 2012
%o A077856 (Magma) I:=[1,3,6,9]; [n le 4 select I[n] else 3*Self(n-1)-3*Self(n-2)+Self(n-4): n in [1..50]]; // _Vincenzo Librandi_, May 27 2016
%Y A077856 Cf. A078001.
%K A077856 sign,easy
%O A077856 0,2
%A A077856 _N. J. A. Sloane_, Nov 17 2002