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 A077828 #20 May 02 2025 14:29:19
%S A077828 1,3,12,48,189,747,2952,11664,46089,182115,719604,2843424,11235429,
%T A077828 44395371,175422672,693160416,2738935377,10822555395,42763953564,
%U A077828 168976333008,667688525901,2638286437419,10424853888984,41192486556912,162766880649945,643152663287523
%N A077828 Expansion of 1/(1-3*x-3*x^2-3*x^3).
%H A077828 Michael De Vlieger, <a href="/A077828/b077828.txt">Table of n, a(n) for n = 0..1675</a>
%H A077828 Yassine Otmani, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL28/Otmani/otmani10.html">The 2-Pascal Triangle and a Related Riordan Array</a>, J. Int. Seq. (2025) Vol. 28, Issue 3, Art. No. 25.3.5. See p. 19.
%H A077828 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,3,3).
%F A077828 a(n) = sum{k=0..n, T(n-k, k)3^(n-k)}, T(n, k) = trinomial coefficients (A027907). - _Paul Barry_, Feb 15 2005
%F A077828 a(n) = sum{k=0..n, sum{i=0..floor((n-k)/2), C(n-k-i, i)C(k, n-k-i)}*3^k}. - _Paul Barry_, Apr 26 2005
%t A077828 CoefficientList[Series[1/(1-3x-3x^2-3x^3),{x,0,30}],x] (* or *) LinearRecurrence[ {3,3,3},{1,3,12},30] (* _Harvey P. Dale_, Dec 25 2018 *)
%o A077828 (PARI) Vec(1/(1-3*x-3*x^2-3*x^3)+O(x^99)) \\ _Charles R Greathouse IV_, Sep 27 2012
%Y A077828 Partial sums of S(n, x), for x=1...12, A021823, A000217, A027941, A061278, A089817, A053142, A092521, A076765, A092420, A097784, A097826-7.
%Y A077828 Cf. A071675.
%K A077828 nonn,easy
%O A077828 0,2
%A A077828 _N. J. A. Sloane_, Nov 17 2002