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 A089978 #14 Apr 30 2017 02:58:59
%S A089978 1,3,9,30,99,324,1062,3483,11421,37449,122796,402651,1320300,4329288,
%T A089978 14195817,46548351,152632917,500486202,1641103659,5381209728,
%U A089978 17645087790,57858574347,189719352225,622093320045,2039855683176
%N A089978 Expansion of 1/(1-3x-3x^3).
%H A089978 G. C. Greubel, <a href="/A089978/b089978.txt">Table of n, a(n) for n = 0..1000</a>
%H A089978 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (3,0,3).
%F A089978 a(n) = Sum_{k=0..floor(n/2)} C(n-2*k, k)3^(n-2*k).
%F A089978 a(0)=1, a(1)=3, a(2)=9, a(n)=3*a(n-1)+3*a(n-3). - _Harvey P. Dale_, Jul 22 2015
%e A089978 G.f. = 1 + 3*x + 9*x^2 + 30*x^3 + 99*x^4 + 324*x^5 + 1062*x^6 + 3483*x^7 + ...
%t A089978 CoefficientList[Series[1/(1-3x-3x^3),{x,0,30}],x] (* or *) LinearRecurrence[ {3,0,3},{1,3,9},30] (* _Harvey P. Dale_, Jul 22 2015 *)
%o A089978 (PARI) {a(n) = sum(k=0, n\3, binomial(n - 2*k, k) * 3^(n - 2*k))}; /* _Michael Somos_, Jan 30 2015 */
%o A089978 (PARI) {a(n) = if( n<0, 0, polcoeff( 1 / (1 - 3*x - 3*x^3) + x * O(x^n), n))}; /* _Michael Somos_, Jan 30 2015 */
%Y A089978 Cf. A052912, A000930.
%K A089978 easy,nonn
%O A089978 0,2
%A A089978 _Paul Barry_, Nov 18 2003