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 A099856 #29 Aug 23 2024 10:03:08
%S A099856 1,6,18,54,162,486,1458,4374,13122,39366,118098,354294,1062882,
%T A099856 3188646,9565938,28697814,86093442,258280326,774840978,2324522934,
%U A099856 6973568802,20920706406,62762119218,188286357654,564859072962,1694577218886,5083731656658,15251194969974,45753584909922
%N A099856 Expansion of (1+3*x)/(1-3*x).
%C A099856 A099858 gives a Chebyshev transform. Binomial transform is A083420.
%C A099856 Hankel transform is 1, -18, 0, 0, 0, 0, 0, 0, 0, ... - _Philippe Deléham_, Dec 13 2011
%H A099856 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (3).
%F A099856 a(n) = 2*3^n - 0^n.
%F A099856 a(n) = A025192(n+1), n > 0. - _R. J. Mathar_, Sep 02 2008
%F A099856 a(n) = Sum_{k=0..n} A093561(n,k)*2^k. - _Philippe Deléham_, Dec 13 2011
%F A099856 From _Elmo R. Oliveira_, Aug 23 2024: (Start)
%F A099856 E.g.f.: 2*exp(3*x) - 1.
%F A099856 a(n) = 3*a(n-1) for n > 1. (End)
%t A099856 CoefficientList[Series[(1+3x)/(1-3x),{x,0,30}],x] (* or *) Join[{1}, NestList[3#&,6,30]] (* _Harvey P. Dale_, Nov 08 2011 *)
%o A099856 (PARI) Vec((1+3*x)/(1-3*x) + O(x^40)) \\ _Michel Marcus_, Dec 11 2015
%Y A099856 Cf. A025192, A083420, A093561, A099858.
%K A099856 easy,nonn
%O A099856 0,2
%A A099856 _Paul Barry_, Oct 28 2004
%E A099856 a(26)-a(28) from _Elmo R. Oliveira_, Aug 23 2024