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 A014920 #30 May 16 2025 07:19:09
%S A014920 1,15,162,1534,13539,114381,937924,7526268,59409477,462945547,
%T A014920 3570173286,27298094202,207234827815,1563680973513,11737027066248,
%U A014920 87698011225336,652657830908553,4840007082678279,35779865442976810,263757769150439670,1939395361400291691,14227404371232539845
%N A014920 a(1)=1, a(n) = n*7^(n-1) + a(n-1).
%H A014920 Vincenzo Librandi, <a href="/A014920/b014920.txt">Table of n, a(n) for n = 1..1000</a>
%H A014920 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (15,-63,49).
%F A014920 From _Vincenzo Librandi_, Oct 23 2012: (Start)
%F A014920 a(n) = 14*a(n-1) - 49*a(n-2) + 1, a(1)=1, a(2)=15.
%F A014920 G.f.: x/((1-x)*(1-7*x)^2). (End)
%F A014920 a(n) = (1/36)*(1 + 7^n*(6*n-1)). - _Vincenzo Librandi_, Oct 26 2012
%F A014920 a(n) = 15*a(n-1) - 63*a(n-2) + 49*a(n-3), a(1)=1, a(2)=15, a(3)=162. - _Harvey P. Dale_, Jun 26 2013
%F A014920 E.g.f.: exp(x)*(1 + exp(6*x)*(42*x - 1))/36. - _Elmo R. Oliveira_, May 15 2025
%p A014920 a:=n->sum (7^n-7^j, j=0..n): seq(a(n)/6, n=1..21); # _Zerinvary Lajos_, Dec 14 2008
%t A014920 CoefficientList[Series[1/((1 - x)(1 - 7*x)^2), {x, 0, 40}], x] (* _Vincenzo Librandi_, Oct 23 2012 *)
%t A014920 LinearRecurrence[{15,-63,49},{1,15,162},30] (* _Harvey P. Dale_, Jun 26 2013 *)
%o A014920 (Magma) I:=[1, 15]; [n le 2 select I[n] else 14*Self(n-1)-49*Self(n-2)+1: n in [1..30]]; // _Vincenzo Librandi_, Oct 23 2012
%o A014920 (Magma) [(1/36)*(1+7^n*(6*n-1)): n in [1..20]]; // _Vincenzo Librandi_, Oct 26 2012
%K A014920 nonn,easy
%O A014920 1,2
%A A014920 _Olivier Gérard_