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 A168572 #26 Sep 08 2022 08:45:49
%S A168572 0,49,392,2793,19600,137249,960792,6725593,47079200,329554449,
%T A168572 2306881192,16148168393,113037178800,791260251649,5538821761592,
%U A168572 38771752331193,271402266318400,1899815864228849,13298711049601992,93090977347213993
%N A168572 a(n) = Sum_{k=2..n}(7^k).
%H A168572 Vincenzo Librandi, <a href="/A168572/b168572.txt">Table of n, a(n) for n = 1..200</a>
%H A168572 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (8,-7).
%F A168572 a(n) = 7^n + a(n-1), with a(1)=0.
%F A168572 From _Robert Israel_, Sep 24 2014: (Start)
%F A168572 a(n) = 7*(7^n - 7)/6.
%F A168572 G.f.: 49*x^2/((1-x)*(1-7*x)).
%F A168572 E.g.f.: 7*(exp(7*x) - 7*exp(x)+42)/6. (End)
%F A168572 a(n) = A104896(n) - 7. - _Michel Marcus_, Sep 25 2014
%F A168572 a(n) = 8*a(n-1) - 7*a(n-2). - _G. C. Greubel_, Jul 26 2016
%t A168572 RecurrenceTable[{a[1] == 0, a[n] == a[n-1] + 7^n}, a, {n, 30}] (* _Vincenzo Librandi_, Sep 24 2014 *)
%t A168572 LinearRecurrence[{8,-7},{0,49}, 25] (* _G. C. Greubel_, Jul 26 2016 *)
%t A168572 Join[{0},Accumulate[7^Range[2,20]]] (* _Harvey P. Dale_, Jul 29 2019 *)
%o A168572 (Magma) [n le 1 select (n-1) else Self(n-1) + 7^n: n in [1..30] ]; // _Vincenzo Librandi_, Sep 24 2014
%o A168572 (PARI) a(n)=7*(7^n - 7)/6 \\ _Charles R Greathouse IV_, Jul 26 2016
%K A168572 nonn,easy
%O A168572 1,2
%A A168572 _Vincenzo Librandi_, Nov 30 2009
%E A168572 Definition and examples simplified by _Jon E. Schoenfield_, Jun 19 2010