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 A120353 #18 Sep 08 2022 08:45:25
%S A120353 121,1089,9801,88209,793881,7144929,64304361,578739249,5208653241,
%T A120353 46877879169,421900912521,3797108212689,34173973914201,
%U A120353 307565765227809,2768091887050281,24912826983452529,224215442851072761,2017938985659654849,18161450870936893641
%N A120353 Sum of 5 consecutive powers of 3, starting with a power of 9.
%C A120353 Always a square.
%H A120353 Vincenzo Librandi, <a href="/A120353/b120353.txt">Table of n, a(n) for n = 1..300</a>
%H A120353 Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>
%H A120353 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (9).
%F A120353 a(n) = 121*3^(2n-2).
%e A120353 a(2) = 3^2 + 3^3 + 3^4 + 3^5 + 3^6 = 121*(3^2) = 1089.
%t A120353 Total/@Select[Partition[3^Range[0,60],5,1],IntegerQ[Log[9, First[#]]]&] (* or *) Table[121 3^(2n-2),{n,30}] (* _Harvey P. Dale_, May 03 2011 *)
%o A120353 (Magma) [121*3^(2*n-2): n in [1..30]]; // _Vincenzo Librandi_, Jun 10 2011
%o A120353 (PARI) a(n)=121*9^(n-1) \\ _Charles R Greathouse IV_, Jul 11 2016
%K A120353 nonn,easy
%O A120353 1,1
%A A120353 _Anton Joha_, Jun 25 2006
%E A120353 Corrected and extended by _Harvey P. Dale_, May 03 2011