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 A013768 #21 Sep 08 2022 08:44:38 %S A013768 21,194481,1801088541,16679880978201,154472377739119461, %T A013768 1430568690241985328321,13248496640331026125580781, %U A013768 122694327386105632949003612841,1136272165922724266740722458520501,10523016528610349434285830688358359761 %N A013768 a(n) = 21^(3*n + 1). %H A013768 Vincenzo Librandi, <a href="/A013768/b013768.txt">Table of n, a(n) for n = 0..200</a> %H A013768 Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a> %H A013768 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (9261). %t A013768 21^(3*Range[0,10]+1) (* or *) NestList[9261#&,21,10] (* _Harvey P. Dale_, Apr 30 2020 *) %o A013768 (Magma) [21^(3*n+1): n in [0..10]]; // _Vincenzo Librandi_, Jun 27 2011 %o A013768 (PARI) a(n)=21^(3*n+1) \\ _Charles R Greathouse IV_, Jul 10 2016 %Y A013768 Subsequence of A009965. %K A013768 nonn,easy %O A013768 0,1 %A A013768 _N. J. A. Sloane_