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 A013726 #30 Jul 11 2025 10:13:40
%S A013726 21,9261,4084101,1801088541,794280046581,350277500542221,
%T A013726 154472377739119461,68122318582951682301,30041942495081691894741,
%U A013726 13248496640331026125580781,5842587018385982521381124421,2576580875108218291929075869661,1136272165922724266740722458520501
%N A013726 a(n) = 21^(2*n + 1).
%H A013726 Vincenzo Librandi, <a href="/A013726/b013726.txt">Table of n, a(n) for n = 0..200</a>
%H A013726 Tanya Khovanova, <a href="http://www.tanyakhovanova.com/RecursiveSequences/RecursiveSequences.html">Recursive Sequences</a>.
%H A013726 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (441).
%F A013726 From _Philippe Deléham_, Nov 28 2008: (Start)
%F A013726 a(n) = 441*a(n-1); a(0)=21.
%F A013726 G.f.: 21/(1-441*x). (End)
%F A013726 a(n) = A009965(A005408(n)). - _Wesley Ivan Hurt_, Feb 10 2014
%F A013726 From _Elmo R. Oliveira_, Jul 10 2025: (Start)
%F A013726 E.g.f.: 21*exp(441*x).
%F A013726 a(n) = A013708(n)*A013712(n). (End)
%p A013726 seq(21^(2*n+1),n=0..10); # _Nathaniel Johnston_, Jun 25 2011
%t A013726 Table[21^(2 n + 1), {n, 0, 15}] (* _Wesley Ivan Hurt_, Feb 10 2014 *)
%o A013726 (Magma) [21^(2*n+1): n in [0..15]]; // _Vincenzo Librandi_, Jun 26 2011
%o A013726 (PARI) a(n)=21^(2*n+1) \\ _Charles R Greathouse IV_, Jul 11 2016
%Y A013726 Bisection of A009965 (21^n).
%Y A013726 Cf. A005408, A013708-A013725, A013727-A013729.
%K A013726 nonn,easy
%O A013726 0,1
%A A013726 _N. J. A. Sloane_