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 A018038 #10 Sep 08 2022 08:44:44
%S A018038 1,3,8,21,58,160,441,1217,3357,9261,25551,70492,194481,536559,1480324,
%T A018038 4084101,11267725,31086799,85766121,236622225,652822777,1801088541,
%U A018038 4969066720,13709278307,37822859361,104350401111,287894844441,794280046581,2191358423330
%N A018038 Powers of cube root of 21 rounded up.
%H A018038 Vincenzo Librandi, <a href="/A018038/b018038.txt">Table of n, a(n) for n = 0..200</a>
%t A018038 Table[Ceiling[21^(n/3)], {n, 0, 40}] (* _Vincenzo Librandi_, Jan 10 2014 *)
%o A018038 (Magma) [Ceiling(21^(n/3)): n in [0..40]]; // _Vincenzo Librandi_, Jan 10 2014
%Y A018038 Cf. powers of cube root of k ceiling up: A017981 (k=2), A017984 (k=3), A017987 (k=4), A017990 (k=5), A017993 (k=6), A017996 (k=7), A018002 (k=9), A018005 (k=10), A018008 (k=11), A018011 (k=12), A018014 (k=13), A018017 (k=14), A018020 (k=15), A018023 (k=16), A018026 (k=17), A018029 (k=18), A018032 (k=19), A018035 (k=20), this sequence (k=21), A018041 (k=22), A018044 (k=23), A018047 (k=24).
%K A018038 nonn
%O A018038 0,2
%A A018038 _N. J. A. Sloane_
%E A018038 More terms from _Vincenzo Librandi_, Jan 10 2014