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 A056506 #12 Oct 02 2019 01:23:49
%S A056506 1,1,2,2,5,5,13,15,41,52,143,202,564,855,2433,3845,11101,18002,52662,
%T A056506 86472,255128,422005,1252898,2079475,6197590,10306752,30796243,
%U A056506 51263942,153411664,255514355,765393943,1275163905,3822001141,6368612302,19095317552,31821472612
%N A056506 Number of periodic palindromic structures using a maximum of five different symbols.
%C A056506 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure.
%D A056506 M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. [See A056391 for pdf file of Chap. 2]
%H A056506 Andrew Howroyd, <a href="/A056506/b056506.txt">Table of n, a(n) for n = 0..200</a>
%F A056506 a(n) = Sum_{k=1..5} A285012(n, k) for n > 0. - _Andrew Howroyd_, Oct 01 2019
%Y A056506 Cf. A056470, A285012.
%K A056506 nonn
%O A056506 0,3
%A A056506 _Marks R. Nester_
%E A056506 Corrected by _T. D. Noe_, Oct 25 2006
%E A056506 a(0)=1 prepended and terms a(17) and beyond from _Andrew Howroyd_, Oct 01 2019