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 A056507 #12 Oct 02 2019 01:23:54
%S A056507 1,1,2,2,5,5,13,15,41,52,144,203,577,876,2587,4111,12589,20648,65519,
%T A056507 109299,356613,601492,2005545,3403127,11522829,19628064,67183531,
%U A056507 114700315,395497312,676207628,2343265049,4010090463,13942486653,23874362200,83192239491,142508723651
%N A056507 Number of periodic palindromic structures using a maximum of six different symbols.
%C A056507 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure.
%D A056507 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 A056507 Andrew Howroyd, <a href="/A056507/b056507.txt">Table of n, a(n) for n = 0..200</a>
%F A056507 a(n) = Sum_{k=1..6} A285012(n, k) for n > 0. - _Andrew Howroyd_, Oct 01 2019
%Y A056507 Cf. A056471, A285012.
%K A056507 nonn
%O A056507 0,3
%A A056507 _Marks R. Nester_
%E A056507 Corrected by _T. D. Noe_, Oct 25 2006
%E A056507 a(0)=1 prepended and terms a(16) and beyond from _Andrew Howroyd_, Oct 01 2019