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 A056505 #12 Oct 02 2019 01:23:46
%S A056505 1,1,2,2,5,5,13,15,40,51,133,187,479,715,1815,2795,7036,11051,27747,
%T A056505 43947,110061,175275,438683,700075,1750959,2798251,6997643,11188907,
%U A056505 27975351,44747435,111876715,178973355,447445692,715860651,1789684267,2863377067,7158491875
%N A056505 Number of periodic palindromic structures using a maximum of four different symbols.
%C A056505 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure.
%D A056505 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 A056505 Andrew Howroyd, <a href="/A056505/b056505.txt">Table of n, a(n) for n = 0..200</a>
%F A056505 a(n) = Sum_{k=1..4} A285012(n, k) for n > 0. - _Andrew Howroyd_, Oct 01 2019
%Y A056505 Cf. A007581, A285012.
%K A056505 nonn
%O A056505 0,3
%A A056505 _Marks R. Nester_
%E A056505 Corrected by _T. D. Noe_, Oct 25 2006
%E A056505 a(0)=1 prepended and terms a(17) and beyond from _Andrew Howroyd_, Oct 01 2019