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 A056504 #17 Oct 02 2019 01:23:37
%S A056504 1,1,2,2,5,5,12,14,33,41,90,122,259,365,756,1094,2233,3281,6642,9842,
%T A056504 19813,29525,59292,88574,177527,265721,532170,797162,1595443,2391485,
%U A056504 4785156,7174454,14352233,21523361,43053282,64570082,129150085,193710245,387440172,581130734,1162291121
%N A056504 Number of periodic palindromic structures of length n using a maximum of three different symbols.
%C A056504 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure.
%D A056504 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 A056504 Andrew Howroyd, <a href="/A056504/b056504.txt">Table of n, a(n) for n = 0..200</a>
%F A056504 a(n) = Sum_{k=1..3} A285012(n, k) for n > 0. - _Andrew Howroyd_, Oct 01 2019
%Y A056504 Cf. A007051, A285012.
%K A056504 nonn
%O A056504 0,3
%A A056504 _Marks R. Nester_
%E A056504 a(17)-a(35) from _Andrew Howroyd_, Apr 07 2017
%E A056504 a(0)=1 prepended and terms a(36) and beyond from _Andrew Howroyd_, Oct 01 2019