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 A056509 #22 Oct 01 2019 18:28:24 %S A056509 0,0,0,1,1,5,6,19,25,64,90,208,301,656,966,2035,3025,6250,9330,19035, %T A056509 28501,57740,86526,174436,261625,525994,788970,1583119,2375101, %U A056509 4760516,7141686,14303011,21457825,42954850,64439010,128953341,193448101,387046700,580606446,1161504423 %N A056509 Number of periodic palindromic structures of length n using exactly three different symbols. %D A056509 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 A056509 Andrew Howroyd, <a href="/A056509/b056509.txt">Table of n, a(n) for n = 1..200</a> %F A056509 a(n) = A056504(n) - A056503(n). %F A056509 Inverse Moebius transform of A056519. - _Andrew Howroyd_, Oct 01 2019 %e A056509 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure. %Y A056509 Column 3 of A285012. %Y A056509 Cf. A000392, A056503, A056504, A056519. %K A056509 nonn %O A056509 1,6 %A A056509 _Marks R. Nester_ %E A056509 a(17)-a(35) from _Andrew Howroyd_, Apr 07 2017 %E A056509 Terms a(36) and beyond from _Andrew Howroyd_, Oct 01 2019