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 A056519 #13 Oct 01 2019 19:49:51 %S A056519 0,0,0,1,1,5,6,18,25,63,90,202,301,650,965,2016,3025,6220,9330,18970, %T A056519 28495,57650,86526,174210,261624,525693,788945,1582462,2375101, %U A056519 4759482,7141686,14300976,21457735,42951825,64439003,128946888,193448101,387037370,580606145,1161485370 %N A056519 Number of primitive (period n) periodic palindromic structures using exactly three different symbols. %D A056519 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 A056519 Andrew Howroyd, <a href="/A056519/b056519.txt">Table of n, a(n) for n = 1..200</a> %F A056519 a(n) = A056514(n) - A056513(n). %F A056519 Moebius transform of A056509. - _Andrew Howroyd_, Oct 01 2019 %e A056519 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome. Permuting the symbols will not change the structure. %Y A056519 Column 3 of A285037. %Y A056519 Cf. A056482, A056509, A056513, A056514. %K A056519 nonn %O A056519 1,6 %A A056519 _Marks R. Nester_ %E A056519 a(17)-a(35) from _Andrew Howroyd_, Apr 08 2017 %E A056519 Terms a(36) and beyond from _Andrew Howroyd_, Oct 01 2019