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 A056500 #11 Sep 29 2019 11:09:09
%S A056500 0,0,0,0,0,12,24,132,240,900,1560,4968,8400,24588,40824,113520,186480,
%T A056500 502248,818520,2157360,3497976,9085452,14676024,37723260,60780720,
%U A056500 155082900,249401640,632947728,1016542800,2569816476,4123173624,10393520640
%N A056500 Number of primitive (period n) periodic palindromes using exactly four different symbols.
%D A056500 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 A056500 Andrew Howroyd, <a href="/A056500/b056500.txt">Table of n, a(n) for n = 1..200</a>
%F A056500 a(n) = Sum_{d|n} mu(d)*A056490(n/d).
%e A056500 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome.
%Y A056500 Column 4 of A327878.
%Y A056500 Cf. A056465, A056490.
%K A056500 nonn
%O A056500 1,6
%A A056500 _Marks R. Nester_
%E A056500 Terms corrected by _Andrew Howroyd_, Sep 28 2019