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 A056280 #20 Apr 15 2021 16:59:25
%S A056280 0,0,0,1,10,65,350,1700,7770,34095,145750,611435,2532530,10391395,
%T A056280 42355940,171797200,694337290,2798799150,11259666950,45232081795,
%U A056280 181509069700,727778478075,2916342574750,11681056021300,46771289738800,187226354413735,749329038527580
%N A056280 Number of primitive (aperiodic) word structures of length n which contain exactly four different symbols.
%C A056280 Permuting the alphabet will not change a word structure. Thus aabc and bbca have the same structure.
%D A056280 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 A056280 Andrew Howroyd, <a href="/A056280/b056280.txt">Table of n, a(n) for n = 1..500</a>
%F A056280 a(n) = Sum_{n > 0, d|n} mu(d)*A000453(n/d).
%F A056280 G.f.: Sum_{k>=1} mu(k) * x^(4*k) / Product_{j=1..4} (1 - j*x^k). - _Ilya Gutkovskiy_, Apr 15 2021
%Y A056280 Column 4 of A137651.
%Y A056280 Cf. A056269.
%K A056280 nonn
%O A056280 1,5
%A A056280 _Marks R. Nester_
%E A056280 Terms a(25) and beyond from _Andrew Howroyd_, Apr 15 2021