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 A056278 #16 Apr 15 2021 16:38:29
%S A056278 0,1,3,6,15,27,63,120,252,495,1023,2010,4095,8127,16365,32640,65535,
%T A056278 130788,262143,523770,1048509,2096127,4194303,8386440,16777200,
%U A056278 33550335,67108608,134209530,268435455,536854005,1073741823,2147450880,4294966269,8589869055,17179869105
%N A056278 Number of primitive (aperiodic) word structures of length n which contain exactly two different symbols.
%C A056278 Permuting the alphabet will not change a word structure. Thus aabc and bbca have the same structure. This is identical to A000740 for n>1.
%D A056278 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 A056278 Andrew Howroyd, <a href="/A056278/b056278.txt">Table of n, a(n) for n = 1..500</a>
%F A056278 a(n) = Sum_{d|n} mu(d)*A000225(n/d-1) where n>0.
%F A056278 G.f.: Sum_{k>=1} mu(k) * x^(2*k) / ((1 - x^k) * (1 - 2*x^k)). - _Ilya Gutkovskiy_, Apr 15 2021
%Y A056278 Apart from initial term, this is a duplicate of A000740.
%Y A056278 Column 2 of A137651.
%Y A056278 Cf. A056267.
%K A056278 nonn
%O A056278 1,3
%A A056278 _Marks R. Nester_
%E A056278 Terms a(30) and beyond from _Andrew Howroyd_, Apr 15 2021