%N Number of primitive (aperiodic) word structures of length n which contain exactly two different symbols.
%C 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 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]
%F Sum mu(d)*A000225(n/d-1) where d|n and n>0.
%Y Apart from initial term, this is a duplicate of A000740.
%Y Column 2 of A137651.
%Y Cf. A056267.
%A _Marks R. Nester_