%I
%S 0,1,3,6,15,27,63,120,252,495,1023,2010,4095,8127,16365,32640,65535,
%T 130788,262143,523770,1048509,2096127,4194303,8386440,16777200,
%U 33550335,67108608,134209530,268435455
%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/d1) where dn and n>0.
%Y Apart from initial term, this is a duplicate of A000740.
%Y Column 2 of A137651.
%Y Cf. A056267.
%K nonn
%O 1,3
%A _Marks R. Nester_
