%I
%S 0,2,6,12,30,54,126,240,504,990,2046,4020,8190,16254,32730,65280,
%T 131070,261576,524286,1047540,2097018,4192254,8388606,16772880,
%U 33554400,67100670,134217216,268419060,536870910,1073708010,2147483646
%N Number of primitive (aperiodic) words of length n which contain exactly two different symbols.
%D M. R. Nester, Mathematical investigations of some plant interaction designs, PhD Thesis, University of Queensland, Brisbane, Australia, 1999 [See A056391 for pdf file of Chap. 2].
%F a(n) = Sum_{dn} mu(d)*A000918(n/d), n>0.
%o (PARI) a(n) = sumdiv(n, d, moebius(d)*(2^(n/d)  2)); \\ _Michel Marcus_, Jun 30 2019
%Y Cf. A000079, A000918.
%Y A027375 and A038199 are essentially the same sequence with different initial terms.
%K nonn
%O 1,2
%A _Marks R. Nester_
%E Corrected and extended by _Franklin T. AdamsWatters_ and _T. D. Noe_, Oct 25 2006
