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%I #16 Apr 15 2021 21:40:49
%S 0,0,0,0,0,1,21,266,2646,22827,179487,1323651,9321312,63436352,
%T 420693273,2734926292,17505749898,110687248392,693081601779,
%U 4306078872557,26585679462783,163305339165738,998969857983405
%N Number of primitive (aperiodic) word structures of length n which contain exactly six different symbols.
%C Permuting the alphabet will not change a word structure. Thus aabc and bbca have the same structure.
%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 a(n) = Sum_{n > 0, d|n} mu(d)*A000770(n/d).
%F G.f.: Sum_{k>=1} mu(k) * x^(6*k) / Product_{j=1..6} (1 - j*x^k). - _Ilya Gutkovskiy_, Apr 15 2021
%Y Column 6 of A137651.
%Y Cf. A056271.
%K nonn
%O 1,7
%A _Marks R. Nester_