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%I #9 Jan 18 2022 15:36:29
%S 1,1,4,13,51,196,854,3830,17997,86419,422004,2079260,10306751,
%T 51263086,255514299,1275160060,6368612301,31821454413,159042661904,
%U 795019250650,3974515029793,19870830290476,99348921288654,496728909635860,2483597478617750,12417846151236799
%N Number of primitive (aperiodic) word structures of length n using a 5-ary alphabet.
%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_{d|n} mu(d)*A056272(n/d).
%Y Cf. A008683, A054720.
%K nonn
%O 1,3
%A _Marks R. Nester_