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%I #10 Aug 22 2017 20:53:08
%S 4,6,36,126,540,2034,8316,32760,131544,524250,2099196,8388450,
%T 33562620,134217594,536903100,2147483520,8590065660,34359735816,
%U 137439477756,549755813250,2199025344348,8796093020154
%N Number of primitive (aperiodic) reversible strings with n beads using a maximum of four different colors.
%C A string and its reverse are considered to be equivalent.
%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)*A032121(n/d) where d|n.
%Y Column 4 of A284871.
%Y Cf. A045625, A056314.
%K nonn
%O 1,1
%A _Marks R. Nester_
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