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%I #9 Aug 22 2017 20:53:12
%S 0,1,1,2,2,5,4,10,9,22,14,78,36,124,180,343,260,1329,804,3732,4428,
%T 9742,8384,47364,33778,108292,138756,408244,331184,2251604,1155734
%N Number of primitive (period n) step cyclic shifted sequence structures using exactly two different symbols.
%C See A056371 for an explanation of step shifts. Under step cyclic shifts, abcde, bdace, bcdea, cdeab and daceb etc. are equivalent. Permuting the symbols will not change the structure. Identical to A056439 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 a(n) = A056439(n) - A000007(n-1).
%Y Cf. A056424.
%K nonn
%O 1,4
%A _Marks R. Nester_