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 A056331 Number of primitive (aperiodic) reversible string structures with n beads using a maximum of two different colors. 3
 1, 1, 2, 4, 9, 16, 35, 66, 133, 261, 527, 1032, 2079, 4123, 8244, 16440, 32895, 65639, 131327, 262380, 524762, 1049071, 2098175, 4195230, 8390646, 16779231, 33558392, 67112892, 134225919, 268443306 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A string and its reverse are considered to be equivalent. Permuting the colors will not change the structure. REFERENCES 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] LINKS FORMULA a(n) = sum mu(d)*A005418(n/d) where d divides n. MATHEMATICA a[n_] := DivisorSum[n, MoebiusMu[#] (2^(n/#-2) + 2^(Floor[n/#/2]-1))&]; Array[a, 30] (* Jean-François Alcover, Jun 29 2018 *) CROSSREFS Cf. A045625. Sequence in context: A203320 A005980 A128093 * A056336 A203318 A282776 Adjacent sequences:  A056328 A056329 A056330 * A056332 A056333 A056334 KEYWORD nonn AUTHOR STATUS approved

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Last modified June 16 04:56 EDT 2021. Contains 345056 sequences. (Running on oeis4.)