This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A056331 #11 Jun 29 2018 05:06:06 %S A056331 1,1,2,4,9,16,35,66,133,261,527,1032,2079,4123,8244,16440,32895,65639, %T A056331 131327,262380,524762,1049071,2098175,4195230,8390646,16779231, %U A056331 33558392,67112892,134225919,268443306 %N A056331 Number of primitive (aperiodic) reversible string structures with n beads using a maximum of two different colors. %C A056331 A string and its reverse are considered to be equivalent. Permuting the colors will not change the structure. %D A056331 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 A056331 a(n) = sum mu(d)*A005418(n/d) where d divides n. %t A056331 a[n_] := DivisorSum[n, MoebiusMu[#] (2^(n/#-2) + 2^(Floor[n/#/2]-1))&]; %t A056331 Array[a, 30] (* _Jean-François Alcover_, Jun 29 2018 *) %Y A056331 Cf. A045625. %K A056331 nonn %O A056331 1,3 %A A056331 _Marks R. Nester_