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 A056332 #12 Jun 29 2018 06:08:10 %S A056332 1,1,3,8,24,65,195,564,1677,4976,14883,44452,133224,399113,1196808, %T A056332 3588840,10764960,32289855,96864963,290580040,871725426,2615132465, %U A056332 7845353475,23535926760,70607649816,211822550576 %N A056332 Number of primitive (aperiodic) reversible string structures with n beads using a maximum of three different colors. %C A056332 A string and its reverse are considered to be equivalent. Permuting the colors will not change the structure. %D A056332 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 A056332 a(n) = Sum mu(d)*A001998(n/d-1) where d|n. %t A056332 a1998[n_] := If[OddQ[n], (1/4)*(3^n + 4*3^((n-1)/2) + 1), (1/4)*(3^n + 2*3^(n/2) + 1)]; %t A056332 a[n_] := DivisorSum[n, MoebiusMu[#] a1998[n/#-1]&]; %t A056332 Array[a, 26] (* _Jean-François Alcover_, Jun 29 2018 *) %Y A056332 Cf. A001998, A056314. %K A056332 nonn %O A056332 1,3 %A A056332 _Marks R. Nester_