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 A056494 #24 Aug 22 2017 20:53:13
%S A056494 3,3,6,12,24,42,78,144,234,456,726,1392,2184,4290,6528,12960,19680,
%T A056494 39078,59046,117600,177060,353562,531438,1061280,1594296,3186456,
%U A056494 4782726,9561552,14348904,28690752,43046718
%N A056494 Number of primitive (period n) periodic palindromes using a maximum of three different symbols.
%C A056494 For example, aaabbb is not a (finite) palindrome but it is a periodic palindrome.
%C A056494 Number of aperiodic necklaces with three colors that are the same when turned over and hence have reflectional symmetry but no rotational symmetry. - _Herbert Kociemba_, Nov 29 2016
%D A056494 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 A056494 a(n) = Sum_{d|n} mu(d)*A038754(n/d+1).
%F A056494 From _Herbert Kociemba_, Nov 29 2016: (Start)
%F A056494 More generally, gf(k) is the g.f. for the number of necklaces with reflectional symmetry but no rotational symmetry and beads of k colors.
%F A056494 gf(k): Sum_{n>=1} mu(n)*Sum_{i=0..2} binomial(k,i)x^(n*i)/(1-k*x^(2*n)). (End)
%t A056494 mx=40;gf[x_,k_]:=Sum[ MoebiusMu[n]*Sum[Binomial[k,i]x^(n i),{i,0,2}]/( 1-k x^(2n)),{n,mx}]; CoefficientList[Series[gf[x,3],{x,0,mx}],x] (* _Herbert Kociemba_, Nov 29 2016 *)
%Y A056494 Column 3 of A284856.
%Y A056494 Cf. A056459.
%K A056494 nonn
%O A056494 1,1
%A A056494 _Marks R. Nester_