OFFSET
0,5
COMMENTS
Clearly in each necklace the number of beads of each of the two colors must be equal and so the number of beads must be even, if a(n) is to be positive.
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
MATHEMATICA
a13[n_] := DivisorSum[n, EulerPhi@(2*#)*2^(n/#)&]/(2*n);
a29[n_] := (1/4)*(Mod[n, 2] + 3)*2^Quotient[n, 2] + DivisorSum[n, EulerPhi[#]*2^(n/#)&]/(2*n);
a[0] = 0; a[n_] := a29[2*n] - a13[2*n] - 2^(n - 1);
Array[a, 34, 0] (* Jean-François Alcover, Nov 05 2017 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Henry Bottomley, Dec 22 2000
EXTENSIONS
More terms from Vladeta Jovovic, Mar 06 2001
STATUS
approved