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A056296
Number of n-bead necklace structures using exactly three different colored beads.
7
0, 0, 1, 2, 5, 18, 43, 126, 339, 946, 2591, 7254, 20125, 56450, 158355, 446618, 1262225, 3580686, 10181479, 29032254, 82968843, 237645250, 682014587, 1960981598, 5647919645, 16292761730, 47069104613, 136166703562, 394418199725, 1143822046786, 3320790074371
OFFSET
1,4
COMMENTS
Turning over the necklace is not allowed. Colors may be permuted without changing the necklace 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
E. N. Gilbert and J. Riordan, Symmetry types of periodic sequences, Illinois J. Math., 5 (1961), 657-665.
FORMULA
a(n) = A002076(n) - A000013(n).
From Robert A. Russell, May 29 2018: (Start)
a(n) = (1/n) * Sum_{d|n} phi(d) * ([d==0 mod 6] * (S2(n/d + 2, 3) - S2(n/d + 1, 3)) + [d==3 mod 6] * (S2(n/d + 2, 3) - 3*S2(n/d + 1, 3) + 3*S2(n/d, 3)) + [d==2 mod 6 | d==4 mod 6] * (2*S2(n/d + 1, 3) - 2*S2(n/d, 3)) + [d==1 mod 6 | d=5 mod 6] * S2(n/d, 3)), where S2(n,k) is the Stirling subset number, A008277.
G.f.: -Sum_{d>0} (phi(d) / d) * ([d==0 mod 6] * (log(1-3x^d) - log(1-x^d)) + [d==3 mod 6] * (log(1-3x^d) - log(1-2x^d) + log(1-x^d)) / 2 + [d==2 mod 6 | d==4 mod 6] * (2*log(1-3x^d) - 3*log(1-2x^d)) / 3 + [d==1 mod 6 | d=5 mod 6] * (log(1-3x^d) - 3*log(1-2x^d) + 3*log(1-x^d)) / 6).
(End)
MATHEMATICA
Adn[d_, n_] := Adn[d, n] = If[1==n, DivisorSum[d, x^# &], Expand[Adn[d, 1] Adn[d, n-1] + D[Adn[d, n-1], x] x]];
Table[Coefficient[DivisorSum[n, EulerPhi[#] Adn[#, n/#] &]/n , x, 3], {n, 1, 40}] (* Robert A. Russell, Feb 23 2018 *)
Table[(1/n) DivisorSum[n, EulerPhi[#] Which[Divisible[#, 6], StirlingS2[n/#+2, 3] - StirlingS2[n/#+1, 3], Divisible[#, 3], StirlingS2[n/#+2, 3] - 3 StirlingS2[n/#+1, 3] + 3 StirlingS2[n/#, 3], Divisible[#, 2], 2 StirlingS2[n/#+1, 3] - 2 StirlingS2[n/#, 3], True, StirlingS2[n/#, 3]] &], {n, 1, 40}] (* Robert A. Russell, May 29 2018*)
mx = 40; Drop[CoefficientList[Series[-Sum[(EulerPhi[d] / d) Which[ Divisible[d, 6], Log[1 - 3x^d] - Log[1 - 2x^d], Divisible[d, 3] , (Log[1 - 3x^d] - Log[1 - 2x^d] + Log[1 - x^d]) / 2, Divisible[d, 2], (2 Log[1 - 3x^d] - 3 Log[1 - 2x^d]) / 3, True, (Log[1 - 3x^d] - 3Log[1 - 2x^d] + 3 Log[1 - x^d]) / 6], {d, 1, mx}], {x, 0, mx}], x], 1] (* Robert A. Russell, May 29 2018 *)
CROSSREFS
Column 3 of A152175.
Sequence in context: A048221 A183365 A295905 * A293964 A275079 A148417
KEYWORD
nonn
STATUS
approved