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 A056284 Number of n-bead necklaces with exactly four different colored beads. 7
 0, 0, 0, 6, 48, 260, 1200, 5106, 20720, 81876, 318000, 1223136, 4675440, 17815020, 67769552, 257700906, 980240880, 3731753180, 14222737200, 54278580036, 207438938000, 793940475900, 3043140078000, 11681057249536, 44900438149296, 172824331826580, 666070256489680 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS Turning over the necklace is not allowed. 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 Alois P. Heinz, Table of n, a(n) for n = 1..1000 FORMULA a(n) = A001868(n) - 4*A001867(n) + 6*A000031(n) - 4. From Robert A. Russell, Sep 26 2018: (Start) a(n) = (k!/n) Sum_{d|n} phi(d) S2(n/d,k), where k=4 is the number of colors and S2 is the Stirling subset number A008277. G.f.: -Sum_{d>0} (phi(d)/d) * Sum_{j} (-1)^(k-j) * C(k,j) * log(1-j x^d), where k=4 is the number of colors. (End) EXAMPLE For n=4, the six necklaces are ABCD, ABDC, ACBD, ACDB, ADBC and ADCB. MATHEMATICA k=4; Table[k!DivisorSum[n, EulerPhi[#]StirlingS2[n/#, k]&]/n, {n, 1, 30}] (* Robert A. Russell, Sep 26 2018 *) PROG (PARI) a(n) = my(k=4); (k!/n)*sumdiv(n, d, eulerphi(d)*stirling(n/d, k, 2)); \\ Michel Marcus, Sep 27 2018 CROSSREFS Cf. A001868. Column k=4 of A087854. Sequence in context: A262354 A052771 A056289 * A293967 A246587 A229504 Adjacent sequences:  A056281 A056282 A056283 * A056285 A056286 A056287 KEYWORD nonn AUTHOR STATUS approved

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Last modified December 10 15:30 EST 2018. Contains 318049 sequences. (Running on oeis4.)