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 A208543 Number of n-bead necklaces of 7 colors allowing reversal, with no adjacent beads having the same color. 2
 7, 21, 35, 231, 777, 4291, 19995, 107331, 559895, 3037314, 16490775, 90782986, 502334385, 2799220041, 15672833365, 88162676511, 497842924845, 2821127825971, 16035782631855, 91404068329560, 522308348593785, 2991403003191771, 17168048327252235, 98716281736491076 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Andrew Howroyd, Table of n, a(n) for n = 1..100 EXAMPLE All solutions for n=3: ..1....1....5....2....1....3....1....4....1....3....3....1....1....2....3....2 ..6....3....6....6....4....6....3....5....3....5....4....4....2....5....5....3 ..7....6....7....7....7....7....4....6....7....7....5....6....4....6....6....4 .. ..1....1....3....1....1....1....2....4....2....4....1....2....1....1....3....2 ..2....2....4....2....2....4....4....6....5....5....5....4....5....3....4....3 ..6....5....7....7....3....5....6....7....7....7....6....5....7....5....6....7 .. ..2....2....2 ..3....3....4 ..6....5....7 MATHEMATICA T[n_, k_] := If[n == 1, k, (DivisorSum[n, EulerPhi[n/#]*(k - 1)^# &]/n + If[OddQ[n], 1 - k, k*(k - 1)^(n/2)/2])/2]; a[n_] = T[n, 7]; Array[a, 24] (* Jean-François Alcover, Nov 01 2017, after Andrew Howroyd *) CROSSREFS Column 7 of A208544. Sequence in context: A045849 A031008 A147587 * A009475 A063292 A045524 Adjacent sequences:  A208540 A208541 A208542 * A208544 A208545 A208546 KEYWORD nonn AUTHOR R. H. Hardin, Feb 27 2012 EXTENSIONS a(18)-a(24) from Andrew Howroyd, Mar 12 2017 STATUS approved

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Last modified October 26 05:15 EDT 2020. Contains 338027 sequences. (Running on oeis4.)