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A208724
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Number of 2n-bead necklaces labeled with numbers 1..5 not allowing reversal, with neighbors differing by exactly 1.
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3
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4, 7, 12, 25, 52, 131, 316, 835, 2196, 5935, 16108, 44369, 122644, 341803, 956636, 2690845, 7596484, 21524543, 61171660, 174342217, 498112276, 1426419859, 4093181692, 11767920119, 33891544420, 97764131647, 282429537948, 817028472961, 2366564736724, 6863038218843
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OFFSET
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1,1
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LINKS
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FORMULA
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EXAMPLE
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All solutions for n=3:
..4....1....3....2....1....2....3....1....2....3....1....2
..5....2....4....3....2....3....4....2....3....4....2....3
..4....1....3....2....3....4....3....3....2....5....1....4
..5....2....4....3....2....3....4....4....3....4....2....5
..4....3....3....2....3....4....5....3....4....5....1....4
..5....2....4....3....2....3....4....2....3....4....2....3
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MATHEMATICA
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a[n_] := (1/n)*DivisorSum[n, EulerPhi[n/#] * (2*3^# + 2) &] / 2;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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