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A208723
Number of 2n-bead necklaces labeled with numbers 1..4 not allowing reversal, with neighbors differing by exactly 1.
2
3, 5, 8, 15, 27, 60, 123, 285, 648, 1529, 3603, 8680, 20883, 50825, 124056, 304575, 750123, 1855100, 4600203, 11442087, 28527448, 71292605, 178526883, 447919420, 1125750147, 2833906685, 7144450568, 18036423975, 45591631803, 115381823348
OFFSET
1,1
LINKS
FORMULA
a(n) = (1/n) * Sum_{d | n} totient(n/d) * A005248(d). - Andrew Howroyd, Mar 18 2017
EXAMPLE
All solutions for n=4:
..1....1....3....2....1....2....2....2....2....1....1....1....1....1....1
..2....2....4....3....2....3....3....3....3....2....2....2....2....2....2
..1....3....3....2....3....2....4....4....2....3....1....1....3....3....1
..2....4....4....3....2....3....3....3....3....2....2....2....4....2....2
..3....3....3....2....1....2....4....2....4....3....1....3....3....3....1
..2....4....4....3....2....3....3....3....3....4....2....4....2....2....2
..3....3....3....2....3....4....4....4....4....3....1....3....3....3....3
..2....2....4....3....2....3....3....3....3....2....2....2....2....2....2
CROSSREFS
Column 4 of A208727.
Sequence in context: A052977 A191633 A215327 * A285010 A352917 A099846
KEYWORD
nonn
AUTHOR
R. H. Hardin, Mar 01 2012
EXTENSIONS
a(26)-a(30) from Andrew Howroyd, Mar 18 2017
STATUS
approved