A208670 Number of 2n-bead necklaces labeled with numbers 1..7 allowing reversal, with neighbors differing by exactly 1.

6, 11, 20, 41, 86, 208, 514, 1398, 3934, 11576, 34850, 107303, 334396, 1053851, 3345320, 10685570, 34292064, 110498897, 357256018, 1158492478, 3766458404, 12274037845, 40082339406, 131144365904, 429838330172, 1411104048106, 4639351259122, 15273992343065

Offset: 1

Author

R. H. Hardin, Feb 29 2012

Keywords

• nonn

Examples

			All solutions for n=3:
..1....5....1....5....3....4....1....6....4....1....2....2....4....5....3....4
..2....6....2....6....4....5....2....7....5....2....3....3....5....6....4....5
..1....5....3....7....3....4....1....6....6....3....4....2....6....5....5....4
..2....6....2....6....4....5....2....7....5....4....3....3....7....6....4....5
..3....5....3....7....5....6....1....6....6....3....4....2....6....7....5....4
..2....6....2....6....4....5....2....7....5....2....3....3....5....6....4....5
..
..2....2....3....3
..3....3....4....4
..2....4....3....5
..3....5....4....6
..4....4....3....5
..3....3....4....4
		

Links

• Andrew Howroyd, Table of n, a(n) for n = 1..100

Crossrefs

Column 7 of A208671.

Formula

a(n) = (2*A208726(n) + A129639(11+n))/4.

Extensions

a(12)-a(28) from Andrew Howroyd, Mar 19 2017