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

1,1

Links

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

Formula

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

Example

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

Crossrefs

Column 7 of A208671.

Sequence in context: A188556 A021011 A000382 * A208726 A192750 A000383

Adjacent sequences: A208667 A208668 A208669 * A208671 A208672 A208673

Keyword

nonn

Author

R. H. Hardin, Feb 29 2012

Extensions

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

Status

approved