A208666 Number of 2n-bead necklaces labeled with numbers 1..n allowing reversal, with neighbors differing by exactly 1.

0, 1, 4, 14, 44, 152, 514, 1866, 6884, 26137, 100442, 390592, 1526272, 5989223, 23548688, 92727898, 365445200, 1441195226, 5686268314, 22444465311, 88622259788, 350040069245, 1383007946774, 5465854718664, 21607909105528, 85444555330132, 337962745845558, 1337094537703089

Offset: 1

Author

R. H. Hardin, Feb 29 2012

Keywords

• nonn

Examples

			All solutions for n=3:
..1....1....1....2
..2....2....2....3
..1....3....1....2
..2....2....2....3
..3....3....1....2
..2....2....2....3
		

Links

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

Crossrefs

Diagonal of A208671.

Formula

a(n) = (2*A208722(n) + A220062(n+1,n))/4. - Andrew Howroyd, Mar 19 2017

Extensions

a(11)-a(28) from Andrew Howroyd, Mar 19 2017