A078676 Number of symmetric ways to lace a shoe that has n pairs of eyelets.

1, 2, 7, 43, 350, 3547, 43011, 607042

Offset: 1

Author

N. J. A. Sloane, Dec 11 2002

Keywords

• nonn

Comments

The lace must pass through each eyelet exactly once, must begin and end at the extreme pair of eyelets and cannot pass in order though three adjacent eyelets that are in a line.

Examples

			a(3) = 7: label the eyelets 1,2,3 from front to back on the left side then 4,5,6 from back to front on the right side. The symmetric lacings are: 124356 154326 153426 142536 145236 132546 135246.
		

Links

• Index entries for sequences related to shoe lacings
• N. J. A. Sloane, FORTRAN program

Crossrefs

Cf. A078602 for directed solutions, A078675 for undirected solutions.

Extensions

a(7) and a(8) from Hugo Pfoertner, Jan 22 2005