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

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

Offset

1,2

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.

Links

Table of n, a(n) for n=1..8.

Index entries for sequences related to shoe lacings

N. J. A. Sloane, FORTRAN program

Example

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.

Crossrefs

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

Sequence in context: A220220 A144059 A352293 * A265229 A286684 A086331

Adjacent sequences: A078673 A078674 A078675 * A078677 A078678 A078679

Keyword

nonn

Author

N. J. A. Sloane, Dec 11 2002

Extensions

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

Status

approved