OFFSET
1,2
COMMENTS
The lace must follow a Hamiltonian path through the 2n eyelets and cannot pass in order though three adjacent eyelets that are in a line.
The lace is "directed": reversing the order of eyelets along the path counts as a different solution (cf. A078674).
LINKS
EXAMPLE
Label the eyelets 1, ..., n from front to back on the left and from n+1, ..., 2n from back to front on the right. For n=2 all 6 lacings are allowed: 1 2 3 4, 2 1 3 4, 3 1 2 4, 1 3 2 4, 2 3 1 4, 3 2 1 4.
a(3) = 100: the first few lacings are: 4 2 1 3 5 6, 2 4 1 3 5 6, 1 4 2 3 5 6, 2 1 4 3 5 6, 1 2 4 3 5 6, 1 3 4 2 5 6, 3 1 4 2 5 6, 4 1 3 2 5 6, 1 4 3 2 5 6, 3 4 1 2 5 6, 4 3 1 2 5 6, 3 4 2 1 5 6, 2 4 3 1 5 6, ...
CROSSREFS
KEYWORD
nonn,more
AUTHOR
N. J. A. Sloane, Dec 12 2002
EXTENSIONS
a(7) from Hugo Pfoertner, Jan 22 2005
STATUS
approved