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 "undirected": reversing the order of eyelets along the path does not count as a different solution (cf. A078629).
Table of n, a(n) for n=1..6.
Index entries for sequences related to shoe lacings
Apart from initial term, A078629/2.
Sequence in context: A279970 A217767 A185157 * A071094 A144987 A304055
Adjacent sequences: A078671 A078672 A078673 * A078675 A078676 A078677
N. J. A. Sloane, Dec 16 2002