

A078702


Number of ways to lace a shoe that has n pairs of eyelets such that each eyelet has at least one direct connection to the opposite side.


4




OFFSET

1,2


COMMENTS

The lace is "undirected": reversing the order of eyelets along the path does not count as a different solution. It must begin and end at the extreme pair of eyelets,


LINKS

Table of n, a(n) for n=1..8.
Index entries for sequences related to shoe lacings


EXAMPLE

a(3) = 13: 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 lacings are: 124356 154326 153426 142536 145236 135246 125346 124536 125436 152346 153246 152436 154236.


CROSSREFS

Cf. A078698, A078700, A078675.
a(n) = ( A078698(n) + A078700(n) ) / 2
Sequence in context: A193550 A102585 A227746 * A259795 A069569 A255882
Adjacent sequences: A078699 A078700 A078701 * A078703 A078704 A078705


KEYWORD

nonn


AUTHOR

Hugo Pfoertner, Dec 18 2002


STATUS

approved



