

A072503


Number of ways to lace a shoe with n eyelet pairs such that there is no direct "horizontal" connection between any adjacent eyelet pair.


0




OFFSET

3,1


COMMENTS

The lacing must not have any "straight connections" between adjacent eyelet pairs (e.g. 2<>2*n1, 3<>2*n2, 4<>2*n3,....). There are no symmetric solutions.


LINKS



EXAMPLE

The 6 nonstraight lacings for n=3 are: 124536, 135426, 142356, 145326, 153246, 154236. Not counting mirror images we get a(3)=3.


PROG

FORTRAN program available at link.


CROSSREFS



KEYWORD

nonn


AUTHOR



STATUS

approved



