ENTANGLEMENT COMPLEXITY OF LATTICE RIBBONS

We consider a discrete ribbon model for double-stranded polymers where the ribbon is constrained to lie in a three-dimensional lattice. The ribbon can be open or closed, and closed ribbons can be orientable or nonorientable. We prove some results about the asymptotic behavior of the numbers of ribbons with n plaquettes, and a theorem about the freq uency of occurence of certain patterns in these ribbons. We use this t o derive results about the frequency of knots in closed ribbons, the l inking of the boundary curves of orientable closed ribbons, and the tw ist and writhe of ribbons. We show that the centerline and boundary of a closed ribbon are both almost surely knotted in the infinite-n limi t. For an orientable ribbon, the expectation of the absolute value of the linking number of the two boundary curves increases at least as fa st as root n, and similar results hold for the twist and writhe.