  A homeomorphism of a 3-manifold M is said to be Dehn twists on the boundary
when its restriction to the boundary of M is isotopic to the identity on the
complement of a collection of disjoint simple closed curves in the boundary of
M. In this paper, we give various results about such collections of curves and
the associated homeomorphisms. In particular, if M is compact, orientable,
irreducible and the boundary of M is a single torus, and M admits a
homeomorphism which is a nontrivial Dehn twist on the boundary of M, then M
must be a solid torus.
