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We present an algorithm for computing one-dimensional stable and unstable manifolds of saddle periodic orbits in a Poincaré section. The computation is set up as a boundary value problem by restricting both end points of orbit segments to the section. Starting from the periodic orbit itself, we use collocation routines from {\sc Auto} to continue the solutions of the boundary value problem such that one end point of the orbit segment varies along a part of the manifold that was already computed. In this way, the other end point of the orbit segment traces out a new piece of the manifold.

As opposed to standard methods that use shooting to compute the Poincaré map as the kth return map, our approach defines the Poincaré map as the solution of a boundary value problem. This enables us to compute global manifolds through points where the flow is tangent to the section---a situation that is typically encountered unless one is dealing with a periodically forced system. Another major advantage of our approach is that it deals effectively with the problem of extreme sensitivity of the Poincaré map to its argument, which is a typical feature in the important class of slow-fast systems.

We illustrate and test our algorithm by computing stable and unstable manifolds for three examples: the forced Van der Pol oscillator, a model of a semiconductor laser with optical injection, and a slow-fast chemical oscillator. All examples are accompanied by animations demonstrating how the manifolds grow during the computation.