INVARIANT CURVES FOR VARIABLE STEP SIZE INTEGRATORS

The behaviour of one-step methods with variable step size applied to x = f(x) is investigated. Usually the step size for the current step depends on one or several previous steps. However, under some natural assumptions it can be shown that the step size asymptotically depends only on the location x. This allows to introduce an x-dependent time transformation taking a variable step size method to a constant step-size method. By means of such a transformation general properties of constant step size methods carry over to variable step size methods. This is used to show that if the differential equation admits a hyperbolic periodic solution the variable step size method admits an invariant closed curve near the orbit of the periodic solution.