**Original manuscript:** 2014/01/26
**Manuscript last revised:** 2014/06/12

Time-varying vector fields on manifolds are considered with measurable time dependence and with varying degrees of regularity in state; e.g., Lipschitz, finitely differentiable, smooth, holomorphic, and real analytic. Classes of such vector fields are described for which the regularity of the flow's dependence on initial condition matches the regularity of the vector field. It is shown that the way in which one characterises these classes of vector fields corresponds exactly to the vector fields, thought of as being vector field valued functions of time, having measurability and integrability properties associated with appropriate topologies. For this reason, a substantial part of the development is concerned with descriptions of these appropriate topologies. To this end, geometric descriptions are provided of locally convex topologies for Lipschitz, finitely differentiable, smooth, holomorphic, and real analytic sections of vector bundles. In all but the real analytic case, these topologies are classically known. The description given for the real analytic topology is new. This description allows, for the first time, a characterisation of those time-varying real analytic vector fields whose flows depend real analytically on initial condition.

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Last Updated: Fri Jul 10 07:46:59 2020