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The purpose of this paper is to prove equivariant versions of some basic theorems in
differential topology for proper Lie group actions. In particular, we study how to
extend equivariant isotopies and then apply these results to obtain equivariant
smoothing and gluing theorems. We also study equivariant collars and tubular
neighbourhoods. When possible, we follow the ideas in the well-known book of
M W Hirsch. When necessary, we use results from the differential topology of Hilbert
spaces.
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