  Suppose M is a non-compact connected n-manifold without boundary, DD(M) is
the group of C^\infty-diffeomorphisms of M endowed with the Whitney
C^\infty-topology and DD_0(M) is the identity connected component of DD(M),
which is an open subgroup in the group DD_c(M) \subset DD(M) of compactly
supported diffeomorphisms of M. It is shown that DD_0(M) is homeomorphic to N
\times IR^\infty for an l_2-manifold N whose topological type is uniquely
determined by the homotopy type of DD_0(M). For instance, DD_0(M) is
homeomorphic to l_2 \times IR^\infty if n = 1, 2 or n = 3 and M is orientable
and irreducible. We also show that for any compact connected n-manifold N with
non-empty boundary \partial N the group DD_0(N - \partial N) is homeomorphic to
DD_0(N; \partial N) \times IR^\infty, where DD_0(N;\partial N) is the identity
component of the group DD(N;\partial N) of diffeomorphisms of N that do not
move points of the boundary \partial N.
