A topological space
is a
*manifold*^{4.4} if for every , an open set
exists such that: 1) , 2) is homeomorphic to
, and 3) is fixed for all . The fixed is
referred to as the *dimension* of
the manifold, . The second condition is the most important. It
states that in the vicinity of any point, , the space behaves
just like it would in the vicinity of any point
;
intuitively, the set of directions that one can move appears the same
in either case. Several simple examples that may or may not be
manifolds are shown in Figure 4.4.

One natural consequence of the definitions is that .
According to Whitney's embedding theorem [449],
. In
other words,
is ``big enough'' to hold any
-dimensional manifold.^{4.5}Technically, it is said that the -dimensional manifold is
*embedded* in
, which means that an
injective mapping exists from to
(if it is not injective,
then the topology of could change).

As it stands, it is impossible for a manifold to include its boundary
points because they are not contained in open sets. A *manifold
with boundary* can be defined requiring
that the neighborhood of each boundary point of is homeomorphic to
a half-space of dimension (which was defined for and
in Section 3.1) and that the interior points must be
homeomorphic to
.

The presentation now turns to ways of constructing some manifolds that frequently appear in motion planning. It is important to keep in mind that two manifolds will be considered equivalent if they are homeomorphic (recall the donut and coffee cup).

Steven M LaValle 2012-04-20