26 1. Various Ways of Representing Surfaces and Examples
a vertex; at a vertex, the angles add up to less than 2π, whereas
everywhere else, they add up to exactly 2π. However, we cannot
tell whether or not we are at an edge; this has to do with the fact
that given two points on adjacent faces, the way to find the shortest
path between them is to unfold the two faces and place them flat
on the plane (at which stage points on an edge look just like points
on a face), draw a straight line between the two points in question,
and then fold the surface back up (Figure 1.18). As far as our two-
dimensional selves are concerned, points on an edge and points on a
face are indistinguishable, since the unfolding process does not change
any distances along the surface.
It is also possible that a surface which can be embedded in
R3
will
lose some of its nicer properties in the process. For example, the usual
embedding of the torus destroys the symmetry between meridians and
parallels; all of the meridians are the same length, but the length of
the parallels varies. We can retain this symmetry by embedding in
R4, the so-called flat torus. Parametrically, this is given by
x = r cos t, y = r sin t,
z = r cos s, w = r sin s,
where s, t [0, 2π]. As we already mentioned, we can also obtain
the flat torus as a factor space, using the same method as in the
definition of the projective plane or Klein bottle. Beginning with a
rectangle, we identify opposite sides (with no reversal of direction);
alternately, we can consider the family of isometries of R2 given by
Tm,n : (x, y) (x + m, y + n), where m, n Z, and mod out by
orbits. This construction of T2 as R2/Z2 is exactly analogous to the
construction of the circle S1 as R/Z.
We have seen that surfaces can be considered from different view-
points: sometimes we treat them as geometric objects, with intrinsi-
cally defined distances, angles, and areas, while other times we treat
them as ‘stretchable’ objects which can be bent and deformed, but
not torn or broken. In mathematical language, this corresponds to
considering different structures on surfaces, and this is the central
theme of this course, which we will take up in earnest in the next
lecture.
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