1.3. Drawing on a Surface 7
Figure 1.5. Loops on the torus
One of the loops, the one represented by the thick vertical line,
is different from the others. Imagine that these loops are made from
rubber bands, and are allowed to compress in a continuous way. The
first 2 loops can shrink continuously to points, whereas the third loop
is “stuck”. It can’t make itself any shorter no matter how it moves.
Such a loop is commonly called essential. There are many essential
loops on the torus. The right side of Figure 1.5 shows another essential
loop. In contrast, the sphere has no essential loops at all.
We will see in Chapter 4 that there is an algebraic object we can
associate to a surface (and many other kinds of spaces) called the fun-
damental group. The fundamental group organizes all the different
ways of drawing loops on the surface into one basic structure. The
nice thing about the fundamental group is that it links the theory
of surfaces to algebra, especially group theory. Beautifully, it turns
out that 2 (compact) surfaces have the same Euler characteristic if
and only if they have the same fundamental group. The Euler char-
acteristic and the fundamental group are 2 entry points into the vast
subject of algebraic topology.
For the most part, studying algebraic topology is beyond the
scope of this book, but we will study the fundamental group and
related constructions, in great detail. After defining the fundamental
group in Chapter 4, we will compute a number of examples in Chapter
5.
Previous Page Next Page