# An Introduction to Topology Linda Green

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An Introduction to Topology Linda Green
Bay Area Circle for Teachers 2012 Summer Workshop Images from virtualmathmuseum.org

Topics What is topology? Surfaces and gluing diagrams The universe
A math auction Most of this material is from The Shape of Space by Jeff Weeks.

Geometry vs. Topology Study of rigid objects
Rotations and reflections are okay No stretching allowed Study of flexible objects Rotations and reflections are okay Stretching and bending are okay No tearing or gluing allowed

Topology vs. Geometry Informal Definitions
The properties of an object that stay the same when you bend or stretch it are called the topology of the object. Two objects are considered the same topologically if you can deform one into the other without tearing, cutting, gluing, or other violent actions. The properties of an object that change when you bend or stretch it are the geometry of the object. For example, distances, angles, and curvature are parts of geometry but not topology.

Deforming an object doesn’t change it’s topology
Image from A topologist is someone who can’t tell the difference between a coffee cup and a doughnut.

Which surfaces are topologically the same?

Gluing diagrams What topological surface do you get when you glue (or tape) the edges of the triangle together as shown?

Gluing diagrams What do you get when you glue the edges of the square together like this? Don’t glue the interior parts of the square together, just the edges!

Gluing diagrams What surface is this? And this? S2 (a sphere)
T2 (a torus)

Life inside the surface of a torus
What happens as this 2-dimensional creature travels through its tiny universe? What does it see when it looks forward? Backward? Left? Right?

Tic-Tac-Toe on a Torus Where should X go to win?
What if it is 0’s turn?

Tic-Tac-Toe on the Torus
Which of the following positions are equivalent in torus tic-tac-toe? How many essentially different first moves are there in torus tic-tac-toe? Is there a winning strategy for the first player? Is it possible to get a Cat’s Game (no winner)?

Another surface What surface do you get when you glue together the sides of the square as shown? K2 (a Klein bottle)

Life in a Klein bottle surface
What happens as this creature travels through its Klein bottle universe? A path that brings a traveler back to his starting point mirror-reversed is called an orientation-reversing path. How many orientation-reversing paths can you find? A surface that contains an orientation-reversing path is called non-orientable.

Tic-Tac-Toe on a Klein Bottle
Where should X go to win?

Tic-Tac-Toe on a Klein bottle
How many essentially different first moves are there in Klein bottle tic-tac-toe? Is there a winning strategy for the first player? Is it possible to get a Cat’s Game?

What happens when you cut a Klein bottle in half?
It depends on how you cut it. Cutting a Klein bottle Another Klein bottle video

Three dimensional spaces
How can you make a 3-dimensional universe that is analogous to the 2-dimensional torus? Is there a 3-dimensional analog to the Klein bottle?

Name that Surface What two surfaces do these two gluing diagrams represent?

What topological surface is this?
Image from Plus Magazine

We have seen that the following gluing diagrams describe a topological sphere, torus, and Klein bottle. How many essentially different ways are their to glue the edges of a square in pairs? How many different topological surfaces result?

Which of these surfaces are orientable? Which represent a torus? A sphere? A Klein bottle? Something new?

Two gluing diagrams can represent the same surface
Try cutting apart surface E along a diagonal and regluing it to get one of the other surfaces. Try the same thing with surface F. Make sure to label the edges of the cut you make to keep track of which new edges get glued to each other.

The Projective Plane

Is there an algorithm to tell which gluing diagrams represent which surfaces?
Look at what happens if you walk around the corners of the square. In some gluing diagrams, all four corners glue up to form only one piece of the surface. In other diagrams, the corners glue in pairs. If you mark the 4 edges and 4 vertices of the square, each pair of edges glues up so that there are only 2 distinct edges on the glued up surface. Some vertices also get glued up to each other, so there may be only 1 or 2 distinct vertices in the final glued up surface. Label each diagram with the number of edges and vertices AFTER gluing.

Edges, vertices, and faces
Torus V = 1 E = 2 F = 1 Klein bottle V = 1 E = 2 F = 1 Projective plane V = 2 E = 2 F = 1 Sphere V = 3 E = 2 F = 1 Projective plane V = 2 E = 2 F = 1 Klein bottle V = 1 E = 2 F = 1

What surfaces do these gluing diagrams represent?
G = sphere, H = torus, I = Klein bottle, J = projective plane, K = torus, L = Klein bottle Compute the number of edges and vertices after gluing for each of these diagrams.

Euler number All gluing diagrams that represent the same surface have the same relationship between number of vertices, edges, and faces Gluing diagram Surface Number of vertices Number of edges Number of faces Euler number A B C D E F G H I J K

Euler number + orientability is enough
Any two gluing diagrams that represent the same surface have the same Euler number V – E + F Amazingly, it is also true that any two gluing diagrams that have the same Euler number and are both orientable represent the same surface! Similarly, any two gluing diagrams that have the same Euler number and are both non-orientable represent the same surface! So Euler number plus orientability is enough to determine the surface

Gluing diagram puzzles
Find a gluing diagram, in which all edges get glued in pairs, that does not represent a sphere, torus, Klein bottle, or projective plane What is the Euler number for these gluing diagrams? What surfaces do they represent?

References and Resources
The Shape of Space by Jeff Weeks Torus games Gluing diagram animations Klein bottle experimentshttp://plus.maths.org/content/os/issue26/features/mathart/applets2/experiments

Dimension Informal Definitions
1-dimensional: Only one number is required to specify a location; has length but no area. Each small piece looks like a piece of a line. 2-dimensional: Two numbers are required to specify a location; has area but no volume. Each small piece looks like a piece of a plane. 3-dimensional: Three numbers are required to specify a location; has volume. Each small piece looks like a piece of ordinary space.