Uni Maths
Taster
Dr. Paul Johnson
Paul
- Topology: what's leftover from Geometry if can't measure distance or angles
- Graph Theory: Studying how "things" are "connected"
Studying "Shapes"
Fun and useful
Topology Joke
Henry Segerman
Videogames
Surface:
Locally looks 2-dimensional
Text
Manifold:
Locally looks n-dimensional
What n-dimensional manifolds are there?
How can we tell different manifolds apart?
What manifold do we live in?
Cosmic Microwave Background
Janna Levin, How the Universe got its Spots
Could we be seeing the same thing from multiple directions?
Could they have made their videogame world a sphere?
QUESTION 1:
QUESTION 2: Utilities
A failed attempt
Utilities question is impossible
But how can we know for sure?
How can we prove it?
Can we check every single possible way of connecting them, or are there infinite many different possibilities to try?
If it were possible, some connections would make a loop.
That loop would have an inside and an outside...
What connections are missing?
What if we were on the torus?
Graphs:
- Vertices: A set of "things"
- Edges: A set of "connections" between pairs of things
Text
"Epidemic dynamics on higher-dimensional small world networks"
The same graph, drawn different ways
Definition: A graph is planar if it can be drawn on a piece of paper without any edges crossing.
Definition: The complete graph on n vertices, written K_n, is the graph with n vertices, and an edge between every pair of vertices
Definition: The complete bipartite graph K_{n,m} has n red vertices, m blue vertices, and an edge between every red vertex and every blue vertex.
Utilities Question: Is K_{3,3} planar?
What about other graphs?
If a graph is drawn on a plane, cuts the plane up into pieces called faces.
Graph Vertices Edges Faces
Cube
Tetrahedron
Octahedron
Square Pyramid
Your Example
Notice any patterns?
The n-cycle C_n
Euler's Theorem
For any graph G drawn on the plane, we have:
E-V+F=2
Back to Question 1
A videogame graph will have V-E+F=0
Video game graph:
- Every vertex has four edges
- Every face has four edges
Locally, it looks like a grid:
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