Graph Theory Lecture 3: Sum of Degrees Formulas, Planar Graphs, and Euler s Theorem Spring 2014 Morgan Schreffler Office: POT 902

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1 Graph Theory Lecture 3: Sum of Degrees Formulas, Planar Graphs, and Euler s Theorem Spring 2014 Morgan Schreffler Office: POT 902

2 Different Graphs, Similar Properties E F J K A B I C D N O G H V R W L M U S P Q X T Y What do the five graphs above have in common? For each graph, write its degree list and add the numbers together. What do you notice? If we add any other edge to each of these graphs, what would happen to the numbers we came up with earlier? Why?

3 Sum of Degrees of Vertices Theorem Theorem (Sum of Degrees of Vertices) Suppose a graph has v vertices with degrees d 1, d 2, d 3,..., d v. Add together all degrees to get a new number d 1 + d 2 + d d v = D V. Then D V = 2e. In words, for any graph the sum of the degrees of the vertices equals twice the number of edges. Stated in a slightly different way, D V = 2e says that D V is ALWAYS an even number. (Using the Sum of Degrees of Vertices Theorem) It is impossible to make a graph with v = 6 where the vertices have degrees 1, 2, 2, 3, 3, 4. This is because the sum of the degrees D V is D V = = 15 D V is always an even number but 15 is odd!

4 Using the Sum of the Degrees of Vertices Formula 1 Consider the following scenarios: A graph has 4 vertices with degrees 0, 0, 0, and 0. What does this graph look like? A graph has 1 vertex with degree 2. What does this graph look like? A graph has 4 vertices with degrees 2, 3, 3, and 4. How many edges are there? A graph has 4 vertices with degrees 2, 2, 2, and 4. Can you say what this graph looks like?

5 Using the Sum of the Degrees of Vertices Formula 2 Consider the following scenarios: Is it possible to have a graph with vertices of degrees: 1 and 1? Is it possible to have a graph with vertices of degrees: 1 and 2? Is it possible to have a graph with vertices of degrees: 1, 1, 2, 3? Is it possible to have a graph with vertices of degrees: 1, 1, 2, 3, 3?

6 Planar Graphs Definition (Planar Graph) A graph is Planar if it is isomorphic to another graph whose edges do not cross. Based on the definition, to determine if a graph is planar we will often have to consider isomorphic versions of the graph. (Using Isomorphisms to Show a Graph is Planar) The graph on the left is definitely planar (edges do not cross). Even though the graph to the right does not appear to be planar, it is because it is isomorphic to the graph on the left!

7 Faces of a Planar Graph Definition (Faces of a Planar Graphs) In any planar graph, drawn with no intersections, the edges divide the planes into different regions. The regions enclosed by the planar graph are called interior faces of the graph. The region surrounding the planar graph is called the exterior (or infinite) face of the graph. When we say faces of the graph we mean BOTH the interior AND the exterior faces. We usually denote the number of faces of a planar graph by f. Be Careful! Before you count the faces of a planar graph, it is important to first draw an isomorphic version of the graph so that no edges cross!

8 Counting Faces of a Planar Graph It is really tempting to try to count the faces in the graph below: Remember, the idea of faces only applies to planar graphs drawn with no edge crossings. Instead we need to redraw the graph to get an isomorphic version: In the bottom version it is easy to see there are 3 interior faces and 1 exterior face for a total of 4 faces.

9 Degree of a Face Definition (Degree of a Face) For a planar graph drawn without edges crossing, the number of edges bordering a particular face is called the degree of the face. In the graph below each of the two interior faces have degree 3. The infinite (exterior) face has degree 4. Adding up degrees gives a result that is curiously similar to one before. Theorem (Sum of the Degrees of Faces) In any planar graph, the sum of the degrees of all faces is equal to twice the number of edges. In symbols, D F = 2e.

10 Counting Faces and Degrees 1 C A B D E F Is this graph planar? How many faces does this graph have? What is the degree of each face? List v, e, and f for this graph.

11 Counting Faces and Degrees 2 C A B D E F Is this graph planar? How many faces does this graph have? What is the degree of each face? List v, e, and f for this graph.

12 Counting Faces and Degrees 3 A B D C E F Is this graph planar? How many faces does this graph have? What is the degree of each face? List v, e, and f for this graph.

13 Making Graphs Look Planar Here are a couple more graphs that don t look planar at first. Can you count the faces? Left Graph (Cube) Right Graph (K 3,2 ) Vertices v 8 5 Edges e 12 6 Faces f 6 3 These numbers are giving more evidence of some big ideas with planar graphs.

14 Euler s Formula Theorem (Euler s Formula) Take any connected planar graph drawn with no intersecting edges. Let v be the number of vertices in the graph. Let e be the number of edges in the graph. Let f be the number of faces in the graph. Then v e + f = 2. Check the planar graphs on the previous slide: Cube v e + f = = 2 K 3,2 v e + f = = 2 Let s see what Euler s Formula can do for us!

15 Using Euler s Formula 1 C B A D E F What is v? What is e? If the graph is planar, what must f be? Redraw the graph with vertices/edges moved around so the graph appears planar.

16 Using Euler s Formula 2 A connected planar graph has 8 vertices and 12 edges. How many faces are there? A connected planar graph has 6 vertices and 4 faces. How many edges are there? A connected planar graph has 8 vertices with degrees: 1, 1, 2, 2, 3, 3, 4, 4. How many edges are there? A connected planar graph has 8 vertices with degrees: 1, 1, 2, 2, 3, 3, 4, 4. How many faces are there?

17 Using Euler s Formula 3 A connected planar graph has 6 faces and 12 edges. How many vertices are there? A connected planar graph has 4 faces with degrees: 3, 3, 4, 4. How many edges are there? A connected planar graph has 4 faces with degrees: 3, 3, 4, 4. How many vertices are there? Draw a connected planar graph with 4 faces with degrees: 3, 3, 4, 4. Is there only one such isomorphic graph?

18 Using Euler s Formula 4 B A C E D F What is v? What is e? Though it doesn t look it, this is graph is planar. What is f?

19 Useful Facts for Planar Graphs We can combine the following theorems to answer questions about planar graphs. Theorem (Sum of the Degrees of Vertices) In any graph, the sum of the degrees of all vertices is equal to twice the number of edges. along with what we just looked at Theorem (Sum of the Degrees of Faces) In any planar graph, the sum of the degrees of all faces is equal to twice the number of edges. along with Euler s Formula Theorem (Euler s Formula) For a connected planar graph with vertices v, edges e, and faces f, the following must hold: v e + f = 2.

20 Another Euler s Formula Find v, e, and f for these two graphs. Find c for these two graphs. Combine c, e, f, and v to make a new Euler s Formula!

21 A Chance to Discover Euler s General Formula Note that Euler s Formula only applies to connected graphs, i.e., graphs that have one component where we write c = 1. See if you can extend Euler s Formula for c = 2 in the graph below! Theorem (Euler s General Formula) For any planar graph with vertices v, edges e, faces f and components c, the following must hold: v e + f c = 1.

22 Another Euler s Formula 2 A connected planar graph has vertices whose degrees are 3, 3, 4, 4, 5, 6, 7. How many vertices are there? A connected planar graph has vertices whose degrees are 3, 3, 4, 4, 5, 6, 7. How many edges are there? A connected planar graph has vertices whose degrees are 3, 3, 4, 4, 5, 6, 7. How many faces are there? A disconnected planar graph with c = 2 has vertices whose degrees are 3, 3, 4, 4, 5, 6, 7. How many faces are there? A disconnected planar graph with c = 4 has vertices whose degrees are 3, 4, 5, 6, 7, 8, 9. How many faces are there?

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