# MATH 22 PLANAR GRAPHS & EULER S FORMULA. Lecture X: 11/25/2003

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1 MATH 22 Lecture X: 11/25/2003 PLANAR GRAPHS & EULER S FORMULA Regions of sorrow,... where peace and rest can never dwell, hope never comes. Milton, Paradise Lost (on Tufts) Ornate rhetoric taught out of the rule of Plato. Milton, Of Education (on Math 22B) Copyright 2003 Larry Denenberg

2 Administrivia Exam followup? Grading policy followup? N.B.: Final homework and projects are due on 12/4 Today: Planar graphs, Euler s Formula, and some consequences and generalizations.

3 Planarity An undirected graph G, which can be a multigraph and can have self-loops, is planar if it can be drawn in the plane in such a way that no edges cross (this is always what drawn in the plane means). Note that edges don t have to be drawn as straight lines. As definitions go, this one is rotten, but a more precise definition isn t really illuminating. Only undirected graphs today, by the way. Any number of multiple edges or self-loops, no matter how many, don t really matter; they can always be drawn small enough not to affect anything. [blackboard examples of definitely-planar and maybenonplanar graphs] Question: How do we know if a graph is planar or not? If we can draw it in the plane, it s planar for sure. But if we can t, maybe we re just not clever enough. How do we know when to quit trying? [This is typical: It s easier to show how to do something than to prove there s no possible way to do it.]

4 Regions When a graph is drawn in the plane, it divides the plane into regions called (surprisingly) regions. Sometimes they re called faces for a good reason that we ll see soon. Note that there is always exactly one infinite region since a finite graph must occupy a finite portion of the plane. The term region is not defined for nonplanar graphs or for nonplanar drawings of planar graphs. When a graph G is drawn in the plane, how many regions does it have? Can it depend on how you draw it? The answer is that the number of regions depends only on the graph and not on any particular drawing. It s a property of G, just like the number of vertices or edges. Theorem: Suppose that G is a planar graph that has v vertices, e edges, and k connected components. Then any planar drawing of G has k + e v +1 regions. This Theorem is more usually written like this: v e + r = k + 1 When G is connected, we get Euler s Formula v e + r = 2 of which we ll see two Awesome Generalizations later.

5 Proof of Euler s Formula The proof is by induction on e, the number of edges. Base case, e = 0: An edgeless graph on v vertices has v connected components, and no matter how you draw it there s only 1 region. Plug in and verify the Formula. Inductive case: Suppose the Formula is true for all graphs with e or fewer edges, and let G be any graph with e+1 edges. Pick any edge e 0 from G and delete it, obtaining graph H, in which the Formula holds by the Inductive Hypothesis. There are two possibilities: Possibility 1: e 0 joins two nodes in distinct connected components of H. In this case G has one fewer connected component and one more edge than H, but the number of regions remains the same [picture]. So the Formula is valid for G. Possibility 2: e 0 joins nodes in the same connected component of H. In this case G has the same number of connected components as H, but has one more edge and one more region, so again the Formula is valid for G. [Blackboard: semi-handwaving proof that G has one more region than H. Special case if e 0 is a self-loop!]

6 Corollaries of Euler s F If G is a planar graph, not a multigraph, with no self loops and at least 2 edges, then 3r 2e. (That is, you can t have lots of regions without lots of edges!) Proof: This is obvious when r = 1. If r > 1, then each region is bounded by at least 3 edges, so the total number of edge-boundings (instances where an edge touches a region) is 3r. But each edge bounds at most 2 regions, so the number of edge-boundings is also 2e. This implies that 3r 2e. [Where did we use the fact that G has neither self-loops nor multiple edges?] Under the same hypotheses, e 3v 3k 3. (That is, a planar graph can t have too many edges.) Proof: From Euler s formula, k + 1 = v e + r v e + 2e/3 = v e/3 from which a little arithemetic gives the result. When the graph is connected, we have e 3v 6. Every planar graph without self loops or multiple edges has at least one vertex with degree 5. Proof: Assume there is a graph where all nodes have degree at least 6. The sum of the node degrees is then at least 6v. This sum is also 2e (remember?) so we have 2e 6v or e 3v. But this contradicts e 3v 3k 3. Corollary: K n is non-planar for every n 7.

7 Two Non-Planar Graphs Fact: K 5, the complete graph on 5 nodes, is nonplanar. Proof: In K 5 we have v = 5 and e = 10, so we don t have e 3v 6 as we must in all connected planar graphs. Definition: For integers u,v 1, the complete bipartite graph on u and v nodes, written K u,v, is the undirected bipartite graph with no multiple edges, with u vertices in one part and v vertices in the other part, and with every possible edge between the parts. [blackboard examples] Fact: K 3,3 is nonplanar. Proof: If K 3,3 were planar, it would have 5 regions by Euler s formula. Now K 3,3 contains no triangles [why?] so each region would touch at least 4 edges, making at least 20 edge-boundings. But K 3,3 has only 9 edges and each bounds at most 2 regions, a contradiction. It turns out that these two nonplanar graphs are more than just examples; they are the very essence of nonplanarity. The next theorem, which we won t prove, says essentially that every nonplanar graph contains one of these two graphs as (sort of) a subgraph!

8 Kuratowski s Theorem Definition: Two graphs are homeomorphic if they can be made isomorphic via the following operations: Delete any edge {u,v}, then add a new vertex w and edges {u,w} and {w,v} Delete any vertex w of degree two, delete (perforce) its adjacent edges {u,w} and {w,v}, then add new edge {u,v} Informally, two graphs are homeomorphic if they can be made the same by putting dots on lines, or erasing dots that have only two lines coming out of them and merging the lines. The phrase isomorphic up to vertices of degree 2 means the same thing. [Blackboard examples] Theorem: A graph is nonplanar if and only if it contains a subgraph homeomorphic either to K 5 or to K 3,3. Proof: Half the proof is easy, since a graph with a subgraph homeomorphic to K 5 or K 3,3 clearly can t be planar. (This can t be plainer.) The converse is harder. This Theorem is called Kuratowski s Theorem. [Blackboard examples of nonplanar graphs]

9 Polyhedra A polyhedron is a solid object bounded by planar polygonal surfaces, each of which is called a face. The vertices and edges of the faces are also the vertices and edges of the polyhedron. [blackboard drawings, insofar as I can draw polyhedra] Note that polyhedra need not be convex. Every polyhedron corresponds to a connected planar graph with the same number of edges and vertices. To get this graph, puncture any face of the polyhedron and pull the hole open until the polyhedron is stretched out flat. [even less coherent drawings; artist needed!] Each face of the polyhedron corresponds to a region of the planar graph; the face we punctured becomes the infinite region. It follows that v e + f = 2 for any polyhedron, where f is the number of its faces. A convex polyhedron is a Platonic solid if its faces are identical regular polygons, and at each vertex the same number of faces come together at the same (solid) angle. [blackboard drawing of at most two Platonic solids] Question: How many Platonic solids exist?

10 All the Platonic Solids Suppose a Platonic solid has v vertices, e edges, and f faces. Suppose each face is a regular m-gon and that n faces meet at each vertex. Fact: m 3 Fact: n 3 Fact: 2e = mf Fact: 2e = nv (each face must be at least a triangle) (each vertex lies in at least three faces) (each edge lies in two faces, each of which has m edges) (there are 2e edge endpoints, and each vertex is the endpoint of n edges) Using these facts and Euler s formula, we get: 2 = v e + f = 2e/n e + 2e/m = (2m mn+2n)e/mn Therefore (2m mn+2n) > 0, hence (mn 2m 2n) < 0, hence (mn 2m 2n+4) < 4, hence finally (m 2)(n 2) < 4. But m and n are integers each at least 3, so the only possibilities are: m = n = 3 (so e=6, v=f=4), the regular tetrahedron m = 4, n = 3 (so e=12, f=6, v=8), the cube m = 3, n = 4 (so e=12, f=8, v=6), the octahedron m = 5, n = 3 (so e=30, f=12, v=20), the dodecahedron m = 3, n = 5 (so e=30, f=20, v=12), the icosahedron

11 Awesome Generalization 1 Suppose P is a solid object bounded by plane polygonal faces, but is not a polyhedron because it has holes. [Blackboard drawing of a cube with a square hole through it. Note that each face must be a polygon, so the front face is really four faces, each a trapezoid, with edges between them. The object has 16 faces, 16 vertices, and 32 edges; it s regular of degree 4.] The number of holes in P is called the genus of P. The object drawn on the blackboard has genus 1. Theorem: Given any such solid object with v vertices, e edges, f faces, and genus g, we have Proof: Omitted. v e + f = 2 2g When the object is a polyhedron there are no holes, so g = 0 and the theorem reduces to Euler s formula.

12 Awesome Generalization 2 The object that generalizes vertex (0 dimensions), edge (1d), polygon (2d), and polyhedron (3d) to an arbitrary number of dimensions is called a polytope. A polygon is bounded by vertices and edges. A polyhedron is bounded by vertices, edges, and faces. Similarly, an n-dimensional polytope is bounded by vertices, edges, faces, polyhedra,..., and polytopes of all dimension up to n 1. We also consider an n-d polytope to be bounded by a single n-d polytope, namely itself. Example: The 4-hypercube is a 4 dimensional polytope bounded by 16 vertices, 32 edges, 24 (square) faces, 8 (cubical) polyhedra, and itself. [blackboard drawing] Theorem: Let P be an n-dimensional polytope bounded by b k polytopes of dimension k, for each 0 k n. (So P has b 0 vertices, b 1 edges, etc., and b n = 1.) Then b 0 b 1 + b 2 b ( 1) n b n = 1 Proof: You must be joking. Check it on the 4- hypercube. In 3 dimensions, this is v e + f 1 = 1, Euler s formula. In 2d, it says any polygon has as many vertices as edges. In 1d, it says that an edge joins two vertices.

13 Duality Again Suppose G is a planar graph or multigraph, possibly including self-loops, and we have a planar drawing of G. A dual graph of G, written G d, is generated from this drawing as follows: Each region in the drawing becomes a node of G d. For each edge e of G we make an edge of G d : If e separates two regions of G, we put an edge between the nodes of G d that correspond to those regions. If e lies wholly within a region of G, we give the corresponding node of G d a self loop. A dual graph of G has as many vertices as G has regions, and has the same number of edges as G. A dual graph of G is connected whether or not G is connected. But if G is connected, it follows from Euler s formula that G d has as many regions as G has vertices. A dual graph of G may be a multigraph and may have self loops, even if G is not a multigraph and has no selfloops. A dual is generated from a drawing of G. Depending on how G is drawn, we may get different results. That is, a graph can have non-isomorphic dual graphs.

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