# Planar graphs. Wednesday, April 20, 2011 Math 55, Discrete Mathematics. Recall...

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1 Planar graphs Wednesday, April 20, 20 Math 55, Discrete Mathematics Recall...

2 Euler s formula face --> portion of the plane cut off by a planar embedding of graph vertices edges faces components n e f c K 4 Euler s formula vertices edges faces components n e f c Theorem (Euler s formula): Let G be a planar graph with n vertices, e edges, and c components. Consider a planar embedding of G with f faces. Then n - e + f - c =. We prove the connected version of Euler s formula......on the chalkboard!

3 More about faces Implication of Euler s formula: Regardless of how you draw a planar graph without edge crossings, the number of faces is always the same. degree of a face = the # of sides of edges on its boundary Proposition: Let G be a planar graph. The sum of the degrees of the faces in a planar embedding of G is 2 E(G). Bounds on number of edges in planar graph Corollary (to Euler s formula): Let G be a planar graph with at least 2 edges. Then E(G)! V(G) - 6. Furthermore, if G is triangle-free, then E(G)! 2 V(G) - 4. We prove the first part of this corollary......on the chalkboard! Corollary (to Euler s formula): Let G be a planar graph. Then δ(g)! 5. Proof also on the chalkboard!

4 Bound on min degree in a planar graph Corollary (to Euler s formula): Let G be a planar graph. Then δ(g)! 5. icosahedron icosahedral graph regular polyhedron with 20 identical equilateral triangular faces Characterizing planar graphs Claim: The graphs K5 and K, are nonplanar. Proof: We have E(K5) = 0 > 9 = V(K5) - 6. Thus, K5 cannot be a planar graph (by previous corollary). K 5 K, Also, K, is a bipartite graph, so it has no odd cycles, and thus, no triangles. Furthermore, E(K,) = 9 > 8 = 2 V(K,) - 4. Thus, K, cannot be a planar graph (by previous corollary).

5 Three Utilities Problem Can you run connections from each utility plant to each home without any connections crossing? Note: You may place the homes and utility plants anywhere you like (except on top of one another). Three Utilities Problem Is K, planar? Note: You may place the homes and utility plants anywhere you like (except on top of one another).

6 Characterizing planar graphs Theorem (Kuratowski s Thm): A graph is planar if and only if it does not contain a subdivision of K5 or K,. A subdivision of a graph G is formed by replacing edges of G with paths. K5 Characterizing planar graphs Theorem (Kuratowski s Thm): A graph is planar if and only if it does not contain a subdivision of K5 or K,. A subdivision of a graph G is formed by replacing edges of G with paths. subdivision of K5

7 Example using subdivisions A subdivision of a graph G is formed by replacing edges of G with paths. Claim: graph has a subdivision of K, Example using subdivisions A subdivision of a graph G is formed by replacing edges of G with paths. Claim: graph has a subdivision of K,

8 Example using subdivisions A subdivision of a graph G is formed by replacing edges of G with paths. Claim: graph has a subdivision of K, Example using subdivisions A subdivision of a graph G is formed by replacing edges of G with paths. Claim: graph has a subdivision of K,

9 Characterizing planar graphs Theorem (Kuratowski s Thm): A graph is planar if and only if it does not contain a subdivision of K5 or K,. A subdivision of a graph G is formed by replacing edges of G with paths. This theorem can also be used to show that the Petersen graph is a nonplanar graph. In your spare time... Planarity is a game based on planar graphs:

10 Planar graph proofs (done on chalkboard) Theorem (Euler s formula). If a connected planar graph G with n vtcs and e edges has a planar embedding with f faces, it follows that n e + f =2. Proof. Proof by induction on e = E(G). Fix the number of vtcs in G as n. Base case: Since G is connected, it must have at least n edges, so base case is e = n. Because G is a connected graph with n vtcs and n edges, it follows that G is a tree. Since G is acyclic, it has exactly one face in any planar embedding. So as desired. n e + f = n (n ) + = 2 Induction hypothesis: Suppose the result holds for any connected, planar graph with n vtcs and e n edges. Let G be a connected planar graph with n vtcs and e + edges. Consider a planar embedding of G with f faces. Since G is not a tree (it has at least n edges), there exists an edge uv of G that is not a cut edge. Consider graph G uv. It is a connected graph with n vtcs and e edges. Furthermore, G uv is planar and the planar embedding of G uv (use the planar embedding of G from above and erase edge uv) has f faces. This is a consequence of the fact that there are two distinct faces in the planar embedding of G that have uv on their boundaries. When uv is removed, these two faces become one face. Applying the induction hypothesis to G uv, wehave n e +(f ) = 2. Formally, this requires some further justification, i.e. the Jordan Curve Theorem.

11 From rearranging terms in this expression, it follows that which is the desired conclusion. n (e +) f =2, Thus, the result holds by induction. We can use Euler s formula to derive upper bounds on the number of edges in planar graphs (in terms of the number of vtcs). Corollary. Let G be a planar graph with n vertices and e 2 edges. Then e n 6. Furthermore, if G is triangle-free, then e 2n 4. Proof. Without loss of generality, assume G is connected. (If not and G has c components, add c edges between the components to make it connected and maintain planarity.) Consider a planar embedding of G. Then, by Euler s formula, we have n e + f =2 = f =2 n + e (). Since G has more than one edge, every face in the embedding has degree at least. Thus, 2e = sum of degrees of faces f =(2 n + e) by (). Rearranging terms, we have 6 n +e 2e = e n 6. The proof of the second part of the corollary is left as an exercise. (Based on the assumption that G is triangle-free, we adjust the bound on the degree of any face.) 2

12 Remark Notice that this necessary condition is not a sufficient condition for a graph to be planar. For example, consider the triangle-free graph G below: graph G It has 7 vtcs and 0 edges. So E(G) = =2 V (G) 4 but this graph contains K,, which we already saw was nonplanar. Corollary. Let G be a planar graph. Then δ(g) 5. Proof. Suppose G is a planar graph with n vertices. If G has less than 2 edges, then clearly the result holds, so assume G has at least 2 edges. Then nδ(g) d(v) = 2 E(G) 2(n 6) = 6n 2. v V (G) We divide both sides by n to obtain δ(g) 6 2 n < 6 where the last inequality follows from the fact that n 2 n δ(g) is an integer, we have δ(g) 5. > 0. Since An example for which δ(g) =5: icosahedral graph

13 Coloring planar graphs Strategy for proof of 6-colorability: proof by induction; color min degree vertex with a leftover color. Theorem (Six Color Theorem). Every planar graph is 6-colorable. Proof. We prove by induction on n = V (G). Base cases (n 6): Let G be a planar graph with 6 vtcs or less. Then by assigning each vertex a distinct color, we have a proper 6-coloring of G. Induction hypothesis: Any planar graph on n vertices is 6-colorable. Consider a planar graph G on n + vertices. By previous corollary, G has a vertex v such that d(v) 5. Let G = G v. Then G is planar and has n vertices. By the IH, G is 6-colorable. Properly color vtcs of G using 6 colors. Extend this coloring to G by assigning v a color that is not present among its neighbors. Note that this is possible since v has at most 5 neighbors, and there are 6 colors to choose from, so at least one color is not present amongst neighbors of v. Hence, G is 6- colorable. Theorem (Five Color Theorem). Every planar graph is 5-colorable. See slides. Theorem (Four Color Theorem). Every planar graph is 4-colorable. 4

14 Coloring planar graphs Wednesday, April 20, 200 Math 55, Discrete Mathematics Coloring planar graphs with 6 colors Theorem: Every planar graph is 6-colorable. Proof type: Induction on n = V(G). Basic argument of inductive step: Let v be a vertex of min degree in a planar graph G on n + vtcs. Consider G-v. Apply IH to G-v to get a 6-coloring of G-v. Color min degree vertex v with a leftover color. Key fact: δ(g) 5.

15 Coloring planar graphs with 5 colors Theorem: Every planar graph is 5-colorable. Proof type: Induction on n = V(G). Basic argument: Color min degree vertex v with a leftover color, if possible. Otherwise, swap two colors to make a color available at v. 5 4 v 2 Key facts: δ(g) 5. Cannot have both a - alternating path and a 2-4 alternating path. Coloring planar graphs with 5 colors Theorem: Every planar graph is 5-colorable. Proof type: Induction on n = V(G). Basic argument: Color min degree vertex v with a leftover color, if possible. Otherwise, swap two colors to make a color available at v. Key facts: δ(g) 5. Cannot have both a - alternating path and a 2-4 alternating path. 5 v 2 4 Kempe chain

16 Coloring planar graphs with 5 colors Theorem: Every planar graph is 5-colorable. Proof type: Induction on n = V(G). Basic argument: Color min degree vertex v with a leftover color, if possible. Otherwise, swap two colors to make a color available at v. Key facts: δ(g) 5. Cannot have both a - alternating path and a 2-4 alternating path. 5 v 2 4 Coloring planar graphs with 5 colors Theorem: Every planar graph is 5-colorable. Proof type: Induction on n = V(G). Basic argument: Color min degree vertex v with a leftover color, if possible. Otherwise, swap two colors to make a color available at v. Key facts: δ(g) 5. Cannot have both a - alternating path and a 2-4 alternating path. 5 w v 2 4 on component of G[2/4 vtcs] containing w, swap all 2/4 colored vtcs

17 Coloring planar graphs with 5 colors Theorem: Every planar graph is 5-colorable. Proof type: Induction on n = V(G). Basic argument: Color min degree vertex v with a leftover color, if possible. Otherwise, swap two colors to make a color available at v. Key facts: δ(g) 5. Cannot have both a - alternating path and a 2-4 alternating path. 5 w v 4 4 on component of G[2/4 vtcs] containing w, swap all 2/4 colored vtcs Coloring planar graphs with 4 colors Theorem (Four Color Thm): Every planar graph is 4-colorable. Proof: Proof by Appel and Haken in 977 (computer-aided proof). Note that it is easy to see that the above bound is best possible by considering K4. There were several early failed attempts at proving the theorem. Timeline 879: Proof by Alfred Kempe, which was widely acclaimed. 880: Proof by Peter Guthrie Tait. 890: Kempe's proof was shown incorrect by Percy Heawood. (Heawood proved 5-color theorem at this time.) To show χ(g) 4 is very difficult. 89: Tait's proof was shown incorrect by Julius Petersen. A new (more simplified) proof of the Four Color Theorem was given in 996 by Robertson, Sanders, Seymour, and Thomas.

18

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