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Graph Theory

Graph Theory. Two Applications D.N. Seppala-Holtzman St. Joseph ’ s College. Here we will consider two applications of graph theory. Euler Circuits Planarity. The 7 Bridges of Konigsburg.

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Graph Theory

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  1. Graph Theory Two Applications D.N. Seppala-Holtzman St. Joseph’s College

  2. Here we will consider two applications of graph theory • Euler Circuits • Planarity

  3. The 7 Bridges of Konigsburg • Konigsburg (now called Kalingrad) is a city on the Baltic Sea wedged between Poland and Lithuania. • A river runs through the city which contains a small island. • There are 7 bridges which connect the various land masses of the city.

  4. The City of Konigsburg

  5. The City of Konigsburg

  6. The Problem • The people of Konigsburg made a sport during the 18th century of trying to cross each and every one of the 7 bridges exactly once. • This was to be done in such a way that one would always end up where one began.

  7. The problem was solved by Leonhard Euler (1707-1783)

  8. Euler (pronounced “oiler”) • Euler was one of the greatest mathematicians of all time. • He contributed to virtually every field of mathematics that existed in his time. • The publication of his collected works (Opera Omnia) is presently up to volume 73 and still not complete.

  9. Euler and Graph Theory • Euler’s solution to the Konigsburg bridge problem was more than a trivial matter. • He didn’t just solve the problem as stated; he made a major contribution to graph theory. Indeed, he essentially invented the subject. • His contribution has many practical applications.

  10. Some Vocabulary • A graph is a set of vertices connected by edges. • The valence of a vertex is the number of edges that meet there. • An Euler Circuit is a path within a graph that covers each and every edge exactly once and returns to its starting point.

  11. Euler’s Theorem • A connected graph has an Euler circuit if and only if every vertex has an even valence. • The Konigsburg bridge problem translated into a graph in which all valences were odd. Thus there was no way to walk on each bridge precisely once.

  12. Euler’s Theorem ---Why is it true? • Any vertex with odd valence must be either a starting point or an ending point. • All points that are neither starting nor ending points must be left as often as they are entered.

  13. Euler’s Theorem ---Why is it important? • There are many, many examples of circuits that one wishes to traverse such that every edge is covered and no edge is repeated. • Routes for snowplows, letter carriers, meter readers, and the like, share these characeristics.

  14. Not all graphs have even valence on all vertices --- What then? • One cannot expect that every street layout or route will translate into a graph with all vertices of even valence. • In these cases, one can try to minimize the number of edges that are repeated. • There is an algorithm to do this. It is called Eulerizing the graph.

  15. Eulerizing a Graph I • Select pairs of vertices in the graph that have odd valence. • Do this in such a way that the vertices are as close together (have the fewest edges between them) as possible. • Neighboring vertices would be the best choice, if possible.

  16. Eulerizing a Graph II • For each edge on the path that connects a pair of odd-valenced vertices, generate a “phantom edge” duplicating that edge. • Do this for each pair of odd-valenced vertices. • In general, there will be more than one Eulerization of a graph. The fewer duplicated edges, the better.

  17. Recall the City of Konigsburg

  18. Let us Eulerize Konigsburg I B D A C

  19. Let us Eulerize Konigsburg II B D A C

  20. Eulerizing Konigsburg III • Here, we have selected pairs of odd-valenced vertices, BD and AC. • We have added a “phantom” edge between these pairs of vertices. These phantom edges are edges that are traversed twice. • Now, with the addition of just two edges, the graph has all even-valenced vertices.

  21. A Troublesome Question • How do we know that we can always do this? • In particular, how do we know that the odd-valenced vertices will occur in pairs?

  22. The Number of Odd-Valenced Vertices is Even ---Here’s a Proof: • Suppose that there are N edges. • Thus, there are 2N “ends” of edges. • The sum of all the valences must be 2N. • Thus, it is not possible to have an odd number of odd-valenced vertices. • Hence, the odd-valence vertices occur in pairs.

  23. Euler Circuits: In Summation • A very simple and elegant idea has led to a wide variety of real-world applications. • Nearly any process which involves routing (and there are many) can be made more efficient by these methods. • Many millions of dollars can be saved in the process!!

  24. Planarity • Another problem in graph theory also has a simple solution that has major consequences. • The question of planarity refers to whether a graph can be drawn in the plane without any edges crossing any other ones.

  25. Connect 3 Houses to 3 Utilities H1 H2 H3 U1 U2 U3 Draw edges from each U to each H without crossing edges.

  26. An Attempted Solution H1 H2 H3 U1 U2 U3 No H2-U2 Connector

  27. K3,3 • The graph connecting all vertices of a set of three to all vertices to another set of three is called K3,3 • This graph is not planar. That is to say, it is not possible draw it in the plane with no edges crossing others.

  28. Kn • Kn is called the complete graph on n vertices. It is the graph one gets by starting with n vertices and drawing an edge between each pair. • Kn is planar or not depending upon n.

  29. K3 Is Just a Triangle, Thus It Is Planar

  30. K4 Is Also Planar

  31. K5 Is Not Planar E A D B No E B Edge C

  32. Kn Is Not Planar for n > 4 • We know that K5 is not planar. • If n is bigger than or equal to 5 then Kn couldn’t possibly be planar.

  33. Planar Graphs --- A Theorem • All non-planar graphs (those that cannot be drawn in the plane without crossing edges) contain either a copy of K5 or K3,3 as a sub-graph. • Conversely, if neither K5 nor K3,3 is to be found embedded anywhere inside a graph, that graph will be planar.

  34. Planar Graphs --- Who Cares? • Whether a graph is planar or not is quite important. • Any physical interpretation of a graph that wants to avoid crossings of edges needs to take this into account. • The most obvious examples are printed circuit boards and micro-chips.

  35. Other Graph Theory Applications • Euler Circuits and Planarity are only two of many applications of graph theory. • Most any system or object which contains a network of some type has an application of graph theory lurking somewhere below the surface. • Roads, routes, phone lines, electrical circuits are all examples.

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