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Some applications of graph theory

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More on the Four Color Problem

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The four color theorem was stated, but not proved, in 1853 by Francis Guthrie.

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The theorem asserts that four colors are enough to color any geographical map in such a way that no neighboring two countries are of the same color.

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Other applications? Beyond the mathematical issues in connection with this theorem, coloring question have wider practical applicationsespecially for mobile phones. They make it possible to effectively reduce the number of used broadcasting frequencies, equivalent to the colors.especially for mobile phones

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Connection with graph theory Vertices represent geographical regions Edges represent borders between regions. Such a graph is planar, meaning it can be drawn on paper in such a way that no two edges (pieces of border) intersect except at a shared vertex (two pieces of border of the same region). Nonplanar graphs are not always 4-colorable.

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A long road… Tait's conjecture:(1886) Every polyhedron has a Hamiltonian cyclepolyhedron ================= (along the edges) through ALL the vertices.

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Polyhedra The Steinitz theorem (1894) for polyhedra states that every simple 3-connected planar graph is a polyhedral graph of a 3-dimensional polytope and vice versa.

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If Tait’s conjecture were true… it would prove the 4-color theorem: any planar trivalent graph with a Hamiltonian cycle has a 3-coloring of the edges, which is equivalent to having a 4-coloring of faces.trivalent graph Proof: color the Hamiltonian cycle alternately yellow and red. All other edges are diagonals of this polygon, & can be blue. code the edge colors as (0,1) (1,0) and (1,1); then color any face (0,0), and color the rest by crossing over edges using vector addition [mod 2]. This gives a face-coloring in (0,0) (0,1) (1,0) & (1,1).

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Tutte’s counterexample A non-Hamiltonian 3- connected cubic graph given by Tutte (1946) as a counterexample to Tait's Hamiltonian graph conjecture using three copies of the Walther graph.

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Appel and Haken’s proof (1972) reduced the infinitude of possible maps to 1,936 reducible configurations (later reduced to 1,476). These had to be checked one by one by computer... This reducibility part of the work was independently double checked with different programs and computers. … the unavoidability part of the proof was verified in over 400 pages of microfiche, which had to be checked by

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Proof by...computer Automated theorem proving has become a cottage industry. Human beings are still required to give meaning to what the computers do… Curiously, “proof assistants” are also used in studying software reliability.software reliability

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Recent work on map coloring

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Finding the shortest path

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Path from vertex V to vertex W f: edge weight function f(p) Minimize sum of f(p) over all possible paths Shortest path algorithms are applied in an obvious way to automatically find directions between physical locations, such as driving directions on web mapping websites like Mapquest.web mappingMapquest For example, if vertices represent the states of a puzzle like a Rubik's Cube and each directed edge corresponds to a single move or turn, shortest path algorithms can be used to find a solution that uses the minimum possible number of movesRubik's Cube

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The traveling salesman problemtraveling salesman problem find shortest path that goes through every vertex exactly once, and returns to the start. Unlike the shortest path problem, this problem is NP-complete and is believed not to be efficiently solvableNP-complete

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A much simpler problem…shortest path Find the shortest path between two vertices on a weighted graph

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Dijkstra’s algorithm

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Dijkstra’s the running time is O(|V| 2 +|E|)=O(|V| 2 ). Here |V|=#of vertices, |E|=# of edges Total run time is at most a fixed multiple of the square of the number of vertices.

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Dijkstra’s algorithm Create distance list, previous vertex list, visited list, and current vertex. All values in distance list set to infinity except starting vertex to zero. All values in visited list are set to false. All values in the previous list are set to a special value signifying that they are undefined, such as null.null Current vertex is set as the starting vertex. Mark the current vertex as visited. Update distance and previous lists based on those vertices which can be immediately reached from the current vertex. Update the current vertex to the unvisited vertex that can be reached by the shortest path from the starting vertex. Repeat (from step 6) until all nodes are visited.

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Dijkstra demo on youtube

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Dijkstra applets

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SPA: the pineapple express

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Honlu toSFChicagoLABostonNYATLLondon 175/Hon105/Hon68/Hon∞∞∞∞ 2 LA75/Hon105/Hon68/Hon∞131/LA125/LA∞ 3 ATL75/Hon105/Hon68/Hon 475/Hon105/Hon68/Hon …75/Hon105/Hon68/Hon

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[Find the shortest path from Honolulu to London] A.[Honolulu -> Chicago -> Boston -> London] B.[Honolulu -> SF -> NY -> London] C.[Honolulu -> LA -> NY -> London] D.[Honolulu -> LA -> ATL -> London]

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Traveling salesman problem The case of Sweden

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The Traveling Salesman Problem

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World tour

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Small world graphs

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What’s your Kevin Bacon number? Bela Lugosi, has a Bacon number of 3: 1. Bela Lugosi was in Abbott and Costello Meet Frankenstein (1948) with Vincent Price 2. Vincent Price was in The Raven (1963) with Jack Nicholson 3. Jack Nicholson was in A Few Good Men (1992) with Kevin Bacon

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Thanks to George Hickenlooper, your instructor is two handshakes from the president

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Cinema Finder Problem: Find an actor who has as large a Kevin Bacon number as possible Hamidreza Tahery

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