Paths and CircuitsThe Seven Bridges of KönigsbergSlide 3Slide 4Euler’s SolutionSlide 6Walks and PathsSlide 8SynopsisEuler CircuitsConnected GraphsSlide 12The Two Bridges of AshlandProofSlide 15Slide 16Slide 17Slide 18Slide 19ExampleSlide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Slide 40Slide 41Slide 42Slide 43Slide 44Slide 45Slide 46Slide 47Slide 48Slide 49Slide 50Slide 51Slide 52Slide 53Slide 54Slide 55Slide 56Slide 57Slide 58Slide 59Slide 60Euler PathsHamiltonian CircuitsSlide 63Slide 64Paths and CircuitsLecture 52Section 11.2Wed, Apr 26, 2006The Seven Bridges of KönigsbergIn the city of Königsberg, two branches of the Pregel River came together, with an island at their junction.The Seven Bridges of KönigsbergThere were seven bridges crossing the river at various places.The Seven Bridges of KönigsbergThe challenge was to start at one point, cross each bridge exactly once, and return to the starting point.?Euler’s SolutionEuler abstracted the bridges as a graph with four vertices and seven edges.Euler’s SolutionEach vertex represents a land mass and each edge represents a bridge.North ShoreSouth ShoreIslandPeninsulaWalks and PathsA walk from vertex v to vertex w is a finite alternating sequence of adjacent vertices and edges from v to w:v0 e1 v1 e2 … en – 1 vn – 1 en vn,where v0 = v and vn = w.A path from v to w is a walk that does not repeat any edge.Walks and PathsA simple path is a path that does not repeat any vertices.A closed walk is a walk that starts and ends at the same vertex.A circuit is a closed path.A simple circuit is a circuit that does not repeat any vertex.Synopsiswalk = from A to B, no restrictions.path = walk, no repeated edge.closed = from A to A.circuit = closed walk.simple = no repeated vertex.Euler CircuitsAn Euler circuit is a circuit that contains every vertex and every edge of the graph.The problem of the Seven Bridges of Königsberg is to find an Euler circuit.Connected GraphsA graph is connected if, for every pair of vertices v and w, there is a walk from v to w.A connected component of a graph is a maximal connected subgraph.Euler’s SolutionTheorem: A graph has an Euler circuit if and only if it is connected and every vertex has even degree.Thus, an Euler circuit over the Seven Bridges of Königsberg does not exist.The Two Bridges of AshlandAt Randolph-Macon College, they have been trying to solve the Two Bridges of Ashland problem for decades.King’s DominionRMCI-95?ProofProof ():Suppose a graph G has an Euler circuit.Let v  V(G).Then as we travel the circuit, each time we pass through v, we “use up” two of the edges incident to v.When we finish the circuit, we have used all the edges incident to v .ProofThus, v had an even number of edges.Obviously, G must be connected.ProofProof ():Now suppose that G is connected and that every vertex of G has even degree.Choose a vertex v at which to begin.deg(v) > 0 since G is connected, so follow one of the edges incident to v.Let w be the next vertex.We used one of w’s edges to get there.Proofw has even degree, so there is at least one more edge available that we can follow.This happens at every vertex that we visit.Thus, the circuit is forced to terminate only when we return to the starting vertex v.This procedure alone does not necessarily produce an Euler circuit.ProofSuppose there are edges that were not used.Follow the original circuit until a vertex is reached that is incident to one of the unused edges.Apply the original procedure to produce a circuit that starts and ends at this vertex.“Splice” it into the original circuit.ProofContinue in this way, splicing circuits into the existing circuit, until there are no unused edges remaining.The result is an Euler circuit.ExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleExampleEuler PathsTheorem: A graph G has an Euler path from v to w if G is connected, v and w have odd degree, and all other vertices have even degree.Proof:Add an edge from v to w.Then the graph has an Euler circuit.Remove the new edge from the circuit.Hamiltonian CircuitsA Hamiltonian circuit is a simple circuit that includes every vertex of the graph.The Traveling Salesman Problem seeks a Hamiltonian circuit of minimal length.Hamiltonian CircuitsTheorem: If a graph G has a nontrivial Hamiltonian circuit, then G has a subgraph H such thatV(H) = V(G).H is connected.|E(H)| = |V(G)|.deg(v) = 2 for all v  V(H).These conditions are necessary, but not sufficient.Hamiltonian CircuitsThe following graph does not have a Hamiltonian