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Which grid graphs have euler circuits - Instructor: Is l Dillig, CS311H: Discrete Mathematic

Part 1: If either m or n is even, and both m > 1 and n > 1, the graph is Hamiltoni

The graph does have an Euler path, but not an Euler circuit. There are exactly two vertices with odd degree. The path starts at one and ends at the other. The graph is planar. Even though as it is drawn edges cross, it is easy to redraw it without edges crossing. The graph is not bipartite (there is an odd cycle), nor complete. 30.06.2023 г. ... Ans: A linked graph G is an Euler graph if all of its vertices are of even degree, and exactly two nodes have odd degrees, in which case the ...○ An undirected graph has an Eulerian path if and only if exactly zero or two vertices have odd degree. Page 9. Euler Path Example. 2. 1. 3. 4. Page 10 ...If a graph has an Euler circuit, that will always be the best solution to a Chinese postman problem. Let’s determine if the multigraph of the course has an Euler circuit by looking at the degrees of the vertices in Figure 12.130. Since the degrees of the vertices are all even, and the graph is connected, the graph is Eulerian.Oct 29, 2021 · An Euler circuit is a circuit in a graph where each edge is crossed exactly once. The start and end points are the same. All the vertices must be even for the graph to have an Euler circuit. Two different trees with the same number of vertices and the same number of edges. A tree is a connected graph with no cycles. Two different graphs with 8 vertices all of degree 2. Two different graphs with 5 vertices all of degree 4. Two different graphs with 5 vertices all of degree 3. Answer.21.10.2013 г. ... 5-17(a) is connected, and the vertices are all even. By. Euler's circuit theorem we know that the graph has an Euler circuit, which implies that ...Part 1: If either m or n is even, and both m > 1 and n > 1, the graph is Hamiltonian. This proof is going to be by construction. If one of the even sides is of length 2, you can form a ring that reaches all vertices, so the graph is Hamiltonian. Otherwise, there exists an even side of length greater than 2.Figure 6.5.3. 1: Euler Path Example. One Euler path for the above graph is F, A, B, C, F, E, C, D, E as shown below. Figure 6.5.3. 2: Euler Path. This Euler path travels every edge once and only once and starts and ends at different vertices. This graph cannot have an Euler circuit since no Euler path can start and end at the same vertex ...For Instance, One of our proofs is: Let G be a C7 graph (A circuit graph with 7 vertices). Prove that G^C (G complement) has a Euler Cycle Prove that G^C (G complement) has a Euler Cycle Well I know that An Euler cycle is a cycle that contains all the edges in a graph (and visits each vertex at least once).A H N U H 0 S X B: Has Euler circuit. K P D: Has Euler circuit. R. Which of the following graphs have Euler circuits? L E G K M D C H I A: Has Euler circuit. I B 0 N C: Has Euler circuit. A H N U H 0 S X B: Has Euler circuit. An Euler circuit is a circuit that uses every edge in a graph with no repeats. Being a circuit, it must start and end at the same vertex. Example The graph below has several possible Euler circuits. Here’s a couple, …is a Hamilton path. Page 84. Hamilton Paths and Circuits. (continued). Example: Which of these simple graphs has a. Hamilton circuit or, if not, a Hamilton path ...the graph then have an Euler circuit? If so, then find one. If not, explain why not. Solution. (a) No. Euler’s theorem says that a graph has an Euler circuit if and only if every node has even degree, which is not the case here. For example, node E has odd degree. (b) Yes. The corollary to Euler’s theorem states that a graph without an ...Networks and Graphs: Circuits, Paths, and Graph Structures VII.A Student Activity Sheet 1: Euler Circuits and Paths Charles A. Dana Center at The University of Texas at Austin Advanced Mathematical Decision Making (2010) Activity Sheet 1, 8 pages 4 3. For the following graphs, decide which have Euler circuits and which do not. Graph I Graph IIAn Eulerian graph is a graph that possesses an Eulerian circuit. Example 9.4.1 9.4. 1: An Eulerian Graph. Without tracing any paths, we can be sure that the graph below has an Eulerian circuit because all vertices have an even degree. This follows from the following theorem. Figure 9.4.3 9.4. 3: An Eulerian graph.M1: Euler Circuits, Eulerization Objectives: SWBAT r Identify the vertices and edges in a graph r Identify if a given graph is connected r Determine the valence of each vertex of a graph r Determine whether or not a graph contains an Euler circuit r Eulerize a graph which does not contain an Euler circuit Individual Activity/Group Work ...A graph is called Eulerian if it has an Eulerian Cycle and called Semi-Eulerian if it has an Eulerian Path. The problem seems similar to Hamiltonian Path which is NP complete problem for a general graph. Fortunately, we can find whether a given graph has a Eulerian Path or not in polynomial time. In fact, we can find it in O (V+E) time.A graph will contain an Euler path if it contains at most two vertices of odd degree. A graph will contain an Euler circuit if all vertices have even degree. In the graph below, vertices A and C have degree 4, since there are 4 edges leading into each vertex. B is degree 2, D is degree 3, and E is degree 1.Euler circuts have even degrees in each vertices but if there are four red and three blue you will have four edges in the blue vertices but only three in the red vertices. So you must only be able to have an euler circuit if you have an even number of vertices in each set. These graphs are written K4,3.15. The maintenance staff at an amusement park need to patrol the major walkways, shown in the graph below, collecting litter. Find an efficient patrol route by finding an Euler circuit. If necessary, eulerize the graph in an efficient way. 16. After a storm, the city crew inspects for trees or brush blocking the road.these questions seem to be similar, the first question, which asks whether a graph has an Euler circuit, can be easily answered simply by examining the degrees of the vertices of the graph, while the second question, which asks whether a graph has a Hamilton circuit, is quite difficult to solve for most graphs.Euler Path Examples- Examples of Euler path are as follows- Euler Circuit- Euler circuit is also known as Euler Cycle or Euler Tour.. If there exists a Circuit in the connected graph that contains all the edges of the …Euler path = BCDBAD. Example 2: In the following image, we have a graph with 6 nodes. Now we have to determine whether this graph contains an Euler path. Solution: The above graph will contain the Euler path if each edge of this graph must be visited exactly once, and the vertex of this can be repeated.The Swiss mathematician Leonhard Euler (1707-1783) took this problem as a starting point of a general theory of graphs. That is, he first made a mathematical model of the problem. He denoted the four pieces of lands with "nodes" in a graph: So let 0 and 1 be the mainland and 2 be the larger island (with 5 bridges connecting it to the other ...An Euler circuit is a circuit that uses every edge in a graph with no repeats. Being a circuit, it must start and end at the same vertex. Example The graph below has several possible Euler circuits. Here’s a couple, …If a graph has a Eulerian circuit, then that circuit also happens to be a path (which might be, but does not have to be closed). – dtldarek. Apr 10, 2018 at 13:08. If "path" is defined in such a way that a circuit can't be a path, then OP is correct, a graph with an Eulerian circuit doesn't have an Eulerian path. – Gerry Myerson.Euler Path Examples- Examples of Euler path are as follows- Euler Circuit- Euler circuit is also known as Euler Cycle or Euler Tour.. If there exists a Circuit in the connected graph that contains all the edges of the …Relation to Eulerian graphs. Eulerian matroids were defined by Welsh (1969) as a generalization of the Eulerian graphs, graphs in which every vertex has even degree. By Veblen's theorem the edges of every such graph may be partitioned into simple cycles, from which it follows that the graphic matroids of Eulerian graphs are examples of Eulerian ... Feb 1, 2013 at 13:37. well every vertex from K has the same number of edges as the number of vertexes in the opposed set of vertexes.So for example:if one set contains 1,2 and another set contains 3,4,5,6,the vertexes 1,2 will have each 4 edges and the vertexes 3,4,5,6 will each have 2 vertexes.For it to be an eulerian graph,also the sets of ...1 Semester, AY 2020-2021. Finals. Mathematics in the Modern World. Module 7: Graphs and Euler Circuits. An Euler Graph is a connected graph whose all vertices are of even degree. Euler Path is a trail. in the connected graph that contains all the edges of the graph. A closed Euler trail is called as. an Euler Circuit.Hamiltonian path in a graph is a simple path that visits every vertex exactly once. The problem of deciding whether a given graph has a Hamiltonian path is a ...Dec 18, 2021 · 0. The graph for the 8 x 9 grid depicted in the photo is Eulerian and solved with a braiding algorithm which for an N x M grid only works if N and M are relatively prime. A general algorithm like Hierholzer could be used but its regularity implies the existence of a deterministic algorithm to traverse the (2N+1) x (2M +1) verticies of the graph. ∗ 24. Devise an algorithm for constructing Euler circuits in di-rected graphs. 25. Devise an algorithm for constructing Euler paths in di-rected graphs. 26. For which values of n do these graphs have an Euler cir-cuit? a) Kn b) Cn c) Wn d) Qn 27. For which values of n do the graphs in Exercise 26 have an Euler path but no Euler circuit? 28.these questions seem to be similar, the first question, which asks whether a graph has an Euler circuit, can be easily answered simply by examining the degrees of the vertices of the graph, while the second question, which asks whether a graph has a Hamilton circuit, is quite difficult to solve for most graphs.I Given graph G , an Euler circuit is a simple circuit containing every edge of G . I Euler path is a simple path containing every edge of G . Instructor: Is l Dillig, CS311H: Discrete Mathematics Graph Theory IV 12/25 2. Theorem about Euler Circuits Theorem: A connected multigraph G with at least two verticesYes there is lots of graphs which can be Euler path but not Euler circuit. just like your graph after removing 4->0.. If a graph has Euler circuit it is easier to find an Euler path, because if you start from every node, you could find an Euler path, because all of them are in the circuit, but if you dont have an Euler circuit you cant start from any …The graph does have Euler circuits. 40. Euler Circuits. Euler's Path Theorem ... The total length of this route is 28 blocks (24 blocks in the grid plus 4 ...Graph theory is an important branch of mathematics that deals with the study of graphs and their properties. One of the fundamental concepts in graph theory is the Euler circuit, which is a path that visits every edge exactly once and returns to the starting vertex. In this blog post, we will explore which grid graphs have Euler circuits. The first problem in graph theory dates to 1735, and is called the Seven Bridges of Königsberg.In Königsberg were two islands, connected to each other and the mainland by seven bridges, as shown in figure 5.2.1.The question, which made its way to Euler, was whether it was possible to take a walk and cross over each bridge exactly once; Euler showed that it is not possible.4. You are a mail deliverer. Consider a graph where the streets are the edges and the intersections are the vertices. You want to deliver mail along each street exactly once without repeating any edges. Would this path be represented by a Euler circuit or a Hamiltonian circuit? 5.Euler path exists if the graph is a connected pattern and the connected graph has exactly two odd-degree vertices. And an undirected graph has an Euler circuit if vertexes in the Euler path were even (Barnette, D et al., 1999). For some type of grid stiffened panels, the graphical of 2D slicing array generally has more than two odd vertices.4. You are a mail deliverer. Consider a graph where the streets are the edges and the intersections are the vertices. You want to deliver mail along each street exactly once without repeating any edges. Would this path be represented by a Euler circuit or a Hamiltonian circuit? 5.Unlike Euler paths and circuits, there is no simple necessary and sufficient criteria to determine if there are any Hamiltonian paths or circuits in a graph. But there are certain criteria which rule out the existence of a Hamiltonian circuit in a graph, such as- if there is a vertex of degree one in a graph then it is impossible for it to have a …Revisiting Euler Circuits Remark Given a graph G, a “no” answer to the question: Does G have an Euler circuit?” can be validated by providing a certificate. Now this certificate is one of the following. Either the graph is not connected, so the referee is told of two specific vertices for which theGraphs which have Euler paths that are not Euler Circuits must have two odd vertices. Let’s figure out if she is correct. We can think of the edges at a vertex as “entries” and “exits”. In other words, edges can be used to “enter” or “exit” a vertex. For a graph that has an Euler path, we have three type of vertices: starting ...Euler’s Formula for plane graphs: v e+ r = 2. Trails and Circuits 1. For which values of n do K n, C n, and K m;n have Euler circuits? What about Euler paths? (F) 2. Prove that the dodecahedron is Hamiltonian. 3. A knight’s tour is a a sequence of legal moves on a board by a knight (moves 2 squares horizontally which says that if the graph is drawn without any edges crossing, there would be \(f = 7\) faces. Now consider how many edges surround each face. Each face must be surrounded by at least 3 edges. Let \(B\) be the total number of boundaries around all the faces in the graph. Thus we have that \(B \ge 3f\text{.}\)Hamiltonian path in a graph is a simple path that visits every vertex exactly once. The prob- lem of deciding whether a given graph has a Hamiltonian path ...There is a theorem: Eulerian cycle in a connected graph exists if and only if the degrees of all vertices are even. If m > 1 m > 1 or n > 1 n > 1, you will have vertices of degree 3 (which is odd) on the borders of your grid, i.e. vertices that adjacent to exactly 3 edges. And you will have lots of such vertices as m m, n n grow. 1. We have the bipartite graph G =K5,9 G = K 5, 9. We construct a new graph G′ G ′ by adding a new vertex u u that is connected with each vertex of G G. Then G′ G ′ has an Euler circuit, because every vertex has an even degree (the degree of u u is 5 + 9 = 14 5 + 9 = 14, the degrees of the old vertices in the new graph G′ G ′ are 9 ...Which of the graphs below have Euler circuits? A. I only. B. II only. C. Both I and II. D. Neither I nor II. 4. Every graph with an even number of vertices has an Euler circuit. Choose: True or False: 5. ... You want to create a mileage grid showing the distances between every pair of the 10 Canadian provincial/territorial capitals. How many numbers …A graph will contain an Euler path if it contains at most two vertices of odd degree. A graph will contain an Euler circuit if all vertices have even degree. Example. In the graph below, vertices A and C have degree 4, since there are 4 edges leading into each vertex. B is degree 2, D is degree 3, and E is degree 1.If a graph has a Eulerian circuit, then that circuit also happens to be a path (which might be, but does not have to be closed). – dtldarek. Apr 10, 2018 at 13:08. If "path" is defined in such a way that a circuit can't be a path, then OP is correct, a graph with an Eulerian circuit doesn't have an Eulerian path. – Gerry Myerson.A connected graph \(G\) has an Euler walk if and only if exactly two vertices have odd degree. Proof Suppose first that \(G\) has an Euler walk starting at vertex \(v\) and …the graph then have an Euler circuit? If so, then find one. If not, explain why not. Solution. (a) No. Euler’s theorem says that a graph has an Euler circuit if and only if every node has even degree, which is not the case here. For example, node E has odd degree. (b) Yes. The corollary to Euler’s theorem states that a graph without an ...Solution. The correct option is C The complement of a cycle on 25 vertices. Whenever in a graph all vertices have even degrees, it will surely have an Euler circuit. (a) Since in a k-regular graph, every vertex has exactly k degrees and if k is even, every vertex in the graph has even degrees. k-regular graph need not be connected, hence k ...Euler’s Theorems Theorem (Euler Circuits) If a graph is connected and every vertex is even, then it has an Euler circuit. Otherwise, it does not have an Euler circuit. Theorem (Euler Paths) If a graph is connected and it has exactly 2 odd vertices, then it has an Euler path. If it has more than 2 odd vertices, then it does not have an Euler path.An Euler path can have any starting point with any ending point; however, the most common Euler paths lead back to the starting vertex. We can easily detect an Euler path in a graph if the graph itself meets two conditions: all vertices with non-zero degree edges are connected, and if zero or two vertices have odd degrees and all other vertices ...Here, This graph is a connected graph and all its vertices are of even degree. Therefore, it is an Euler graph. Alternatively, the above graph contains an Euler circuit BACEDCB, so it is an Euler graph. Also Read- Planar Graph Euler Path- Euler path is also known as Euler Trail or Euler Walk.Investigate! An Euler path, in a graph or multigraph, is a walk through the graph which uses every edge exactly once. An Euler circuit is an Euler path which starts and stops at the …An Euler circuit is a circuit in a graph where each edge is crossed exactly once. The start and end points are the same. All the vertices must be even for the graph to have an Euler circuit.4.07.2014 г. ... The method is applied to grid graphs, king's graphs, triangular grids, and three-dimensional grid graphs, and results are obtained for larger ...Just as Euler determined that only graphs with vertices of even degree have Euler circuits, he also realized that the only vertices of odd degree in a graph with an Euler trail are the starting and ending vertices. For example, in Figure 12.132, Graph H has exactly two vertices of odd degree, vertex g and vertex e.Two different trees with the same number of vertices and the same number of edges. A tree is a connected graph with no cycles. Two different graphs with 8 vertices all of degree 2. Two different graphs with 5 vertices all of degree 4. Two different graphs with 5 vertices all of degree 3. Answer.Look back at the example used for Euler paths—does that graph have an Euler circuit? A few tries will tell you no; that graph does not have an Euler circuit. When we were working with shortest paths, we were interested in the optimal path. With Euler paths and circuits, we’re primarily interested in whether an Euler path or circuit exists.In the graph shown below, there are several Euler paths. One such path is CABDCB. The path is shown in arrows to the right, with the order of edges numbered. Euler Circuit An Euler circuit is a circuit that uses every edge in a graph with no repeats. Being a circuit, it must start and end at the same vertex. Seattle LA Chicago Dallas Atlanta ...Definitions: Euler Paths and Circuits. A graph has an Euler circuit if and only if the degree of every vertex is even. A graph has an Euler path if and only if there are at most two vertices with odd degree. Since the bridges of Königsberg graph has all four vertices with odd degree, there is no Euler path through the graph.The task is to find minimum edges required to make Euler Circuit in the given graph. Examples: Input : n = 3, m = 2 Edges [] = { {1, 2}, {2, 3}} Output : 1. By connecting 1 to 3, we can create a Euler Circuit. For a Euler Circuit to exist in the graph we require that every node should have even degree because then there exists an edge that can ...Which of the following graphs have Euler circuits or Euler trails? U R H A: Has Euler trail. A: Has Euler circuit. T B: Has Euler trail. B: Has Euler circuit. S R U X H TU C: Has …Otherwise, the algorithm will stop when if nds an Euler circuit of a connected component of the graph. If this is the whole graph, great, we found an Euler circuit for the original graph. Otherwise, we have shown that the graph is not connected. In this modi ed form, the algorithm tells you if a graph is Eulerian or not, and if so it produces ...6.4: Euler Circuits and the Chinese Postman Problem. Page ID. David Lippman. Pierce College via The OpenTextBookStore. In the first section, we created a graph of the Königsberg bridges and asked whether it was possible to walk across every bridge once. Because Euler first studied this question, these types of paths are named …Euler's Formula for plane graphs: v e + r = 2. Trails and Circuits For which values of n do Kn, Cn, and Km;n have Euler circuits? What about Euler paths? Kn has an Euler circuit for odd numbers n 3, and also an Euler path for n = 2. (F) Prove that the dodecahedron is Hamiltonian. One solution presented in Rosen, p. 699An Euler circuit is a circuit that uses every edge in a graph with no repeats. Being a circuit, it must start and end at the same vertex. The graph below has several possible Euler circuits. Here's a couple, starting and ending at vertex A: ADEACEFCBA and AECABCFEDA. The second is shown in arrows.For which values of n do the graphs have a Hamilton circuit? a) K_n K n b) C_n C n c) W_n W n d) Q_n Qn. discrete math. Let G = (V, E) be a loop-free connected undirected graph, and let {a, b} be an edge of G. Prove that {a, b} is part of a cycle if and only if its removal (the vertices a and b are left) does not disconnect G.* Euler Circuits 5.2 Graphs * Euler Circuits Vertices- dots Edges- lines The edges do not have to be straight lines. But they have to connect two vertices. Loop- an edge connecting a vertex back with itself A graph is a picture consisting of: * Euler Circuits Graphs A graph is a structure that defines pairwise relationships within a set to objects.On small graphs which do have an Euler path, it is usually not difficult to find one. Our goal is to find a quick way to check whether a graph has an Euler path or circuit, even if the graph is quite large. One way to guarantee that a graph does not have an Euler circuit is to include a “spike,” a vertex of degree 1. A graph will contain an Euler path if it contains at most two vertices of odd degree. A graph will contain an Euler circuit if all vertices have even degree. In the graph below, vertices A and C have degree 4, since there are 4 edges leading into each vertex. B is degree 2, D is degree 3, and E is degree 1.Section 4.5 Euler Paths and Circuits Investigate! An Euler path, in a graph or multigraph, is a walk through the graph which uses every edge exactly once.An Euler circuit is an Euler path which starts and stops at the same vertex. Our goal is to find a quick way to check whether a graph (or multigraph) has an Euler path or circuit. Which of the …An Euler path can have any starting point with any ending point; however, the most common Euler paths lead back to the starting vertex. We can easily detect an Euler path in a graph if the graph itself meets two conditions: all vertices with non-zero degree edges are connected, and if zero or two vertices have odd degrees and all other vertices ...Investigate! An Euler path, in a graph or multigraph, is a walk through the graph which uses every edge exactly once. An Euler circuit is an Euler path which starts and stops at the …You'll get a detailed solution from a subject matter expert that helps you learn core concepts. Question: (8 points) [01] Assume n > 3. For which values of n do these graphs have an Euler circuit? (a) Complete graph Kn. (b) Cycle graph Cn. (c) Wheel graph Wn as defined in the lecture. (d) Complete bipartite graph Kn,n.This problem has been solved! You'll get a detailed solution from a subject matter expert that helps you learn core concepts. Question: (1 point) Consider the graph given above. The graph doesn't have an Euler circuit. However, if we added one more (specific) edge to the graph, then it would have an Euler circuit.Math. Advanced Math. Advanced Math questions and answers. Consider the following. A B D E F (a) Determine whether the graph is Eulerian. If it is, find an Euler circuit. If it is not, explain why. Yes. D-A-E-B-E-A-D is an Euler circuit. O Not Eulerian. There are more than two vertices of odd degree.This problem has been solved! You'll get a detailed solution from a subject matter expert that helps you learn core concepts. Question: 26. For which values of n do these graphs have an Euler circuit? a) Kn b) Cn c) Wn d) Qn 27. For which values of n do the graphs in Exercise 26 have an Euler path but no Euler circuit?Math. Advanced Math. Advanced Math questions and answers. Consider the following. A B D E F (a) Determine whether the graph is Eulerian. If it is, find an Euler circuit. If it is not, explain why. Yes. D-A-E-B-E-A-D is an Euler circuit. O Not Eulerian. There are more than two vertices of odd degree. when a graph is guaranteed to have a Euler circuit. 3. Apply conjecture to the Königsberg Bridge problem. 4. Most student conjectures are probably existence conjectures. That is, they help you decide if a given graph has a Euler circuit. If a graph has a Euler circuit, trying to find it may be another matter entirely! Questions 8 and 9 ...Part 1: If either m or n is even, and both m > 1 and n > 1, the graph is Hamiltonian. This proof is going to be by construction. If one of the even sides is of length 2, you can form a ring that reaches all vertices, so the graph is Hamiltonian. Otherwise, there exists an even side of length greater than 2.A graph will contain an Euler path if it contains at most two vertices of odd degree. A graph will contain an Euler circuit if all vertices have even degree. Example. In the graph …This page titled 5.5: Euler Paths and Circuits is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Oscar Levin. An Euler path, in a graph or multigraph, is a walk through the graph which uses every edge exactly once. An Euler circuit is an Euler path which starts and stops at the same vertex.This problem has been solved! You'll get a detailed solution from a subject matter expert that helps you learn core concepts. Question: 26. For which values of n do these graphs have an Euler circuit? a) Kn b) Cn c) Wn d) Qn 27. For which values of n do the graphs in Exercise 26 have an Euler path but no Euler circuit?3-June-02 CSE 373 - Data Structures - 24 - Paths and Circuits 8 Euler pat, Mar 15, 2023 · The task is to find minimum edges required to make Euler Circuit in the given , Euler's cycle or circuit theorem shows that a connected gra, Connected graphs, Euler circuits and paths, vertices of odd degree. 0. Proving a certain , Oct 29, 2021 · An Euler circuit is a circuit in a graph where each edge is, graphs with 6 vertices with an Euler circuits. Solution. By convention we say, The definition says "A directed graph has an eulerian path if and only if it is connected and each v, Part 1: If either m or n is even, and both m > , , Expert Answer. 1)Given graphs namely A, B, C and D does n, Otherwise, the algorithm will stop when if nds an Euler circuit , Euler's Formula for plane graphs: v e + r = 2. Trails and Circui, Euler path exists if the graph is a connected pattern and the, An Eulerian graph is a graph that possesses an Eulerian, Theorem 13.2.1. If G is a graph with a Hamilton cycle, then for every, What is an Euler Path and Circuit? For a graph to be an Eul, Graph theory is an important branch of mathematics that d, Advanced Math questions and answers. Which of the following graphs .