When faced with a difficult problem in mathematics, it often helps to draw a picture. If the problem involves a discrete collection of interrelated objects, it is natural to sketch the objects and draw lines between them to indicate the relationships. A graph (composed of dots called vertices connected by lines or curves called edges) is the mathematical version of such a sketch. The edges of a graph may have arrows on them; in this case, the graph is called a directed graph. When we draw a graph, it doesn’t really matter where we put the vertices or whether we draw the edges as curved or straight; rather, what matters is whether or not two given vertices are connected by an edge (or edges). The degree of a vertex is the number of edges incident to it (i.e., the number of times an edge touches it). This is different than the number of edges touching it, because an edge my form a loop; for instance, vertex ? in graph ? (above) has degree 5. In a directed graph, we can speak of the indegree (the number of edges coming in to the vertex) and the outdegree (the number of edges going out). Vertex ? in graph ? (above) has indegree 1 and outdegree 2. Several departments around campus have wireless access points, but problems arise if two wireless access points within 200 feet of each other are operating on the same frequency. Suppose the departments with wireless access points are situated as follows: a. Draw an undirected graph that represents the table above. b. What is the fewest number of frequencies needed? How could these frequencies be assigned to departments? (Hint: if you need help, Google “graph coloring.”)
When faced with a difficult problem in mathematics, it often helps to draw a picture. If the problem involves a discrete collection of interrelated objects, it is natural to sketch the objects and draw lines between them to indicate the relationships. A graph (composed of dots called vertices connected by lines or curves called edges) is the mathematical version of such a sketch. The edges of a graph may have arrows on them; in this case, the graph is called a directed graph.
When we draw a graph, it doesn’t really matter where we put the vertices or whether we draw the edges as curved or straight; rather, what matters is whether or not two given vertices are connected by an edge (or edges). The degree of a vertex is the number of edges incident to it (i.e., the number of times an edge touches it). This is different than the number of edges touching it, because an edge my form a loop; for instance, vertex ? in graph ? (above) has degree 5. In a directed graph, we can speak of the indegree (the number of edges coming in to the vertex) and the outdegree (the number of edges going out). Vertex ? in graph ? (above) has indegree 1 and outdegree 2.
Several departments around campus have wireless access points, but problems arise if two wireless access points within 200 feet of each other are operating on the same frequency. Suppose the departments with wireless access points are situated as follows:
a. Draw an undirected graph that represents the table above.
b. What is the fewest number of frequencies needed? How could these frequencies be assigned to departments? (Hint: if you need help, Google “graph coloring.”)
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