A. Notes

We learned that graphs are made of vertices and edges.

We use a set of all vertices and all edges to describe a graph.

For the elementary division, we only study undirected graphs.

B. HW

Draw a graph using the first letters of each person’s name: Alice talks to Bob, Bob talks to Carol, Carol talks to Alice, Alice talks to Doug, and Doug talks to Eve. The graph shows all of the conversations.

Draw an undirected graph that has vertices {P, Q, R, S, T} and edges {PQ, RS, ST, PT, QT, RQ, TR}. Can you give a meaning of this graph?

Draw a graph to show the cleaning robot's trace according to the description below. Can you think of a different moving strategy to ensure that the robot will pick up all pieces of trash?

4. There are 5 pieces of toys showing below. How many ways are there to rearrange them?