## What is a graph?

A graph is a set of nodes connected by edges. When there is a relation between two nodes, these nodes are connected by an edge. Figure 4 shows a graph example that has four nodes. In the figure, the node 1 ($$N_1$$) and the node 2 ($$N_2$$) is connected by the edge $$E_{1,2}$$. A graph is only concerned with the connections between nodes, so how the nodes are arranged in a figure doesn't matter. Figure 5 has three graphs, but all the graphs are considered to be the same in graph theory.

 Figure 4. Edges connect nodes.

 Figure 5. The same graphs. Graph only cares the connections between nodes.

I will show you some examples next time.