deletions | additions       diff --git a/A_graph_is_a_structure__.md b/A_graph_is_a_structure__.md  index 28bdd92..1dc87db 100644  --- a/A_graph_is_a_structure__.md  +++ b/A_graph_is_a_structure__.md 
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A graph is a structure used in discrete math that is used to show the relationship between various objects. The objects form the vertices of the graph. Two vertices are connected by a line if they satisfy a certain relationship. We call these lines edges. Graphs can be used to model a variety of things such as  + *  study groups in a class, + *  showing genetic relationships between organisms, + *  sudoku puzzles, + *  committee assignments, and + *  computer program dependencies. If we make the edges of a graph point in only one direction then we get what is called a digraph (or a directed graph). A digraph is perfect for displaying a heirarchy of objects. For example, we can use a digraph to see the math courses and prerequisites at NSC. In the digraph below the class numbers are the vertices, and one class points to another if it is listed as a prerequisite in the NSC course catalog.