5. Graphs and Networks.   

       Recursion.  
       Recursion Principle.  
       Graphs.  
       Networks.  
       Representations for Graphs.  

A Graph and a Network are Abstract Data Types - they could be implemented using different base data types but we, as users, would not care. We want to use them as complete entities - we only deal with them via procedures and functions defined by the Abstract Data Type.