determine if a graph is a function
Since this is not my area of expertise, I searched Google under the key words "determine if a graph is a function" to get these possible sources:
http://www.google.com/search?client=safari&rls=en&q=determine+if+a+graph+is+a+function&ie=UTF-8&oe=UTF-8
In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.