Oswego City School District
Regents Exam Prep Center: Determining Relations and Functions
This resource offers definitions for "relation", "function", and "functional relation", and examples to illustrate each.
Oswego City School District
Regents Exam Prep Center: Definition of a Relation and a Function
Very basic definitions of mathematical relations and functions can be found here. The single example provided shows a relation which is not a function and explains what makes it so.