Curated OER
Closure Property
In this closure property activity, students solve 10 different problems using the closure property. First, they determine whether the numbers and the answer are real numbers. Then, students determine if the given sets are either closed...
Washington State University
Washington State University: Sets & Other Basic Concepts
Washington State University provides an introduction to sets and subsets and the symbols used to represent them.
Wolfram Research
Wolfram Math World: Closed Set
Site gives four different definitions of a closed set in mathematics. Each of the definitions has at least one link for further clarification. The site also expresses which of these definitions is most commonly encountered.