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.
OpenStax
Open Stax: Catherine Schmidt Jones: Keys and Scales Are Sets
Help your students identify sets and intersections of sets with this musical activity. Using the concept of key signatures, students will learn that scales are sets of notes. Included with this lesson plan are downloadable worksheets and...