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.
Illustrative Mathematics
Illustrative Mathematics: A sse.b.4: Cantor Set
This tasks requires learners to explore properties of the Cantor Set, which has many uncountable points, no intervals of real numbers, and has length of zero. Aligns with A-SSE.B.4.
CK-12 Foundation
Ck 12: Geometry: Self Similarity Study Guide
[Free Registration/Login may be required to access all resource tools.] This study guide looks at several examples of self-similarity.