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.
University of Utah
University of Utah: Intervals
University of Utah includes the definition, symbol meanings, and examples regarding intervals.
CK-12 Foundation
Ck 12: Analysis: Intervals and Interval Notation
[Free Registration/Login may be required to access all resource tools.] Students learn a definition of open and closed intervals and introduction to interval notation. Students examine guided notes, review guided practice, watch...
Other
Jfm: The Definition of Riemann Definite Integral
This site introduces Riemann definite integral on the closed interval of real. If you are interested in exploring definite integrals, this site may interest you.
Varsity Tutors
Varsity Tutors: Hotmath: Algebra Review: Interval Notation
Find a quick, concise explanation of how to write interval notation to express subsets of the real number line. Examples are given and clearly explained.
Northeast Parallel Architectures Center, Syracuse University
Npac Reu Program: Intermediate Value Theorem
This site, from the Northeast Parallel Architectures Center at Syracuse University, explains the intermediate value theorem.