**Introduction to Sets - Math is Fun**
https://www.mathsisfun.com/sets/sets-introduction.html

Set of prime numbers: {2, 3, 5, 7, 11, 13, 17, ...} And so on. We can come up with all different types of sets. We can also define a set by its properties, such as {x|x>0} which means "the set of all x's, such that x is greater than 0", see Set-Builder Notation to learn more.Web

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