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

DefinitionNotationNumerical SetsWhy Are Sets Important?Some More NotationEqualitySubsetsProper SubsetsEven More NotationEmpty (or null) Set So what does this have to do with mathematics? When we define a set, all we have to specify is a common characteristic. Who says we can't do so with numbers? 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...

So what does this have to do with mathematics? When we define a set, all we have to** specify** is a common characteristic. Who says we can't do so with numbers? 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...

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