University of California, San Diego: Edward Bender and S. Williamson's "Sets, Equivalence and Order: Sets and Functions"

Study set operations and the property of set operations Section 1 on pages SF-2 - SF-3. Set Union is defined on page SF-2. 

Set Union is defined on page SF-2. Note that A \cup B = {x : x ∈ A or x ∈ B}. You will read and reread this section several times, each time focusing on a basic set operation. These basic operations are so fundamental and ubiquitous (i.e. occur so frequently in mathematics, computer science, and their applications) that they deserve more than a quick reading. Work a lot of examples.
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