Sets:Operations

Definitions

The two main set operations that we deal with are union and intersection. The union of two sets Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle A } and $B$ is defined as $A\cup B=\{x:x\in A$ or $x\in B\}$. For example:

• The union of $A=\{1,3,5,7,9\}$ and $B=\{0,1,2,4,6,8,9\}$ is $A\cup B=\{0,1,2,3,4,5,6,7,8,9\}$
• The union of the even integers and odd integers is $\mathbb {Z}$.
• The union of the set of rational numbers and the set of irrational numbers is $\mathbb {R}$.
• $A\cup A=A$, and $\emptyset \cup A=A$.
• For sets $A$ and $B$ such that $A\subseteq B$, $A\cup B=B$, since all elements of $A$ are already in $B$ if $A\subseteq B$.

The intersection of $A$ and $B$ is defined as $A\cap B=\{x:x\in A$ and $x\in B\}$; that is, the intersection of $A$ and $B$ is the set of all elements shared by the two sets. For example:

• The intersection of $A=\{1,3,5,7,9\}$ and $B=\{0,1,2,4,6,8,9\}$ is $A\cap B=\{1,9\}$.
• The intersection of the even integers and odd integers is the empty set, since no element between these two sets is shared (an integer cannot be both even and odd).
• $A\cap A=A$, and $\emptyset \cap A=\emptyset$.
• For sets $A$ and $B$ such that $A\subseteq B$, $A\cup B=A$.

Sets $A$ and $B$ are "disjoint" if $A\cap B=\emptyset$.

Resources

• Course Textbook, pages 101-115