Functions:Surjective

A function is surjective, or "onto", if for all $b\in B$, there exists at least one $a\in A$ such that $f(a)=b$; that is, every element in the codomain is mapped to by at least one element in the domain. Another way to state this is that a function is surjective if and only if its range (all outputs of the function) equals its codomain.

Examples:

• Let $A=\{-2,-1,0,1,2\}$ and $B=\{1,3,9\}$, and let $f:A\to B$ such that $f(-2)=9$, $f(-1)=3$, $f(0)=1$, $f(1)=3$, and $f(2)=9$. Each element of $B$ is mapped to by at least one element of $A$, so $f$ is surjective.

• For the same $A$ and $B$ as in the previous example, let $g:A\to B$ be a function such that $g(-2)=1$, $g(-1)=1$, $g(0)=1$, $g(1)=1$, and $g(2)=1$. This is not a surjective function since there are elements in the codomain that are not mapped to by any elements of the domain.

• Let $f:\mathbb {R} \to \mathbb {R} ,f(x)=x^{3}$. For every $y\in \mathbb {R}$, there exists 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 x = \sqrt[3]{y} \in\R } such that 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 f(x) = (\sqrt[3]{y})^3 = y } . So, 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 f } is a surjective function.

• Let 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 g:\R\to\R, g(x) = \frac{1}{x} } . 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 0 } is in the codomain, but there is no 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 x\in\R } such that 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 \frac{1}{x} = 0 } . Thus, 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 g } is not surjective.

Resources

• Course Textbook, pages 154-164