Surjection/Examples/Non-Surjection/Arbitrary Mapping on Sets
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Example of Mapping which is Not a Surjection
Let $A = \set {a, b, c}$.
Let $B = \set {1, 2, 3}$.
Let $f \subseteq {A \times B}$ be the mapping defined as:
- $f = \set {\tuple {a, 2}, \tuple {b, 1}, \tuple {c, 1} }$
Then $f$ is not a surjection.
Proof
For $f$ to be a surjection, it would be necessary that:
- $\forall y \in B: \exists x \in A: \map f x = y$
However $3 \in B$ does not fit this condition.
Hence $f$ is not a surjection.
$\blacksquare$
Sources
- 1977: Gary Chartrand: Introductory Graph Theory ... (previous) ... (next): Appendix $\text{A}.4$: Functions