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Definition: Partial and Total Maps (Functions)

Let \(A\) and \(B\) be two (not necessarily different) sets and let \(f \subseteq A \times B\) be a binary relation

  • a (total) function (or a map), if it is left-total and right-unique. This is equivalant to saying that for every element \(x\in A\) there is exactly one element \(y\in B\) with \((x,y)\in f\). To express this, we write $f(x)=y$ for functions instead of writing $(x,y)\in f$ as we wrote for general relations.
  • a (partial) function (or a map), if it is right-unique.

The following terms are strongly related to functions:

  • $A$ is called the domain of $f$.
  • $B$ is called the codomain of $f$.
  • The element \(f(x)=y\) for some $x\in A$ and $b\in B$ is called the value of $f$ at the point $x$.
  • The set \(f[A]:=\{y\in B:f(x)=y\;\text{ for all }x\in A\}\) is called the range: (or image) of $f$.
  • The set $f^{-1}(y):=\{x\in A:f(x)=y\text{ for some }y\in B\}$ is called the fiber of $y$ under $f$.
  • The set $f^{-1}[B]:=\{x\in A:f(x)=y\text{ for all }y\in B\}$ is called the inverse image of $f$.

| | | | | created: 2014-05-02 22:30:29 | modified: 2017-08-15 22:00:32 | by: bookofproofs | references: [577], [979], [6823], [6907]

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[6907] Brenner, Prof. Dr. rer. nat., Holger: “Various courses at the University of Osnabrück”, https://de.wikiversity.org/wiki/Wikiversity:Hochschulprogramm, 2014

Bibliography (further reading)

[6823] Kane, Jonathan: “Writing Proofs in Analysis”, Springer, 2016

[577] Knauer Ulrich: “Diskrete Strukturen – kurz gefasst”, Spektrum Akademischer Verlag, 2001

[979] Reinhardt F., Soeder H.: “dtv-Atlas zur Mathematik”, Deutsche Taschenbuch Verlag, 1994, 10

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