A function \(f:A\mapsto B\) is called **injective**, (or **injection**, or **embedding**, or, less formally, **one-to-one**), if for all \(a_1,a_2\in A\) with $f(a_1)=f(a_2)$ it follows that $a_1=a_2.$ This corresponds to the left-unique property, in addition to the defining properties of a function.

By a contraposition argument the following definition is equivalent: For all \(a_1,a_2\in A\), it follows from $a_1\neq a_2$ that $f(a_1)\neq f(a_2).$

- An injective function allows for every $b\in B$
*at most*an $a\in A$ such that $f(a)=b.$ - In other words, either there is exactly one or no such element $a\in A.$

| | | | | created: 2014-07-11 19:59:54 | modified: 2020-06-01 07:41:41 | by: *bookofproofs* | references: [577], [979], [6823]

[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