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An injection, or "one-to-one function," is a function that takes distinct values on distinct inputs. Equivalently, an injection is a function for which every value in the range is the image of exactly one value in the domain.

Alternative definition: A function $f:A\to B$ is an injection if for all $x,y\in A$, if $f(x)=f(y)$ then $x=y$.

The binary relation $|X|\leq|Y|$ iff there is an injection $f:X\rightarrow Y$ forms a partial order on the class of cardinals: $X\leq X$, $X\leq Y$ and $Y\leq X$ implies $|X|=|Y|$ by the Cantor-Schroeder-Bernstein theorem, and $|X|\leq|Y|$ and $|Y|\leq|Z|$ implies $|X|\leq|Z|$ because the composition of injections is again an injection.


Linear functions are injections: $f:\mathbb R \to \mathbb R$, $f(x)= ax+b$, $a\neq 0$. The domain choosing is also important. For example, while $f:\mathbb R \to \mathbb R$, $f(x)=x^2$ is not an injection ($f(-1)=f(1)=1$), the function $g:[0,\infty)\to\mathbb R$, $g(x)=x^2$, is an injection.

See also

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