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Toolbox:
• A function $f: A \rightarrow B$ where for every $x1, x2 \in X, f(x1) = f(x2) \Rightarrow x1 = x2$ is called a one-one or injective function
Given$A=\{1,2,3,4\}\qquad B=\{2,5,6,7\}$
Let $f:A \to B$ be a mapping from A to B $f=\{(2,5)(3,6)(4,7)\}$
f is an injective mapping. Since for every element $a \in A$ there is an unique element $b \in B$
Let us define a mapping $g:A\to B$ given by $g=\{(2,2)(2,5)(3,6)(4,7)\}$
g is not an injective mapping. since the element $2 \in A$ is not uniquely mapped
Since (2,2) and (2,5) both belong to the mapping g, g is not injective
Let us define a mapping $h:A\to B$ given by $h=\{(2,2),(5,3),(7,4)\}$
h is a mapping from$B\ to A$ since the every ordered puts $\{2,5,7\} \in B$ to elements in $\;\{2,3,4\} \in A$

edited Mar 27, 2013 by meena.p