2, we observe that the images of distinct elements of X1 under the function
f1 are distinct, but the image of two distinct elements 1 and 2 of X1 under f2 is same,
namely b Further, there are some elements like e and f in X2 which are not images of
any element of X1 under f1, while all elements of X3 are images of some elements of X1
under f3 The above observations lead to the following definitions:
Definition 5 A function f : X → Y is defined to be one-one (or injective), if the images
of distinct elements of X under f are distinct, i e