# Thread: Problem involving Indentity, Coposition functoin

1. ## Problem involving Indentity, Coposition functoin

Hey, I don't know how to prove this.
So... help me

Let f: X -> Y and g: Y -> X be functions so that g o f = 1x(which means Identity function). Prove that f is injective and g is surjective. Need either be bijective?

On hints sheet, This has 3 parts. Justify your answer to last part w/ a proof or counterexample.

Thank you

2. Originally Posted by 892king
Hey, I don't know how to prove this.
So... help me

Let f: X -> Y and g: Y -> be functions so that g o f = 1x(which means Identity function). Prove that f is injective and g is surjective. Need either be bijective?

On hints sheet, This has 3 parts. Justify your answer to last part w/ a proof or counterexample.

Thank you
For Part 1: Suppose f is not injective. Then there exist $a, b \in X$ such that a is not equal to b and $f(a) = f(b) = c \in Y$. By definition, $g(c) = a$ and $g(c) = b$. But in order for g to be a function, $a = b$, which is a contradiction.

3. Originally Posted by icemanfan
For Part 1: Suppose f is not injective. Then there exist $a, b \in X$ such that a is not equal to b and $f(a) = f(b) = c \in Y$. By definition, $g(c) = a$ and $g(c) = b$. But in order for g to be a function, $a = b$, which is a contradiction.
Suppose g is not surjective. Then there exists $a \in Y$ such that a is not contained in the image of set Y. By definition, g(a) doesn't exist. But in order for g to be a function, g(a) should be defined(not sure about this part).