# Identity Element in Groups

1. Nov 25, 2014

### HyperActive

Hi
I'm taking a math course at university that covers introductory group theory. The textbook's definition of the identity element of a group defines it as two sided; that is, they say that a group $G$ must have an element $e$ such that for all $a \in G$, $e \cdot a = a = a \cdot e$ .

Is it possible to define a group's identity element as one-sided, and then prove two-sidedness as a theorem? Or is it an intrinsic property of group identities?

I started with a left-identity $e \cdot a = a$ and tried to prove that $e \cdot a = a = a \cdot e$ and kept hitting walls, so I though I'd better check if it's doable.

Thanks.

Last edited: Nov 25, 2014
2. Nov 25, 2014

### Stephen Tashi

If you change the assumption that there exists a two-sided identity, how are you going to modify the assumption that each element has an inverse? That assumption mentions the two-sided identity.

3. Nov 26, 2014

### HyperActive

Thanks for the reply Stephen Tashi. :)
In my book, the assumption that each element has an inverse is stated as follows: for all $a \in G$ , there exists a $b \in G$ such that $a \cdot b = e$. That doesn't seem to rely on the two-sidedness of the identity element (and neither it seems to me does the left-handed version of the statement, for all $a \in G$ , there exists a $b \in G$ such that $b \cdot a = e$). So other than noting that in these definitions $e$ refers to a left identity, I'm not seeing how I would need to modify them.

4. Nov 26, 2014

### rubi

You can weaken the group axioms to require only the existence of a left inverse and a left neutral element:
(i) $e g = g$
(ii) $g^{-1} g = e$

Then every left inverse is a right inverse:
$g g^{-1} = e g g^{-1} = (g^{-1})^{-1} g^{-1} g g^{-1} = (g^{-1})^{-1} g^{-1} = e$
And every left neutral element is a right neutral element:
$g e = g g^{-1} g = e g = e$.
(The proof uses the fact that $g^{-1}$ is a right inverse, which I proved before.)

You can of course replace "left" by "right" and get similar results. You can't mix them, though: Having left inverses and a right neutral element doesn't suffice.

5. Nov 26, 2014

### Stephen Tashi

The point is that the meaning of the statement $a \cdot b = e$ depends on what you mean by the notation "$e$".

6. Nov 26, 2014

### HyperActive

Thank you both very much!