# Math Help - invertible?

1. ## invertible?

Hi
Let $A$ be a ring, $a,b \in A$ such that $ab=1$. Suppose that $X=\{x \in A;\ ax=0\}$ is finite.
Prove that $ba=1$

We can see that $1-ba \in X$, so if $X=\{0\}$, it's over.
Any idea?

2. Originally Posted by clic-clac
Hi
Let $A$ be a ring, $a,b \in A$ such that $ab=1$. Suppose that $X=\{x \in A;\ ax=0\}$ is finite.
Prove that $ba=1$

We can see that $1-ba \in X$, so if $X=\{0\}$, it's over.
Any idea?
we have: $a^{k-1} - ba^k \in X, \ \forall k \in \mathbb{N}.$ hence, since $X$ is finite, there exist $n, m \in \mathbb{N}$ such that $a^n-ba^{n+1}=a^m -ba^{m+1}, \ n > m,$ which gives us: $(1-ba)a^n=a^m-ba^{m+1}. \ \ \ \ (1)$

now multiply both sides of (1) from the right by $b^n$ and note that $\forall k \in \mathbb{N}: \ a^kb^k=1.$ then you'll get: $1-ba=(a^m-ba^{m+1})b^n=a^mb^n - ba^{m+1}b^n=b^{n-m}-b^{n-m}=0. \ \ \ \ \ \ \ \ \ \ \Box$

3. Thanks for your answer.

I don't see why there exists $n$ such that $a^n-ba^{n+1}=a^{n-1}-ba^n$.
The conclusion is still the same with $a^n-ba^{n+1}=a^{k}-ba^{k+1}$ for a $k but there is something I'm missing...

4. Originally Posted by clic-clac
Thanks for your answer.

I don't see why there exists $n$ such that $a^n-ba^{n+1}=a^{n-1}-ba^n$.
The conclusion is still the same with $a^n-ba^{n+1}=a^{k}-ba^{k+1}$ for a $k but there is something I'm missing...
you're right, thanks! it was just one of those really weird mistakes that i make sometimes! it's fixed now!

5. You're welcomed.
But for me the problem is that doing this, we assume that every element of $X$ is equal to a $a^{q}-ba^{q+1}$, $q\in \mathbb{N}$.
Why can we say that?

6. Originally Posted by clic-clac

we assume that every element of $X$ is equal to a $a^{q}-ba^{q+1}$, $q\in \mathbb{N}$.
Why can we say that?
i didn't say that! what we have is this: $A=\{a^{k-1} - ba^k: \ k \in \mathbb{N} \} \subseteq X,$ and we're given that $X$ is finite. so $A$ must also be finite and thus we can find two elements of $A$ which are equal.

7. Great NonCommAlg, I finally got it! Thank you.