Can the product of two non invertible elements in a ring be invertible? Let $A$ be a unitary ring. The question is simply: can the product of two non invertible elements in $A$ be invertible?
I proved that the answer is negative if $A$ does not have zero divisors, because if you have $a,b$ non invertible elements in $A$ and $abx=xab=1$, then $a(bx)=1$ so $a$ is right invertible and if we put $d=bxa  \implies db=b(xab)=b1=b  \implies d=1$ (because obviously $b$ is not $0$), so $a$ is left invertible, as desired. If $A$ does have zero divisors, I don't see how to adapt the proof...
Thank you!
 A: Here is a more algebraic example. Let $R$ be the ring of all $\infty\times\infty$ matrices with real entries such that in each column there is only a finite number of non-zero entries. 
Let $A = \left(\begin{array}{CCCcC} 0 & 1 & 0 & 0&  \cdots \\ 0 & 0 & 2& 0& \cdots \\ 0 & 0 & 0 & 3 & \cdots  \\ \vdots & \vdots & \vdots &\end{array}\right)$ and $B = \left(\begin{array}{CCCcC} 0 & 0 & 0 & 0&  \cdots \\ 1 & 0 & 0 & 0& \cdots \\ 0 & \frac 12 & 0 & 0 & \cdots  \\ \vdots & \vdots & \vdots &\end{array}\right)$. Then it is clear that $AB = I$ therefore $AB$ is invertible but neither $A$ nor $B$ are not invertible. (since for example the first row of $B$ is zero this mean that for any matrix $C \in R$  the $(1,1)$ entries of $BC$ is zero so this is impossible to have $BC = I$.) 
A: Yes.  Consider the ring of bounded linear operators on $\ell^2$.  Let
$$ a = (x_1,x_2,\dots) \mapsto (0,x_1,x_2,\dots), \\
b = (x_1,x_2,\dots) \mapsto (x_2,x_3,\dots) .$$
Then $ba = I$, but neither $a$ nor $b$ are invertible.
