TITLE: the left inverse and right inverse in a ring
QUESTION [1 upvotes]: It is well-known that if $R$ is an Artin ring,and $ab=1$ in $R$ where $a,b\in R$,then $ba=1a$.(This is not difficult)this is a very hot in Mathematics. If $AB = I$ then $BA = I$
It seems it is not right for arbitrary ring that if $ab=1$,then $ba=1$.
Can someone helps to give an example.
Thanks in advance!

REPLY [3 votes]: Staying at linear endomorphisms, this is no longer true for infinite dimensional spaces. 
Let e.g. $e_1,e_2,\dots$ be a basis, and consider $B:=e_k\mapsto e_{k+1}$ and $A:=e_k\mapsto e_{k-1}, \ e_1\mapsto 0$

2.Take any monoid $M$ that satisfies this, then consider its 'group ring' $\Bbb ZM$.
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