|May28-09, 10:58 PM||#1|
For a monoid, if uv = 1, do we know vu = 1?
If [itex]M[/itex] is a monoid and [itex]u,v\in M[/itex], and [itex]uv = 1[/itex], do we know [itex]vu = 1[/itex]? Can someone prove this or provide a counterexample? I tried to come up with one (a counterexample, that is) using 2 x 2 matrices but was unsuccessful.
|May29-09, 10:55 AM||#2|
If there exists a w such that:
Then by associativity:
Thus if such a w exists it must be u.
This isn't quite enough but I can't find a short proof either. I'll think on it.
|May29-09, 12:49 PM||#3|
The canonical example is left and right shift of a sequence or countable dimensional vector space.
L(a,b,c,d,...) = (b,c,d,....)
LR=id, and RL=/=id.
You won't find one in 2x2 matrices - the invertible ones form a group, so there's no point looking.
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