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If $A$ and $B$ are two conformable matrices,and $AB = A$ and $BA = B$ then prove that A and B are idempotent matrices.

This may be trivial but I am not sure how to proceed on this.

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up vote 6 down vote accepted

$$Ax = ABx = A(BA)x = (AB)Ax = AAx$$

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Tell me if I do something like this :$A = AB = A(BA) = (AB)A = AA = A^2$, any problem? :) – Quixotic Dec 26 '10 at 13:33
That's of course also fine – kahen Dec 26 '10 at 13:40
Thanks for your time :-) – Quixotic Dec 26 '10 at 13:49

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