# For square matrices $A$, $B$, is $AB=I$ sufficient that $A$ and $B$ are inverse of each other? [duplicate]

Possible Duplicate:
If $AB = I$ then $BA = I$

If $A$ and $B$ are two square matrices, and we know $AB=I$ where $I$ is the identity matrix. Is it sufficient that $BA=I$ as well so that $A$ and $B$ are inverse matrices of each other?

Just found out that this is a duplicate question.

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## marked as duplicate by EuYu, Patrick Li, Austin Mohr, draks ..., Asaf KaragilaDec 12 '12 at 9:07

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

@EuYu Good to know that. Thank you. –  Patrick Li Dec 12 '12 at 4:53

## 1 Answer

In fact, something stronger is true. Suppose that $A$ is invertible, and that $AB=I$. Then, $B=A^{-1}$ so that $BA=A^{-1}A=I$. Now, if $AB=I$, then $\det(A)\det(B)=1$ so that $\det(A)\ne 0$. So, $A$ is invertible. From this, you see that if just $AB=I$ then $A$ is invertible and $B=A^{-1}$.

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Just to comment that this only works for square matrices of finite order. –  Ittay Weiss Dec 12 '12 at 7:07