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I came across the following problem that says:

Let $A$ and $B$ be $n \times n$ real matrices such that $AB=BA=0$ and $A+B$ is invertible. Then how can I prove the following:

  1. rank $A$+ rank $B$= $n$

  2. nullity $A$ + nullity $B$ =$n$

  3. $A-B$ is invertible.

Can someone point me in the right direction? Thanks in advance for your time.

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3 Answers 3

up vote 8 down vote accepted
  1. First $AB=0$ means $\mbox{Im}B\subseteq \mbox{Ker}A$, hence $\mbox{rank}B\leq \mbox{null} A=n-\mbox{rank} A$. Now $\mathbb{R}^n=\mbox{Im}(A+B)\subseteq \mbox{Im}A+\mbox{Im}B$, hence $n\leq \mbox{rank}A+\mbox{rank}B\leq \mbox{rank}A+n-\mbox{rank} A=n$. So $\mbox{rank}A+\mbox{rank}B=n$.

  2. Rank-nullity theorem on $A$ and on $B$.

  3. Observe that $(A-B)^2=A^2+B^2=(A+B)^2$.

Note: we only need $AB=0$ to obtain 1 and 2.

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can you explain the point more clearly why $AB = 0$ means $Im B \subset Ker A$? –  analysis89 Apr 17 '13 at 16:33
    
and why $\mathbb R^n =Im (A+B)$? –  analysis89 Apr 17 '13 at 16:42
    
@analysis89 If $x$ belongs to $\mbox{Im} B$, we can find $y$ such that $x=By$. Then $Ax=ABy=0$. So $x$ belongs to $\mbox{Ker} A$. –  1015 Apr 17 '13 at 16:42
    
$A+B$ invertible implies in particular $A+B$ surjective (and that's equivalent by the rank-nullity theorem). –  1015 Apr 17 '13 at 16:44
1  
The meaning of the Note is not clear to me. Do you mean that nothing but $AB=0$ is needed for 1. and 2. (contradicted by the fact that $A+B$ invertible is also used, as confirmed by the comments), or that $AB=0$ is not used to obtain 3. (but it would seem that $(A-B)^2=A^2+B^2$ requires $AB=BA=0$, or at least $AB+BA=0$). Maybe it just means that 1. and 2. can be obtained without the hypothesis that $BA=0$? It would be a rather cryptic way to say that. –  Marc van Leeuwen Aug 28 '13 at 8:50

$BA = 0$ implies that the nullity of $B \geq$ the rank of $A$. $AB = 0$ implies that the nullity of $A \geq$ the rank of $B$.

$$ n(B) \geq r(A)$$ $$ n(A) \geq r(B)$$ $$ n(A) + r(A) + n(B) + r(B) = 2n$$

The final piece of the puzzle is that $A + B$ is invertible, this means that $r(A) + r(B) \geq n$ since we can't add two sets of vectors together and produce more linearly independent vectors than the sum of the rank of the two sets. The only conclusion is that $r(A) + r(B) = n$ and $n(A) + n(B) = n$.

To see that $A-B$ is invertible, $(A - B)^2 = A^2 + B^2 = (A + B)^2$

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$\operatorname{rank}(AB) \geq \operatorname{rank} A+\operatorname{rank} B-n$ as $AB=0$ rank of $AB=0$ $$0 \geq \operatorname{rank} A+\operatorname{rank} B-n=0$$

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You may want to add some explanations and format your answer to make it more readable. –  mrf Aug 28 '13 at 8:52

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