I would like to prove the following inequalities:

$|\text{rank}A-\text{rank}B|\leq \text{rank}(A+B)$

I know that $\text{rank}(A+B)\leq \text{rank}A + \text{rank} B,$ but I can't tackle the problem.

Any help would be appreciated.



Use the inequality you have already.

We have $$\def\rank{\mathop{\rm rank}}\rank(A)=\rank((A+B)+(-B))\le\rank(A+B)+\rank(-B)\ .$$ See if you can finish it from here.

  • $\begingroup$ Ahhh. Thanks so much.. $\endgroup$ – Balai_Indah Mar 7 '19 at 3:11

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