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<p>if , then</p>

<p>Note the change in order of  and  (we have  instead of ). Similarly, if , then  should be replaced with  (i.e. we get ) and if , then we should have . Note for the last two results, you have to use commutativity of the underlying ring, but not for the first two.</p>

<p>This formula has been generalized to matrices composed of more than  blocks, again under appropriate commutativity conditions among the individual blocks.</p>

<p>For  and , the following formula holds (even if  and  do not commute)</p>

<p><big>Special types of block matrices</big></p><p>
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