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Show that the set of all orthogonal matrices in the set of all $n \times n$ matrices endowed with any norm topology is compact.

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    It would also be expeditious to use the operator norm. What is the operator norm of an orthogonal matrix?2011-01-23

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Recall a compact subset of $R^{n \times n}$ is a set that is closed and bounded. One way to show closedness is to observe that the orthogonal matrices are the inverse image of the element $I$ under the continuous map $M \rightarrow MM^T$. Boundedness follows for example from the fact that each column or row is a vector of magnitude $1$.

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    Why is the map continuous? Can you explain it a little bit more? I am thinking of a clear and concise way to prove it. (I have background in functional analysis.)2017-04-22