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mathmonkey
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Homework Statement
Let ##O(3)## denote the set of all orthogonal 3 by 3 matrices, considered as a subspace of ##\mathbb{R}^9##.
(a) Define a ##C^\infty## ##f:\mathbb{R}^9 \rightarrow \mathbb{R}^6## such that ##O(3)## is the solution set of the equation ##f(x) = 0##.
(b) Show that ##O(3)## is a compact 3-manifold in ##\mathbb{R}^9## without boundary. [Hint: Show the rows of ##Df(x)## are independent if ##X \in O(3)##.
Homework Equations
I think the following theorem:
Let ##O## be open in ##\mathbb{R}^n##; let ##f:O \rightarrow \mathbb{R}## be of class ##C^r##. Let ##M## be the set of points ##x## for which ##f(x) = 0##; let ##N## be the set of points for which ##f(x) \geq 0##. Suppose ##M## is non-empty and ##Df(x)## has rank 1 at each point of ##M##. Then ##N## is an n-manifold in ##\mathbb{R}^n## and the boundary of ##N## is ##M##.
might be useful, but I don't see an opening for how I can apply it to the problem.
The Attempt at a Solution
I'm stuck on part (a) of the question, I think because the construction of the function seems kind of open-ended to me, as well as unmotivated in relation to part (b) (e.g. Why ##f:\mathbb{R}^9 \rightarrow \mathbb{R}^6##?). I feel like the function satisfying the required conditions may not be unique, like maybe something like
##f(\textbf{x},\textbf{y},\textbf{z}) = (\textbf{x} \cdot \textbf{y}, \textbf{x} \cdot \textbf{z}, \textbf{y} \cdot \textbf{z}, \textbf{x} \cdot \textbf{y}, \textbf{x} \cdot \textbf{z}, \textbf{y} \cdot \textbf{z} ) ##
or some permutation of it will satisfy the conditions for ##f## but is not unique (where ##\textbf{x}, \textbf{y} , \textbf{z}## are the orthogonal vectors making up the orthogonal matrix). Am I completely off the mark? I'm quite lost on this problem, any help would be greatly appreciated!
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