# Reaching all of R^N by rotations from a linear subspace

#### uekstrom

Hi all,

I have a problem related to quantum mechanical description of vibrational motion in molecules. I would like, for efficiency, to integrate over symmetries (global rotations) of the molecule.

I would like to prove (or disprove) that all points in $$R^N$$ can be reached by rotations from a linear subspace (let's call it L). In my particular case L has dimension N-3, and my rotations are generated by three matrices $$T_i$$, of the form

$$T_i = \mathrm{diag}(L_i,L_i,..)$$,

where $$L_x,L_y,L_z$$ are the 3x3 generators of SO(3). $$T_x$$ is then a block-diagonal matrix with copies of $$L_x$$ on the diagonal.

So what I want to prove (or disprove) is if, for a given x,

$$x = exp\left( c_x T_x + c_y T_y + c_z T_z \right) y$$

has a solution for x from $$R^N$$ and y from the linear subspace L. L can be chosen arbitrarily, but should not depend on x. From numerical experiments is seems like this rotation is always possible, but I would like to have a formal proof. I will add more details if necessary.

Last edited:
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#### fresh_42

Mentor
2018 Award
Let $v\in \mathbb{R}^N$ be any given vector. Since $L$ is a linear subspace, there is an element in $w\in L$ with $|v|=|w|$. Then there is always a rotation which transforms $w$ into $v$. It can be solved in the two dimensional subspace spanned by $v,w$ and then extended in any way to $\mathbb{R}^N$.

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