# Does every rotation vector has an one-to-one corresponding rotation matrix?

Given a rotation vector, can we always get the same corresponding rotation matrix computed by the Rodrigue's rotation formula?

Mathematically a rotation vector(or axis angle) representation will always convert to the same rotation matrix.

However, multiple different rotation vectors can lead to the same rotation matrix.

A rotation vector represents a rotation by an axis and an angular rotation around that axis(axis angle).

$$\begin{bmatrix} e_x \\ e_y \\ e_z \end{bmatrix},\theta$$

Thinking about it for a bit it is pretty easy to see where different axis angle representations will lead to the same rotation.

One is when the angle($$\theta$$) is the same angle but in the opposite direction. E.g $$\frac{\pi}{2}$$ and $$\frac{-3\pi}{2}$$.

The other is when the axis points in the opposite direction, and the angle is also in the opposite direction.

$$\{\begin{bmatrix} e_x \\ e_y \\ e_z \end{bmatrix},\theta\}=\{\begin{bmatrix} -e_x \\ -e_y \\ -e_z \end{bmatrix},-\theta\}$$

Finally for numerical reason oftentimes the Rodrigues formula will also set some values constant/Identity if the rotation is small enough. So while mathematically they should lead to distinct values, on your computer they might end up being the same thing.
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