# Rotation Matrix

## The special orthogonal group: SO(3)

The special orghogonal group SO(3), also known as the group of rotation matrices, is the set of all 3 \times 3 real matrices R that satisfy (i) RR^T = I and (ii) \det{R} = 1

SO(n) is called groups because they satisfy the properties required of a mathematical group. More specifically, the SO(n) groups are also called matrix Lie groups as the elements of the group form a differentiable manifold (?).

Definition of a group

A group consists of a set of elements and an operation on two elements such that, for all A, B in the group:

1. closure: AB is also in the group
2. associativity: (AB)C = A(BC)
3. identity element existence: There exists an element I in the group (the identity matrix for SO(n)) such that AI=IA=A
4. inverse element existence: There exists an element A^{-1} in the group such that AA^{-1} = A^{-1}A = I
Definition of Lie group

A Lie group is a (non-empty) subset G of \mathbb{R}^N that fulfills:

1. G is a group
2. G is a manifold in \mathbb{R}^N
3. Both the group product operation and its inverse are smooth functions
What is Manifold??

An N-dimensional manifold M is a topological space where every point p \in M is endowed with local Euclidean structure.

Intuitively, this means that in an infinitely small vicinity of a point p, the space looks "flat", or a \mathbb{R}^2 Euclidean space.

## Exponential Coordinate Representation of Rotation (3.2.3; P.77)

The exponential coordinates parameterize a rotation matrix in terms of:

• a rotation axis (as a unit vector \hat{\omega} \in \mathbb{R}^3)
• an angle of rotation \theta \in \mathbb{R} about the axis

A rotation with this is represented as \hat{\omega}\theta \in \mathbb{R}^3.

There are two views to explain the connection with a rotation matrix R:

1. If a frame was rotated by \theta about \hat{\omega}, its final orientation (relative to the original frame) would be expressed by R
2. If a frame followed angular velocity \hat{\omega} for \theta units of time, its final orientation (relative to the original frame) would be expressed by R

Let's think about rotating a vector p(0) by \theta about \hat{\omega} to p(\theta).
p(t) forms the path traced by the tip of the vector.

We can see it as p(0) rotates at a constant rate (1 [rad/s]) from time t=0 to t=\theta. Then, its velocity is given by:

\dot{p} = \hat{\omega} \times p

Then we use 3 \times 3 skew-symmetric matrix to transform it into a familiar form

skew-symmetric matrix

Given a vector x = [x_1, x_2, x_3]^T \in \mathbb{R}^3,

[x] \vcentcolon= \begin{bmatrix}0 & {\color{green} -x_3} & {\color{blue} x_2}\\ {\color{green} x_3} & 0 & {\color{purple} -x_1}\\ {\color{blue} -x_2} & {\color{purple} x_1} & 0 \end{bmatrix}

It's called skew-symmetric because [x] = -[x]^T.

The set of all 3 \times 3 skew-symmetric matrices is called so(3), and is also called Lie algebra of the Lie group SO(3).

\dot{p} = [\hat{\omega}] p

This is a linear differential equation (\dot{x} = Ax), whose solution is given by:

p(t) = e^{[\hat{\omega}]t}p(0)

## Summary

\hat{\omega}\theta \in \mathbb{R}^3 \longleftrightarrow [\hat{\omega}]\theta \in so(3) \longleftrightarrow e^{[\hat{\omega}\theta]} = R \in SO(3)

Rodrigues' formula

e^{[\hat{\omega}\theta]} = I + \sin\theta[\hat{\omega}] + (1 - \cos\theta)[\hat{\omega}] \in SO(3)