Special Orthogonal Group is Subgroup of Orthogonal Group
Theorem
Let $k$ be a field.
Let $\map {\operatorname O} {n, k}$ be the $n$th orthogonal group on $k$.
Let $\map {\operatorname {SO} } {n, k}$ be the $n$th special orthogonal group on $k$.
Then $\map {\operatorname {SO} } {n, k}$ is a subgroup of $\map {\operatorname O} {n, k}$.
Proof
We have that Unit Matrix is Proper Orthogonal, so $\map {\operatorname {SO} } {n, k}$ is not empty.
Let $\mathbf A, \mathbf B \in \map {\operatorname {SO} } {n, k}$.
Then, by definition, $\mathbf A$ and $\mathbf B$ are proper orthogonal.
Then by Inverse of Proper Orthogonal Matrix is Proper Orthogonal:
- $\mathbf B^{-1}$ is a proper orthogonal matrix.
By Product of Proper Orthogonal Matrices is Proper Orthogonal Matrix:
- $\mathbf A \mathbf B^{-1}$ is a proper orthogonal matrix.
Thus by definition of special orthogonal group:
- $\mathbf A \mathbf B^{-1} \in \map {\operatorname {SO} } {n, k}$
Hence the result by One-Step Subgroup Test.
$\blacksquare$