Solution
= Solution
Writing $n$ as a column vector, the reflection is
$$
\Phi(x)=x-2n(n^Tx),
$$
so its <reflection matrix> is
$$
\boxed{H=I-2nn^T},
\qquad
H_{ij}=\delta_{ij}-2n_in_j.
$$
= Solution
Writing $n$ as a column vector, the reflection is
$$
\Phi(x)=x-2n(n^Tx),
$$
so its <reflection matrix> is
$$
\boxed{H=I-2nn^T},
\qquad
H_{ij}=\delta_{ij}-2n_in_j.
$$