Orthogonal invariance of Brownian motion (source code)

= Orthogonal invariance of Brownian motion

If $B$ is Brownian motion in $\mathbb R^d$ and $U$ is an orthogonal matrix, then $UB$ is Brownian motion. Its increments remain independent centered Gaussian vectors and have covariance $(t-s)UIU^T=(t-s)I$.