  We show the closed-form solution to the maximization of trace(A'R), where A
is given and R is unknown rotation matrix. This problem occurs in many computer
vision tasks involving optimal rotation matrix estimation. The solution has
been continuously reinvented in different fields as part of specific problems.
We summarize the historical evolution of the problem and present the general
proof of the solution. We contribute to the proof by considering the degenerate
cases of A and discuss the uniqueness of R.
