I spent a couple hours today working out a Newton method for finding the closest rotation to a given matrix A, i.e. min ‖R − A‖² over R ∈ SO(3). Then I found out that Kugelstadt et al. already figured it out: https://animation.rwth-aachen.de/publication/0561/
Oh well. Glad to verify that I got the same result, but I like my derivation better; it's much shorter :)
