I'm reading the following paper:

"Least-Squares Fitting of Two 3-D Point Sets"

by Arun, Huang, and Bolstein

IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. 9, No. 5, September 1987, pages 698-700

link: http://www.google.com/url?sa=t&rct=j...PNelCS9sTejndw

I can't understand the equation 14 of this paper, where Trace is used.

Thank you in advance!