Walter Ferrer Santos is a professor of mathematics at the University of the Republic, Montevideo, Uruguay. Alvaro Rittatore is an associate professor of mathematics at the University of the Republic, Montevideo, Uruguay.
[The new edition of this book] retains all of the attractive features of the first edition and adds new chapters on observable actions (Chapter 15) and geometric invariant theory (Chapter 16). - Frank D. Grosshans, Mathematical Reviews, August 2017