It is denoted by " I " " n ", or simply by " I " if the size is immaterial or can be trivially determined by the context . ( In some fields, such as quantum mechanics, the identity matrix is denoted by a boldface one, "'1 "'; otherwise it is identical to " I " . ) Less frequently, some mathematics books use " U " or " E " to represent the identity matrix, meaning " unit matrix " and the German word " Einheitsmatrix ", respectively.
