and since three applications of map each basis vector back onto itself, that angle of rotation is 360/3 = 120. Moreover, this observation amounts to saying that A^3 = I, so that A^-1 = A^2. This can be seen geometrically as well in that A^-1 will effect a rotation of 120 in the opposite sense, which is the same as a rotation of 240 in original direction, which is of course the effect of the matrix A^2