1. The problem statement, all variables and given/known data [itex]T((x_0, x_1, x_2)) = (0, x_0, x_1, x_2)[/itex] 2. Relevant equations None 3. The attempt at a solution I'm getting hung up on definitions. My book says that T is an is isomorphism if T is linear and invertible. But it goes on to say that for T of finite dimension, T can only be an isomorphism if dim(T(M)) = dim(M). The T as stated is linear and invertible, but dim(T(M)) = 4 and dim(M) = 3 which indicates it isn't an isomprphism. Rank = dim(M) = 4. If I were to define T as [itex]T((x_0, x_1, x_2, \cdots))=(0,x_0, x_1, x_2, \cdots)[/itex] the dim test doesn't apply because T deals with an infinite sequence, and we can say T is an isomorphism. It doesn't make sense that in the finite case, T isn't an isomorphism but it is in the infinite case. What am I doing wrong?