# Finite dimensional representation of infinite group cannot be unitary: example with $\mathbb R$

Consider the representation of the group of real numbers $\mathbb R$ given by $$\rho (x) = \begin{pmatrix} 1 & x \\ 0 & 1 \end{pmatrix}$$ for $x \in \mathbb R$. How can we see that this representation cannot be equivalent to a unitary representation?

• Certainly there are finite-dimensional unitary representations of infinite groups in general. – whacka Oct 4 '15 at 23:27

Every unitary matrix is diagonalizable, but $\rho(x)$ is not diagonalizable unless $x=0$ (its only eigenvector (up to scaling) is $(1,0)$).