Solution (source code)

= Solution

The <eigenvalue> equation is $u_{i+1}=\lambda u_i$, so
$$
u=c(1,\lambda,\lambda^2,\ldots)^T.
$$
This is a nonzero square-summable <vector> exactly when $c\ne0$ and $|\lambda|<1$. Hence every point of the open unit disc is an <eigenvalue>, with the displayed one-dimensional eigenspace.