On linear operators with an invariant subspace of functions
  Let us denote ${\cal V}$, the finite dimensional vector spaces of functions of the form $\psi(x) = p_n(x) + f(x) p_m(x)$ where $p_n(x)$ and $p_m(x)$ are arbitrary polynomials of degree at most $n$ and $m$ in the variable $x$ while $f(x)$ represents a fixed function of $x$. Conditions on $m,n$ and $f(x)$ are found such that families of linear differential operators exist which preserve ${\cal V}$. A special emphasis is accorded to the cases where the set of differential operators represents the envelopping algebra of some abstract algebra. 
Yves Brihaye