Quasisimilarity of invariant subspaces for $C_0$ operators with   multiplicity two
  For an operator $T$ of class $C_0$ with multiplicity two, we show that the quasisimilarity class of an invariant subspace $M$ is determined by the quasisimilarity classes of the restriction $T|M$ and of the compression $T_{M^\perp}$. We also provide a canonical form for the subspace $M$. 
Rapha\"el Clou\^atre
math.FA math.OA