On Indecomposable triples associated with nilpotent operators

We consider in this paper the family of triples $(V, T, U),$ where $V$ is a finite dimensional space, $T$ is a nilpotent linear operator on $V$ and $U$ is an invariant subspace of $T$. Denote $[U]= ker(T_{|U})$, and $n_U= dim([U] )$. Our main goal is to investigate possible classification of indecomposable triples. The obtained classification depends on the order of nilpotency $p$, on $n_U$ and on $n_V$. Complete classifications are given for arbitrary $p$, when $n_U=1$, and when $n_U=2$ and $n_V \le 3$. The case $p \le 5$, treated in \cite{ring} is recaptured by using constructive proofs based on linear algebra tools. The case $p\ge 6$, where the number of indecomposable triples is infinite, is also investigated.