  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.
