of minimal and maximal operator for an ordinary differential expression [@Nai]. At once certain difficulties arise in the case of an *indefinite system* , which can be explained as follows. In the definite case the quotient mapping $\pi$ isomorphically maps $\dom\tma$ onto $\dom\Tma$, which enables one to identify in fact the relations $\tma$ and $\Tma$. If the system is indefinite, then the mapping $\pi\up\dom\tma$ has as nontrivial kernel the null manifold $\cN$, so that the immediate identifying of $\tma$ and $\Tma$ becomes impossible.

The above difficulties were partially overcome in the papers by Kac [@Kac83; @Kac84] (the case $\dim\bH=2$) and Lesch and Malamud [@LesMal03] (the case $\dim\bH=n<\infty$), where general (not necessarily definite) systems were studied. In these papers first the equality $T_0^*=\Tma$ is proved and then the minimal relation is defined as closure of $T_0$.

In the present paper we show that for the general system the minimal relation in $\LI$ can be also defined by the first equality in with $\tmi$ in the form . Moreover in the case of the regular endpoint $a$ the minimal relation coincides with the relation $T_a$ defined by and the second equality in . Observe also that $\cT_a\subset \tmi$ and an interesting in 