  Let $l[y]$ be a formally selfadjoint differential expression of an even order
on the interval $[0,b> \;(b\leq \infty)$ and let $L_0$ be the corresponding
minimal operator. By using the concept of a decomposing boundary triplet we
consider the boundary problem formed by the equation $l[y]-\l y=f\;(f\in
L_2[0,b>)$ and the Nevanlinna $\l$-depending boundary conditions with constant
values at the regular endpoint 0. For such a problem we introduce the concept
of the $m$-function, which in the case of selfadjoint decomposing boundary
conditions coincides with the classical characteristic (Titchmarsh-Weyl)
function. Our method allows one to describe all minimal spectral functions of
the boundary problem, i.e., all spectral functions of the minimally possible
dimension. We also improve (in the case of intermediate deficiency indices
$n_\pm(L_0)$ and not decomposing boundary conditions) the known estimate of the
spectral multiplicity of the (exit space) selfadjoint extension $\wt A\supset
L_0$. The results of the paper are obtained for expressions $l[y]$ with
operator valued coefficients and arbitrary (equal or unequal) deficiency
indices $n_\pm(L_0)$.
