  Starting from fixed-order perturbation theory (FOPT) we derive expressions
for the heavy-flavour components of the deep-inelastic structure functions FL
and F2 in the variable-flavour number scheme (VFNS). These expressions are
valid in all orders of perturbation theory. This derivation establishes a
relation between the parton densities parametrized at N and N light flavours.
The consequences for the existing parametrizations of the parton densities are
discussed. Further we show that in charm electroproduction the exact and
asymptotic expressions for the heavy-quark coefficient functions yield
identical results for F2 when Q^2>20 (GeV/c)^2. We also study the differences
between the FOPT and the VFNS descriptions for F2. It turns out that the charm
structure function in the VFNS is larger than the one obtained in FOPT over the
whole Q^2-range. Furthermore inspection of the perturbation series reveals that
the higher order corrections in the VFNS are smaller than those present in FOPT
for Q^2>10 (GeV/c)^2. Therefore the VFNS gives a better prediction for the
charm structure function at large Q^2-values than FOPT.
