It is a fundamental problem in algebraic geometry to understand
the behavior of a multiple linear system |nD| on a projective complex manifold
X for large n. For example, the well-known Riemann-Roch problem is to compute
the function
n ↦ h0(OX(nD)) := dimC H0(X,OX(nD)).
In the introduction to his collected works [33], Zariski cited the Riemann-Roch problem
as one of the four "difficult unsolved questions concerning projective varieties
(even algebraic surfaces)". The other natural problems about |nD| are to find the fixed
part and base points (see [32]), the very ampleness, the properties of the associated
rational map and its image variety, the finite generation of the ring of sections.
¶ For a genus g curve X, Riemann-Roch theorem gives good and effective solutions
to these problems.