  The relation of the Weierstrass semigroup with several invariants of a curve
is studied. For Galois covers of curves with group $G$ we introduce a new
filtration of the group decomposition subgroup of $G$. The relation to the
ramification filtration is investigated in the case of cyclic covers. We relate
our results to invariants defined by Boseck and we study the one point
ramification case. We also give applications to Hasse-Witt invariant and
symmetric semigroups.
