On the index of the Weierstrass semigroup of a pair of points on a curve

We obtain the exact formulas for the cardinality of the complement of the Weierstrass semigroup of a pair (p, q) of points on a curveC. Using these formulas we obtain lower bounds and upper bounds on the cardinalities of such sets. Moreover, considering examples, we show that these bounds are sharp.Partially supported by Korea Science and Engineering Foundation and by Global Analysis Research Center.