Hyperelliptic curve residual divisors
Let be the smooth projective complex curve obtained by completing the affine curve over . Let be its unique point at infinity. For each integer with , fix one square root of , and define two labeled points and on . Let . Divisors supported on are integral linear combinations of these labeled points, and an effective divisor has all coefficients nonnegative.
Let be the divisor . For any divisor on , write ; this convention also applies when has negative coefficients. Let be the set of effective divisors supported on with degree such that and satisfying the following condition. There must exist an effective divisor supported on with degree such that is linearly equivalent to .
For such an , let be the set of points of with positive coefficient in , not counted with multiplicity. Let be the number of unordered pairs , with and repetitions allowed, such that ; when , the divisor means . The required condition is that at least one such has and congruent modulo to .
Let be the involution , extended by . It acts on divisors supported on by , , and , preserving coefficients. Two divisors in are in the same -orbit if and only if one is equal to the image of the other under this action; the indices are not permuted. A divisor in is -fixed if as a labeled divisor.
Determine the ordered pair whose first entry is the number of -orbits in and whose second entry is the number of -fixed divisors in .