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03

Hyperelliptic curve residual divisors

Let XX be the smooth projective complex curve obtained by completing the affine curve y2=x33xy^2=x^{33}-x over C\mathbb{C}. Let OO be its unique point at infinity. For each integer ii with 2i92\le i\le 9, fix one square root aiCa_i\in\mathbb{C} of i33ii^{33}-i, and define two labeled points Pi=(i,ai)P_i=(i,a_i) and Qi=(i,ai)Q_i=(i,-a_i) on XX. Let A={O}{Pi,Qi:2i9}A=\{O\}\cup\{P_i,Q_i:2\le i\le 9\}. Divisors supported on AA are integral linear combinations of these 1717 labeled points, and an effective divisor has all coefficients nonnegative.

Let KK be the divisor 30O30O. For any divisor DD on XX, write (D)=dimCH0(X,OX(D))\ell(D)=\dim_{\mathbb{C}}H^0(X,\mathcal{O}_X(D)); this convention also applies when DD has negative coefficients. Let SS be the set of effective divisors EE supported on AA with degree 1616 such that (E)>1\ell(E)>1 and satisfying the following condition. There must exist an effective divisor FF supported on AA with degree 1414 such that E+FE+F is linearly equivalent to KK.

For such an FF, let Supp(F)\operatorname{Supp}(F) be the set of points of AA with positive coefficient in FF, not counted with multiplicity. Let NFN_F be the number of unordered pairs {r,s}\{r,s\}, with r,sSupp(F)r,s\in\operatorname{Supp}(F) and repetitions allowed, such that (F+r+s)>1\ell(F+r+s)>1; when r=sr=s, the divisor F+r+sF+r+s means F+2rF+2r. The required condition is that at least one such FF has NF>0N_F>0 and NFN_F congruent modulo 22 to Supp(F)+(KE)|\operatorname{Supp}(F)|+\ell(K-E).

Let τ:XX\tau:X\to X be the involution τ(x,y)=(x,y)\tau(x,y)=(x,-y), extended by τ(O)=O\tau(O)=O. It acts on divisors supported on AA by τ(Pi)=Qi\tau(P_i)=Q_i, τ(Qi)=Pi\tau(Q_i)=P_i, and τ(O)=O\tau(O)=O, preserving coefficients. Two divisors in SS are in the same τ\tau-orbit if and only if one is equal to the image of the other under this action; the indices i=2,,9i=2,\ldots,9 are not permuted. A divisor EE in SS is τ\tau-fixed if τ(E)=E\tau(E)=E as a labeled divisor.

Determine the ordered pair whose first entry is the number of τ\tau-orbits in SS and whose second entry is the number of τ\tau-fixed divisors in SS.