I hope this question is good to be here.
Let $J$ be the Jacobian of a hyperelliptic curve $H$ of genus $2$ given by $y^2 = x^5 + h$.
I was calculating an explicit formula in Mumford coordinates of $[\sqrt{5}]\in \text{End}(J)$. When I say Mumford coordinates I mean divisors $D:=(x_1+x_2, x_1x_2, \tfrac{y_1-y_2}{x_1-x_2},\tfrac{x_2y_1-x_1y_2}{x_1-x_2})=:(A,B,C,D)\in J$ where $\text{Supp}(D)=\{(x_1,y_1), (x_2,y_2)\}$ and $(x_i,y_i)\in H$.
I used these coordinates in order to work with MAGMA.
The thing is, I got the formulae, the problem is that I do not know how to handle an special situation.
What I did is to calculate $[\zeta_5+\zeta_5^4] \in \text{Aut}(J)$ where the action of $\zeta_5^i$ on the curve $H$ is given by $\zeta_5^i(x,y)=(\zeta_5^i x,y)$ and then extend to the whole Jacobian.
After this, I calculated the formula for $2[\zeta_5+\zeta_5^4]+1=[\sqrt{5}]$ using Mumford coordinates and then if $D=(A,B,C,D)$ then I got $\sigma_i\in k(J)$ where $[\sqrt{5}](D)=(\sigma_1(A,B,C),\sigma_2(A,B,C), \sigma_3(A,B,C), \sigma_4(A,B,C))\in J$
Also I have formulae when the input divisor is of the form $[(x,y)-\infty]\in \Theta\subset J$
My problem is when the input is not in $\Theta$ (an ordinary, generic Divisor) but the output is in $\Theta$.
My question is, if $D:=(A,B,C,D)$ how can I compute if $[\sqrt{5}](D)\in \Theta$, and more... how can I compute the resulting divisor?
Detecting it I think is easy, I just need to calculate a determinant of a $4\times 4$ matrix in terms of $A,B,C$. But I do not know how to compute the divisor using my coefficients $(A,B,C,D)$. Is there a way to get explicit formulae for this "exceptional case" when is already known that the image will be in $\Theta$?
Thanks
