
> 
> Given a positive integer $n$, is there an algorithm (or even better a closed formula) that provides me with a hyperelliptic curve $C/\mathbb Q$, such that its Jacobian $J:=Jac(C)$ possesses a $\mathbb Q$-rational torsion point $P$ of exact order $n$?
> 
> 
> 


The genus of $C$ may very well vary with different values of $n$.


If it makes the situation simpler, please feel free to assume that $n$ is prime.



Is there a formula for a hyperelliptic curve over QQ, such that its Jacobian contains a rational torsion point of extact order n, for any given n?
One can take
$$
\begin{array}{llllll}
 y^2 + (a\_g x^g+...+a\_0) y &=& x^{2g+1} &&& \text{ ($n=2g+1$ odd)} \cr
 y^2 + (2cx\_{g+1} +a\_g x^g+...+a\_0) y &=& -c^2x^{2g+2} &&& \text{ ($n=2g+2$ even).} \cr
\end{array}
$$
The divisor of the $y$ function is 
$$
 \text{div}(y) = n(0,0)-n(\infty),
$$
and so $D=(0,0)-(\infty)$ is an $n$-torsion point on the Jacobian.


Note that in both cases, completing the square on the left gives an equation $y^2=$ polynomial of degree $2g+1$, so the curve has a unique point at infinity. Also, the order of $D$ is exactly $n$, for otherwise $y$ would be (up to a constant) a power of a rational function which has a point of order $<n$ at $\infty$ and no other poles, and all such functions are polynomials in $x$. 


P.S. [We needed such `interesting' torsion points to construct elements of $K\_2$ on hyperelliptic curves](http://arxiv.org/abs/math/0405040)


