**Background.** Let $C$ be a smooth projective curve of genus $g$. Denote by $C^d$
its d-th symmetric product and by $G^r_d(C)$ its associated variety
of linear series of type $\mathfrak{g}^r_d$, i.e.
$$ G^r_d(C):= \left \{ \mathcal{L} = (L, V) \: | \: L \in Pic^d(C), V \in G(r + 1, H^0(C, L)) \right \}$$


**Definition** A linear series $\mathcal{L}$ on $C$ is said to admit a length $n$ de Jonquières divisor if
$$ a_1 p_1 + \ldots + a_n p_n \in \mathbb{P}(V) $$
for some points $p_1,\ldots , p_n \in C$ and some positive integers $a_1, \ldots, a_n$ whose sum is $d$.  

We call the corresponding divisor 
$$D := p_1 + \ldots + p_n \in C^n$$
a de Jonquières divisor of length $n$.

De Jonquières formula (In [ACGH](http://www.springer.com/us/book/9780387909974) chapter VIII, §5) states that, if we
expect there to be a finite number of de Jonquières divisors of length $n$, then
this virtual number is given by the coefficient of the monomial $t_1 \cdot \ldots \cdot t_n$ in
$$(1 + a_1^2 t_1 + \ldots + a_n^2 t_n)^g(1 + a_1 t_1 + \ldots + a_n t_n)^{d−r−g}.$$
.

**My question.** 

 1. Why is the virtual count in this formula "wrong"? I heard there are supposed to be some counter-examples showing that the formula does not always give the "correct number", I'm curious to know: (1) what they are, and (2) what then is the formula actually counting...
 2. Is there a way to relate the virtual count in this formula to *relative Gromov-Witten invariants* (in the sense of [Li-Ruan](https://arxiv.org/abs/math/9803036), [Ionel-Parker](http://annals.math.princeton.edu/wp-content/uploads/annals-v157-n1-p02.pdf) and [Jun Li](http://www.emis.de/journals/NYJM/JDG/p/2001/57-3-6.pdf)) of $\mathbb{P}^r$ with respect to the curve $C$ embedded by the linear series $\mathcal{L}$?