Hi,


Given two (smooth, projective) curves $X$ and $Y$ over a field $k$, define a *correspondence* to be a line
bundle $L$ on $X\times Y$. A *trivial* correspondence is a correspondence of the form $p\_1^\*L\_1\otimes p\_2^\*L\_2$. Define a function $N$ on the set of correspondences by
$$N(L) = -\chi(L)+\chi\_1(L)\chi\_2(L),$$
where $\chi\_1(L)$ (resp. $\chi\_2(L)$) is the Euler characteristic of $L\mid X\times y$ (resp. $L\mid x\times Y$) for any $y\in Y$ (resp. $x\in X$) (the Euler characteristic is constant).
Then it turns out that $N$ is well defined on the set of correspondences modulo trivial corresopndences, and it is a non-degenerate quadratic form. By this I mean that $N(L)\geq 0$
and $N(L) = 0$ if and only if $L$ is a trivial correspondence ('$N$ is non-degenerate') and the form
$$
[L\_1,L\_2] = N(L\_1\otimes L\_2) - N(L\_1) - N(L\_2) - N(\mathcal O\_{X\times Y})
$$
is bilinear ('$N$ is quadratic').


* Question: what is going on here? It just seems magic that this works. Does it work more generally? In higher dimensions? Is there a complex analytic analog of this?


Thanks!



non degenerate quadratic form on the group of correspondences on an algebraic curve?
The Riemann--Roch says that there is a cohomology class $a \in H^{4}(X\times Y,Q)$ such that $$
\chi(L) = a - c\_1(L)((g\_X-1)[Y] + (g\_Y - 1)[X]) + c\_1(L)^2/2.
$$ 
Let $c\_1(L) = d\_X[Y] + d\_Y[X] + c\_{10}(L)$ be the Kunneth decomposition. 
Note that $d\_X = c\_1(L)\cdot [X]$ and $d\_Y = c\_1(L)\cdot [Y]$.
Then 
$$
\chi\_1(L) = d\_X - g\_X + 1,
\qquad
\chi\_2(L) = d\_Y - g\_Y + 1.
$$
Substituting all this we see that
$$
\chi(L) - \chi\_1(L)\chi\_2(L) = 
a - d\_Y(g\_X - 1) - d\_X(g\_Y - 1) + (2d\_Xd\_Y + c\_{10}(L)^2)/2 - d\_Xd\_Y -d\_X(g\_Y - 1) - d\_Y(g\_X + 1) + (g\_X - 1)(g\_Y - 1).
$$
One can see in fact that everything cancels with the only exception of the term $c\_{10}(L)^2$ which gives your quadratic form. Now you can see that it is important that we work on a product (we use the Kuneth decomposition) and that the dimension of the product is 2 (for higher dimension there would be other terms of higher degree in Riemann--Roch). So, basically this fact is true only for product of curves. 


On the other hand, you kight have something similar for two varieties of odd dimension $X$ and $Y$ such that $H^{odd}(X,Q)$ and $H^{odd}(Y,Q)$ are concentrated in degrees $\dim X = \dim Y$.


