Sum of residues formula

In mathematics, the residue formula says that the sum of the residues of a meromorphic differential form on a smooth proper algebraic curve vanishes.

Statement

In this article, X denotes a proper smooth algebraic curve over a field k. A meromorphic (algebraic) differential form $\omega$ has, at each closed point x in X, a residue which is denoted ${\mathrm{res}}_x\omega$. Since $\omega$ has poles only at finitely many points, in particular the residue vanishes for all but finitely many points. The residue formula states:

$\sum _x{\mathrm{res}}_x\omega =0.$

Proofs

A geometric way of proving the theorem is by reducing the theorem to the case when X is the projective line, and proving it by explicit computations in this case, for example in (Altman Kleiman).

(Tate 1968) proves the theorem using a notion of traces for certain endomorphisms of infinite-dimensional vector spaces. The residue of a differential form $fdg$ can be expressed in terms of traces of endomorphisms on the fraction field $K_x$ of the completed local rings ${\widehat{{𝒪}}}_{X,x}$ which leads to a conceptual proof of the formula. A more recent exposition along similar lines, using more explicitly the notion of Tate vector spaces, is given by (Clausen 2009).

References

• Altman, Allen; Kleiman, Steven (1970), Introduction to Grothendieck duality theory, Lecture Notes in Mathematics, 146, Springer, doi:10.1007/BFb0060932 
• Clausen, Dustin (2009), Infinite-dimensional linear algebra, determinant line bundle and Kac–Moody extension, Harvard 2009 seminar notes, http://www.math.harvard.edu/~gaitsgde/grad_2009/ 
• Tate, John (1968), "Residues of differentials on curves", Annales scientifiques de l'École Normale Supérieure, 4 1 (1): 149-159, doi:10.24033/asens.1162, http://www.numdam.org/item/?id=ASENS_1968_4_1_1_149_0