Processing math: 100%

Sunday, 22 October 2006

ag.algebraic geometry - Euler-Maclaurin formula and Riemann-Roch

Let Df denote the derivative of a function f(x) and bigtriangledownf=f(x)f(x1) be the discrete derivative. Using the Taylor series expansion for f(x1), we easily get bigtriangledown=1eD or, by taking the inverses,
frac1bigtriangledown=frac11eD=frac1DcdotfracD1eD=frac1D+frac12+suminftyk=1B2kfracD2k1(2k)!,
where B2k are Bernoulli numbers.



(Edit: I corrected the signs to adhere to the most common conventions.)



Here, (1/D)g is the opposite to the derivative, i.e. the integral; adding the limits this becomes a definite integral intn0g(x)dx. And (1/bigtriangledown)g is the opposite to the discrete derivative, i.e. the sum sumnx=1g(x). So the above formula, known as Euler-Maclaurin formula, allows one, sometimes, to compute the discrete sum by using the definite integral and some error terms.



Usually, there is a nontrivial remainder in this formula. For example, for g(x)=1/x, the remainder is Euler's constant gammasimeq0.57. Estimating the remainder and analyzing the convergence of the power series is a long story, which is explained for example in the nice book "Concrete Mathematics" by Graham-Knuth-Patashnik. But the power series becomes finite with zero remainder if g(x) is a polynomial. OK, so far I am just reminding elementary combinatorics.



Now, for my question. In the (Hirzebruch/Grothendieck)-Riemann-Roch formula one of the main ingredients is the Todd class which is defined as the product, going over Chern roots alpha, of the expression fracalpha1ealpha. This looks so similar to the above, and so suggestive (especially because in the Hirzebruch's version
chi(X,F)=h0(F)h1(F)+dots=intXch(F)Td(TX)
there is also an "integral", at least in the notation) that it makes me wonder: is there a connection?



The obvious case to try (which I did) is the case when X=mathbbPn and F=mathcalO(d). But the usual proof in that case is a residue computation which, to my eye, does not look anything like Euler-Maclaurin formula.



But is there really a connection?




An edit after many answers: Although the connection with Khovanskii-Pukhlikov's paper and the consequent work, pointed out by Dmitri and others, is undeniable, it is still not obvious to me how the usual Riemann-Roch for X=mathbbPn and F=mathcalO(d) follows from them. It appears that one has to prove the following nontrivial



Identity: The coefficient of xn in Td(x)n+1edx equals
frac1n!Td(partial/partialh0)dotsTd(partial/partialhn)(d+h0+dots+hn)n|h0=dotshn=0



A complete answer to my question would include a proof of this identity or a reference to where this is shown. (I did not find it in the cited papers.) I removed the acceptance to encourage a more complete explanation.

No comments:

Post a Comment