Friday, 19 October 2007

nt.number theory - Modular forms and the Riemann Hypothesis

I know two statements about modular forms that are Riemann Hypothesis-ish.



First, note that the constant term of the level-one non-holomorphic Eisenstein series Es is ys+c(s)ys, and that the poles of c(s) are the same as the poles of Es. We can directly calculate that c(s)=Lambda(s)overLambda(1+s) (this depends on your precise normalization of the Eisenstein series), where Lambda is the completed zeta function. We can actually say something about the location of the poles of Es (using the spectral theory of automorphic forms). Unfortunately, we only know how to control poles for rmRe(s)ge0. This does give an alternate proof of the nonvanishing of zeta(s) at the edge of the critical strip (from the lack of poles of Lambda(it)overLambda(1+it)), but it doesn't seem possible to go further to the left (though it does generalize to other L-functions appearing as the constant term of cuspidal-data Eisenstein series).



Second, the values of modular forms at certain (Heegner) points in the upper-half plane can be related to zeta functions. For example, Es(i)=LambdamathbbQ(i)(s)overLambdamathbbQ(2s). The general statement is simple to express adelically. Take a quadratic extension k1 of k, and let H denote kt1imes as a k-group and Es the standard level-one Eisenstein series on G=GL2(k). Take a character chi on ZmathbbAHkbackslashHmathbbA then
intZmathbbAHkbackslashHmathbbAEs(h)chi(h)dh=Lambdak1(s,chi)overLambdak(2s)
where Z denotes the center of G, and we have normalized the measure on the quotient space to be 1. Note that since H is a non-split torus in G, the quotient is compact, so the integral is finite. In fact, the integrand is invariant (on the right) under a compact open subgroup K of HmathbbA, so the integral is actually over the double coset space ZmathbbAHkbackslashHmathbbA/K, which is actually a finite group.
In order to get the Riemann zeta function in the numerator on the right-hand-side, you would need to integrate over a split torus, which is precisely the Mellin transform, and you would have convergence issues. Note that if it did converge, the Mellin transform of Es would be
intZmathbbAMkbackslashMmathbbAEs(a)|a|vda=Lambda(v+s)Lambda(v+1s)overLambda(2s)



The second idea is more commonly discussed in the context of subconvexity problems for general L-functions. (See Iwaniec's Spectral Methods of Automorphic Forms, especially Chp 13.) A class of subconvexity results is the Lindelof Hypothesis, which is one of the stronger implications of the Riemann Hypothesis.

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