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Non-vanishing of class group \(L\)-functions for number fields with a small regulator. (English) Zbl 1473.11210

Author’s abstract: “Let \(E/\mathbb{Q}\) be a number field of degree \(n\). We show that if \(\operatorname{Reg}(E)\ll_n |\!\operatorname{Disc}(E)|^{1/4}\) then the fraction of class group characters for which the Hecke \(L\)-function does not vanish at the central point is \(\gg_{n,\varepsilon} |\!\operatorname{Disc}(E)|^{-1/4-\varepsilon}\). The proof is an interplay between almost equidistribution of Eisenstein periods over the toral packet in \(\mathbf{PGL}_n(\mathbb{Z})\backslash \mathbf{PGL}_n(\mathbb{R})\) associated to the maximal order of \(E\), and the escape of mass of the torus orbit associated to the trivial ideal class.”
Reviewer’s remarks: The precise formulation of the main result of the paper is follows:
Theorem 1.1. Let \(E/\mathbb{Q}\) be a number field of degree \(n\). Denote by \(D\) its discriminant, by \(R\) the regulator of its ring of integers and by \(h\) the class number. For every class group character \(\chi\in\widehat{\text{Cl}(E)}\) let \(L(s,\chi)\) be the associated Hecke \(L\)-function.
Fix a real number \(1/2\le s<1\). There are effectively computable constants \(A,B>0\) that depend only on \(s\), \(n\) such that for every \(1/2>\varepsilon>0\), \[h^{-1}\#\{\chi\in \widehat{\text{Cl}(E)}\mid L(s,\chi)\ne 0\}\ge|D|^{-(1-s+\varepsilon)/2}\left(A-B\frac{R}{|D|^{s/2}}\right)\varepsilon^n.\] In the Introduction the author sets out his aims and compares his results to related earlier investigations of others. Then, in the subsection 1.2 he describes his working out of the paper. In Section 2 properties of the Epstein zeta function are dealed with and sharpened, whereas other means like Weil chambers and Fourier series are applied, in order to achieve his main results. In the proof of Theorems 2.6 one encounters the notion (and notation) of \(erfc\sqrt{\pi t}\), perhaps known to specialists, but perhaps not so well-known in general. It is only a little inconvenience. The whole paper is a good essay in fact, to be studied carefully, and very well suited for seminars also.
Nice stuff, the reviewer enjoined it; a very welcome to modern number theory.

MSC:

11R42 Zeta functions and \(L\)-functions of number fields
11M20 Real zeros of \(L(s, \chi)\); results on \(L(1, \chi)\)
11R04 Algebraic numbers; rings of algebraic integers
11M41 Other Dirichlet series and zeta functions
11E45 Analytic theory (Epstein zeta functions; relations with automorphic forms and functions)
11F66 Langlands \(L\)-functions; one variable Dirichlet series and functional equations
11F67 Special values of automorphic \(L\)-series, periods of automorphic forms, cohomology, modular symbols

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