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Roelof W. Bruggeman

Publications and source records attributed to Roelof W. Bruggeman.

7 recordsLinked to original sources
arXiv2021

Representations of the unitary group SU(2,1) in Fourier term modules

We study Fourier term modules on $\mathrm{SU}(2,1)$, which are the modules arising in Fourier expansions of automorphic forms. Maximal unipotent subgroups $N$ of $\mathrm{SU}(2,1)$ are non-abelian, and we consider the ``abelian'' Fourier term modules connected to characters of $N$, and also the ``non-abelian'' modules described with theta functions. Poincaré series for $\mathrm{SU}(2,1)$ have in general exponential growth. To deal with such generalized automorphic forms we allow exponential growth for the functions in Fourier term modules. We give a complete description of the submodule structure of all Fourier term modules, and discuss the consequences for Fourier expansions of automorphic forms.

math.RT↗
arXiv2021

Generalized Poincaré series for $\mathrm{SU}(2,1)$

We define and study 'non-abelian' Poincaré series for the group $G=\mathrm{SU} (2,1)$, i.e. Poincaré series attached to a Stone-Von Neumann representation of the unipotent subgroup $N$ of $G$. Such Poincaré series have in general exponential growth. In this study we use results on abelian and non-abelian Fourier term modules obtained in arXiv:1912.01334. We compute the inner product of truncations of these series and those associated to unitary characters of $N$ with square integrable automorphic forms, in connection with their Fourier expansions. As a consequence, we obtain general completeness results that, in particular, generalize those valid for the classical holomorphic (and antiholomorphic) Poincaré series for $\mathrm{SL}(2,\mathbb{R})$.

math.NT↗
arXiv2015

The inhomogeneous Fermi-Pasta-Ulam chain

The inhomogeneous Fermi-Pasta-Ulam chain is studied by identifying the mass ratios that produce prominent resonances. This is a technically complicated problem as we have to solve an inverse problem for the spectrum of the corresponding linearized equations of motion. In the case of the inhomogeneous periodic Fermi-Pasta-Ulam chain with four particles each mass ratio determines a frequency ratio for the quadratic part of the Hamiltonian. Most prominent frequency ratios occur but not all. In general we find a one-dimensional variety of mass ratios for a given frequency ratio. For the resonance 1:2:3 a small cubic term added to the Hamiltonian leads to a dynamical behaviour that shows a difference between the case that two masses are equal and the more general case of four different masses. For two equal masses the normalized system is integrable and chaotic behaviour is small-scale. In the transition to four different masses we find a Hamiltonian-Hopf bifurcation of one of the normal modes leading to complex instability and Shilnikov-Devaney bifurcation. The other families of short-periodic solutions can be localized from the normal forms together with their stability characteristics. For illustration we use action simplices and the behaviour with time of the H_2 integral of the normal forms.

math.DS↗
arXiv2012

Harmonic lifts of modular forms

It is shown that each complex conjugate of a meromorphic modular form for $\mathrm{SL}_2(\mathbb{Z})$ of any complex weight $p$ occurs as the image of a harmonic modular form under the operator $2i y^p \, \partial_{\bar z}$. These harmonic lifts occur in holomorphic families with the weight as the parameter.

math.NT↗
arXiv2004

A New Approach to the Spectral Theory of the Fourth Moment of the Riemann Zeta-Function

The aim of the present work is to exhibit a new proof of the explicit spectral expansion for the fourth moment of the Riemann zeta-function that was established by the second named author a decade ago. Our proof is new, particularly in the sense that it dispenses completely with the Kloostermania, the spectral theory of sums of Kloosterman sums that was used in the former proof. The argument is now constructed precisely upon the spectral structure of the Lie group PSL(2,R). Main ingredients in our argument are the theory of automorphic representations as well as the harmonic analysis on the big Bruhat cell. In essence, this work of ours indicates a new way to view the Riemann zeta-function.

math.NT↗
arXiv1995

Automorphic hyperfunctions and period functions

We consider invariant hyperfunctions associated to automorphic forms on the upper half plane. We give two interpretations of the period function of Maass forms introduced by Lewis. The first interpretation shows that the period function arises from the explicit description of a representative of the hyperfunction associated to the Maass form. Under certain conditions, automorphic forms determine cohomology classes in a cohomology group with values in the hyperfunctions with bounded support on the line. A map from hyperfunctions to holomorphic functions leads to a second, cohomological, interpretation of the period function.

math.RT↗