## Mathematik

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#### Keywords

- Heat kernel (2)
- Krein space (2)
- Laplace operator on graphs (2)
- Quantum Zeno dynamics (2)
- W*-dynamical system (2)
- X-Y model (2)
- quantum spin systems (2)
- return to equilibrium (2)
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- Infinite Infrared Regularization in Krein Spaces (2003)
- Presentation at the AMS Southeastern Sectional Meeting 14-16 March 2003, and the Workshop Asymptotic Analysis, Stability, and Generalized Functions', 17-19 March 2003, Louisiana State University, Baton Rouge, Louisiana. See the corresponding papers "Mathematical Problems of Gauge Quantum Field Theory: A Survey of the Schwinger Model" and "Infinite Infrared Regularization and a State Space for the Heisenberg Algebra".

- Estimates for the Heat Kernel Estimates on weighted graphs (2001)
- Presentation at the University of Natal, Pietermaritzburg, South Africa, 25 May 2001. See the corresponding paper "A Note on Heat Kernel Estimates on Weighted Graphs with Two-Sided Bounds on the Weights".

- Zeno Dynamics of von Neumann Algebras (2002)
- The dynamical quantum Zeno effect is studied in the context of von Neumann algebras. It is shown that the Zeno dynamics coincides with the modular dynamics of a localized subalgebra. This relates the modular operator of that subalgebra to the modular operator of the original algebra by a variant of the Kato--Lie--Trotter product formula. PACS - Klassifikation: 03.65.Xp, 03.65.Db, 02.30.Tb . See the following papers: Schmidt, Andreas U.: "Zeno Dynamics in Quantum Statistical Mechanics" and "Mathematics of the Quantum Zeno Effect" :

- Mathematics of the quantum Zeno effect (2004)
- We present an overview of the mathematics underlying the quantum Zeno effect. Classical, functional analytic results are put into perspective and compared with more recent ones. This yields some new insights into mathematical preconditions entailing the Zeno paradox, in particular a simplified proof of Misra's and Sudarshan's theorem. We empahsise the complex-analytic structures associated to the issue of existence of the Zeno dynamics. On grounds of the assembled material, we reason about possible future mathematical developments pertaining to the Zeno paradox and its counterpart, the anti-Zeno paradox, both of which seem to be close to complete characterisations. PACS-Klassifikation: 03.65.Xp, 03.65Db, 05.30.-d, 02.30.T . See the corresponding presentation: Schmidt, Andreas U.: "Zeno Dynamics of von Neumann Algebras" and "Zeno Dynamics in Quantum Statistical Mechanics"

- Zeno dynamics in quantum statistical mechanics (2004)
- We study the quantum Zeno effect in quantum statistical mechanics within the operator algebraic framework. We formulate a condition for the appearance of the effect in W*-dynamical systems, in terms of the short-time behaviour of the dynamics. Examples of quantum spin systems show that this condition can be effectively applied to quantum statistical mechanical models. Furthermore, we derive an explicit form of the Zeno generator, and use it to construct Gibbs equilibrium states for the Zeno dynamics. As a concrete example, we consider the X-Y model, for which we show that a frequent measurement at a microscopic level, e.g. a single lattice site, can produce a macroscopic effect in changing the global equilibrium. PACS - Klassifikation: 03.65.Xp, 05.30.-d, 02.30. See the corresponding papers: Schmidt, Andreas U.: "Zeno Dynamics of von Neumann Algebras" and "Mathematics of the Quantum Zeno Effect" and the talk "Zeno Dynamics in Quantum Statistical Mechanics" - http://publikationen.ub.uni-frankfurt.de/volltexte/2005/1167/

- A note on heat kernel estimates on weighted graphs with two-sided bounds on the weights (2002)
- We reconsider estimates for the heat kernel on weighted graphs recently found by Metzger and Stollmann. In the case that the weights satisfy a positive lower bound as well as a finite upper bound, we obtain a specialized lower estimate and a proper generalization of a previous upper estimate. Reviews: Math. Rev. 1979406, Zbl. Math. 0934.46042

- On Phragmén-Lindelöf principles of type L^p (2003)
- Integrability conditions of type L^p along straight lines within a strip in mathbf{C} are derived for holomorphic functions which are integrable over the area of the strip.

- Euclidean reconstruction in quantum field theory: between tempered distributions and Fourier hyperfunctions (1997)
- In this short note on my talk I want to point out the mathematical difficulties that arise in the study of the relation of Wightman and Euclidean quantum field theory, i.e., the relation between the hierarchies of Wightman and Schwinger functions. The two extreme cases where the reconstructed Wightman functions are either tempered distributions - the well-known Osterwalder-Schrader reconstruction - or modified Fourier hyperfunctions are discussed in some detail. Finally, some perpectives towards a classification of Euclidean reconstruction theorems are outlined and preliminary steps in that direction are presented.

- Infinite infrared regularization and a state space for the Heisenberg algebra (2003)
- We present a method for the construction of a Krein space completion for spaces of test functions, equipped with an indefinite inner product induced by a kernel which is more singular than a distribution of finite order. This generalizes a regularization method for infrared singularities in quantum field theory, introduced by G. Morchio and F. Strocchi, to the case of singularites of infinite order. We give conditions for the possibility of this procedure in terms of local differential operators and the Gelfand-Shilov test function spaces, as well as an abstract sufficient condition. As a model case we construct a maximally positive definite state space for the Heisenberg algebra in the presence of an infinite infrared singularity. See the corresponding paper: Schmidt, Andreas U.: "Mathematical Problems of Gauge Quantum Field Theory: A Survey of the Schwinger Model" and the presentation "Infinite Infrared Regularization in Krein Spaces"

- Mathematical problems of gauge quantum field theory: A survey of the Schwinger model (2002)
- This extended write-up of a talk gives an introductory survey of mathematical problems of the quantization of gauge systems. Using the Schwinger model as an exactly tractable but nontrivial example which exhibits general features of gauge quantum field theory, I cover the following subjects: The axiomatics of quantum field theory, formulation of quantum field theory in terms of Wightman functions, reconstruction of the state space, the local formulation of gauge theories, indefiniteness of the Wightman functions in general and in the special case of the Schwinger model, the state space of the Schwinger model, special features of the model. New results are contained in the Mathematical Appendix, where I consider in an abstract setting the Pontrjagin space structure of a special class of indefinite inner product spaces - the so called quasi-positive ones. This is motivated by the indefinite inner product space structure appearing in the above context and generalizes results of Morchio and Strocchi [J. Math. Phys. 31 (1990) 1467], and Dubin and Tarski [J. Math. Phys. 7 (1966) 574]. See the corresponding paper: Schmidt, Andreas U.: "Infinite Infrared Regularization and a State Space for the Heisenberg Algebra" and the presentation "Infinite Infrared Regularization in Krein Spaces".