TY  - JOUR
A1  - Roos, Saskia
T1  - The Dirac operator under collapse to a smooth limit space
JF  - Annals of global analysis and geometry
N2  - Let (M-i, g(i))(i is an element of N) be a sequence of spin manifolds with uniform bounded curvature and diameter that converges to a lower-dimensional Riemannian manifold (B, h) in the Gromov-Hausdorff topology. Then, it happens that the spectrum of the Dirac operator converges to the spectrum of a certain first-order elliptic differential operator D-B on B. We give an explicit description of D-B and characterize the special case where D-B equals the Dirac operator on B.
KW  - Collapse
KW  - Dirac operator
KW  - Spin geometry
Y1  - 2019
U6  - https://doi.org/10.1007/s10455-019-09691-8
SN  - 0232-704X
SN  - 1572-9060
VL  - 57
IS  - 1
SP  - 121
EP  - 151
PB  - Springer
CY  - Dordrecht
ER  - 
TY  - JOUR
A1  - Leung, Tsz Yan
A1  - Leutbecher, Martin
A1  - Reich, Sebastian
A1  - Shepherd, Theodore G.
T1  - Atmospheric Predictability: Revisiting the Inherent Finite-Time Barrier
JF  - Journal of the atmospheric sciences
N2  - The accepted idea that there exists an inherent finite-time barrier in deterministically predicting atmospheric flows originates from Edward N. Lorenz’s 1969 work based on two-dimensional (2D) turbulence. Yet, known analytic results on the 2D Navier–Stokes (N-S) equations suggest that one can skillfully predict the 2D N-S system indefinitely far ahead should the initial-condition error become sufficiently small, thereby presenting a potential conflict with Lorenz’s theory. Aided by numerical simulations, the present work reexamines Lorenz’s model and reviews both sides of the argument, paying particular attention to the roles played by the slope of the kinetic energy spectrum. It is found that when this slope is shallower than −3, the Lipschitz continuity of analytic solutions (with respect to initial conditions) breaks down as the model resolution increases, unless the viscous range of the real system is resolved—which remains practically impossible. This breakdown leads to the inherent finite-time limit. If, on the other hand, the spectral slope is steeper than −3, then the breakdown does not occur. In this way, the apparent contradiction between the analytic results and Lorenz’s theory is reconciled.
KW  - Atmosphere
KW  - Turbulence
KW  - Error analysis
KW  - Spectral analysis
KW  - models
KW  - distribution
KW  - Numerical weather prediction
KW  - forecasting
Y1  - 2019
U6  - https://doi.org/10.1175/JAS-D-19-0057.1
SN  - 0022-4928
SN  - 1520-0469
VL  - 76
IS  - 12
SP  - 3883
EP  - 3892
PB  - American Meteorological Soc.
CY  - Boston
ER  - 
TY  - GEN
A1  - Beckus, Siegfried
A1  - Bellissard, Jean
A1  - De Nittis, Giuseppe
T1  - Corrigendum to:  Spectral continuity for aperiodic quantum systems I. General theory. - [Journal of functional analysis. - 275 (2018), 11, S. 2917 – 2977]
T2  - Journal of functional analysis
N2  - A correct statement of Theorem 4 in [1] is provided. The change does not affect the main results.
KW  - Haar system
Y1  - 2019
U6  - https://doi.org/10.1016/j.jfa.2019.06.001
SN  - 0022-1236
SN  - 1096-0783
VL  - 277
IS  - 9
SP  - 3351
EP  - 3353
PB  - Elsevier
CY  - San Diego
ER  - 
TY  - JOUR
A1  - Fernandes, Vitor H.
A1  - Koppitz, Jörg
A1  - Musunthia, Tiwadee
T1  - The Rank of the Semigroup of All Order-Preserving Transformations on a Finite Fence
JF  - Bulletin of the Malaysian Mathematical Sciences Society volume
N2  - A zig-zag (or fence) order is a special partial order on a (finite) set. In this paper, we consider the semigroup TFn of all order-preserving transformations on an n-element zig-zag-ordered set. We determine the rank of TFn and provide a minimal generating set for TFn. Moreover, a formula for the number of idempotents in TFn is given.
KW  - Transformation semigroups
KW  - Rank of semigroup
KW  - Idempotents
KW  - Order-preserving
KW  - Fence
KW  - Zig-zag order
Y1  - 2019
U6  - https://doi.org/10.1007/s40840-017-0598-1
SN  - 0126-6705
SN  - 2180-4206
VL  - 42
IS  - 5
SP  - 2191
EP  - 2211
PB  - Malaysian mathematical sciences sciences soc
CY  - Pulau Punang
ER  - 
TY  - JOUR
A1  - Shcherbakov, Robert
A1  - Zhuang, Jiancang
A1  - Zöller, Gert
A1  - Ogata, Yosihiko
T1  - Forecasting the magnitude of the largest expected earthquake
JF  - Nature Communications
N2  - The majority of earthquakes occur unexpectedly and can trigger subsequent sequences of events that can culminate in more powerful earthquakes. This self-exciting nature of seismicity generates complex clustering of earthquakes in space and time. Therefore, the problem of constraining the magnitude of the largest expected earthquake during a future time interval is of critical importance in mitigating earthquake hazard. We address this problem by developing a methodology to compute the probabilities for such extreme earthquakes to be above certain magnitudes. We combine the Bayesian methods with the extreme value theory and assume that the occurrence of earthquakes can be described by the Epidemic Type Aftershock Sequence process. We analyze in detail the application of this methodology to the 2016 Kumamoto, Japan, earthquake sequence. We are able to estimate retrospectively the probabilities of having large subsequent earthquakes during several stages of the evolution of this sequence.
Y1  - 2019
U6  - https://doi.org/10.1038/s41467-019-11958-4
SN  - 2041-1723
VL  - 10
PB  - Nature Publishing Group
CY  - London
ER  - 
TY  - JOUR
A1  - Conforti, Giovanni
A1  - Kosenkova, Tetiana
A1  - Roelly, Sylvie
T1  - Conditioned Point Processes with Application to Levy Bridges
JF  - Journal of theoretical probability
N2  - Our first result concerns a characterization by means of a functional equation of Poisson point processes conditioned by the value of their first moment. It leads to a generalized version of Mecke’s formula. En passant, it also allows us to gain quantitative results about stochastic domination for Poisson point processes under linear constraints. Since bridges of a pure jump Lévy process in Rd with a height a can be interpreted as a Poisson point process on space–time conditioned by pinning its first moment to a, our approach allows us to characterize bridges of Lévy processes by means of a functional equation. The latter result has two direct applications: First, we obtain a constructive and simple way to sample Lévy bridge dynamics; second, it allows us to estimate the number of jumps for such bridges. We finally show that our method remains valid for linearly perturbed Lévy processes like periodic Ornstein–Uhlenbeck processes driven by Lévy noise.
KW  - Ornstein-Uhlenbeck
Y1  - 2019
U6  - https://doi.org/10.1007/s10959-018-0863-8
SN  - 0894-9840
SN  - 1572-9230
VL  - 32
IS  - 4
SP  - 2111
EP  - 2134
PB  - Springer
CY  - New York
ER  - 
TY  - JOUR
A1  - Salamat, Mona
A1  - Zöller, Gert
A1  - Amini, Morteza
T1  - Prediction of the Maximum Expected Earthquake Magnitude in Iran:
BT  - from a Catalog with Varying Magnitude of Completeness and Uncertain Magnitudes
JF  - Pure and applied geophysics
N2  - This paper concerns the problem of predicting the maximum expected earthquake magnitude μ in a future time interval Tf given a catalog covering a time period T in the past. Different studies show the divergence of the confidence interval of the maximum possible earthquake magnitude m_{ max } for high levels of confidence (Salamat et al. 2017). Therefore, m_{ max } should be better replaced by μ (Holschneider et al. 2011). In a previous study (Salamat et al. 2018), μ is estimated for an instrumental earthquake catalog of Iran from 1900 onwards with a constant level of completeness ( {m0 = 5.5} ). In the current study, the Bayesian methodology developed by Zöller et al. (2014, 2015) is applied for the purpose of predicting μ based on the catalog consisting of both historical and instrumental parts. The catalog is first subdivided into six subcatalogs corresponding to six seismotectonic zones, and each of those zone catalogs is subsequently subdivided according to changes in completeness level and magnitude uncertainty. For this, broad and small error distributions are considered for historical and instrumental earthquakes, respectively. We assume that earthquakes follow a Poisson process in time and Gutenberg-Richter law in the magnitude domain with a priori unknown a and b values which are first estimated by Bayes' theorem and subsequently used to estimate μ. Imposing different values of m_{ max } for different seismotectonic zones namely Alborz, Azerbaijan, Central Iran, Zagros, Kopet Dagh and Makran, the results show considerable probabilities for the occurrence of earthquakes with Mw ≥ 7.5 in short Tf , whereas for long Tf, μ is almost equal to m_{ max }
KW  - Maximum expected earthquake magnitude
KW  - completeness levels
KW  - magnitude errors
KW  - Bayesian method
KW  - Iran
Y1  - 2019
U6  - https://doi.org/10.1007/s00024-019-02141-3
SN  - 0033-4553
SN  - 1420-9136
VL  - 176
IS  - 8
SP  - 3425
EP  - 3438
PB  - Springer
CY  - Basel
ER  - 
TY  - JOUR
A1  - Bär, Christian
A1  - Strohmaier, Alexander
T1  - An index theorem for Lorentzian manifolds with compact
spacelike Cauchy boundary
JF  - American Journal of Mathematics
N2  - We show that the Dirac operator on a compact globally hyperbolic Lorentzian spacetime with spacelike Cauchy boundary is a Fredholm operator if appropriate boundary conditions are imposed. We prove that the index of this operator is given by the same expression as in the index formula of Atiyah-Patodi-Singer for Riemannian manifolds with boundary. The index is also shown to equal that of a certain operator constructed from the evolution operator and a spectral projection on the boundary. In case the metric is of product type near the boundary a Feynman parametrix is constructed.
Y1  - 2019
U6  - https://doi.org/10.1353/ajm.2019.0037
SN  - 0002-9327
SN  - 1080-6377
VL  - 141
IS  - 5
SP  - 1421
EP  - 1455
PB  - Johns Hopkins Univ. Press
CY  - Baltimore
ER  - 
TY  - JOUR
A1  - Denecke, Klaus-Dieter
T1  - The partial clone of linear formulas
JF  - Siberian mathematical journal
N2  - A term t is linear if no variable occurs more than once in t. An identity s ≈ t is said to be linear if s and t are linear terms. Identities are particular formulas. As for terms superposition operations can be defined for formulas too. We define the arbitrary linear formulas and seek for a condition for the set of all linear formulas to be closed under superposition. This will be used to define the partial superposition operations on the set of linear formulas and a partial many-sorted algebra Formclonelin(τ, τ′). This algebra has similar properties with the partial many-sorted clone of all linear terms. We extend the concept of a hypersubstitution of type τ to the linear hypersubstitutions of type (τ, τ′) for algebraic systems. The extensions of linear hypersubstitutions of type (τ, τ′) send linear formulas to linear formulas, presenting weak endomorphisms of Formclonelin(τ, τ′).
KW  - term
KW  - formula
KW  - superposition
KW  - linear term
KW  - linear formula
KW  - clone
KW  - partial clone
KW  - linear hypersubstitution
Y1  - 2019
U6  - https://doi.org/10.1134/S0037446619040037
SN  - 0037-4466
SN  - 1573-9260
VL  - 60
IS  - 4
SP  - 572
EP  - 584
PB  - Pleiades Publ.
CY  - New York
ER  - 
TY  - JOUR
A1  - Lekkoksung, Nareupanat
A1  - Denecke, Klaus-Dieter
T1  - The partial clone of linear tree languages
JF  - Siberian mathematical journal
N2  - A term, also called a tree, is said to be linear, if each variable occurs in the term only once. The linear terms and sets of linear terms, the so-called linear tree languages, play some role in automata theory and in the theory of formal languages in connection with recognizability. We define a partial superposition operation on sets of linear trees of a given type and study the properties of some many-sorted partial clones that have sets of linear trees as elements and partial superposition operations as fundamental operations. The endomorphisms of those algebras correspond to nondeterministic linear hypersubstitutions.
KW  - linear term
KW  - linear tree language
KW  - clone
KW  - partial clone
KW  - linear hypersubstitution
KW  - nondeterministic linear hypersubstitution
Y1  - 2019
U6  - https://doi.org/10.1134/S0037446619030121
SN  - 0037-4466
SN  - 1573-9260
VL  - 60
IS  - 3
SP  - 497
EP  - 507
PB  - Pleiades Publ.
CY  - New York
ER  - 
TY  - THES
A1  - Jakobs, Friedrich
T1  - Dubrovin–rings and their connection to Hughes–free skew fields of fractions
T1  - Dubrovinringe und ihre Verbindung zu Hughes-freien Quotientenschiefkörpern
N2  - One method of embedding groups into skew fields was introduced by A. I. Mal'tsev and B. H. Neumann (cf. [18, 19]). If G is an ordered group and F is a skew field, the set F((G)) of formal power series over F in G with well-ordered support forms a skew field into which the group ring F[G] can be embedded. Unfortunately it is not suficient that G is left-ordered since F((G)) is only an F-vector space in this case as there is no natural way to define a multiplication on F((G)). One way to extend the original idea onto left-ordered groups is to examine the endomorphism ring of F((G)) as explored by N. I. Dubrovin (cf. [5, 6]). It is possible to embed any crossed product ring F[G; η, σ] into the endomorphism ring of F((G)) such that each non-zero element of F[G; η, σ] defines an automorphism of F((G)) (cf. [5, 10]). Thus, the rational closure of F[G; η, σ] in the endomorphism ring of F((G)), which we will call the Dubrovin-ring of F[G; η, σ], is a potential candidate for a skew field of fractions of F[G; η, σ]. The methods of N. I. Dubrovin allowed to show that specific classes of groups can be embedded into a skew field. For example, N. I. Dubrovin contrived some special criteria, which are applicable on the universal covering group of SL(2, R). These methods have also been explored by J. Gräter and R. P. Sperner (cf. [10]) as well as N.H. Halimi and T. Ito (cf. [11]). Furthermore, it is of interest to know if skew fields of fractions are unique. For example, left and right Ore domains have unique skew fields of fractions (cf. [2]). This is not the general case as for example the free group with 2 generators can be embedded into non-isomorphic skew fields of fractions (cf. [12]). It seems likely that Ore domains are the most general case for which unique skew fields of fractions exist. One approach to gain uniqueness is to restrict the search to skew fields of fractions with additional properties. I. Hughes has defined skew fields of fractions of crossed product rings F[G; η, σ] with locally indicable G which fulfill a special condition. These are called Hughes-free skew fields of fractions and I. Hughes has proven that they are unique if they exist [13, 14]. This thesis will connect the ideas of N. I. Dubrovin and I. Hughes. The first chapter contains the basic terminology and concepts used in this thesis. We present methods provided by N. I. Dubrovin such as the complexity of elements in rational closures and special properties of endomorphisms of the vector space of formal power series F((G)). To combine the ideas of N.I. Dubrovin and I. Hughes we introduce Conradian left-ordered groups of maximal rank and examine their connection to locally indicable groups. Furthermore we provide notations for crossed product rings, skew fields of fractions as well as Dubrovin-rings and prove some technical statements which are used in later parts. The second chapter focuses on Hughes-free skew fields of fractions and their connection to Dubrovin-rings. For that purpose we introduce series representations to interpret elements of Hughes-free skew fields of fractions as skew formal Laurent series. This 1 Introduction allows us to prove that for Conradian left-ordered groups G of maximal rank the statement "F[G; η, σ] has a Hughes-free skew field of fractions" implies "The Dubrovin ring of F [G; η, σ] is a skew field". We will also prove the reverse and apply the results to give a new prove of Theorem 1 in [13]. Furthermore we will show how to extend injective ring homomorphisms of some crossed product rings onto their Hughes-free skew fields of fractions. At last we will be able to answer the open question whether Hughes--free skew fields are strongly Hughes-free (cf. [17, page 53]).
N2  - In dieser Arbeit beschäftigen wir uns mit Quotientenschiefkörpern von verschränkten Produkten F [G; η, σ], wobei G eine Gruppe und F ein Schiefkörper ist. Eine Methode Gruppen in Schiefkörper einzubetten stammt von A. I. Mal’tsev und B. H. Neumann. Ist G eine beidseitig geordnete Gruppe, so lässt sich die Menge der formalen Potenzreihen F ((G)) über F in G mit wohlgeordnetem Träger als Schiefkörper interpretieren. In diesen lässt sich jedes verschränkte Produkt F [G; η, σ] einbetten. Möchte man die Klasse der einzubettenden Gruppen erweitern, so bieten sich links–geordnete Gruppen an. In diesem Fall hat F ((G)) keine natürliche Ringstruktur, aber man kann nutzen, dass F ((G)) ein rechter F–Vektorraum ist und seinen Endomorphismenring untersuchen. Jedes Verschränkte Produkt F [G; η, σ] lässt sich derart in den Endomorphismenring einbetten, dass die zugehörigen von Null verschiedenen Endomorphismen Automorphismen sind. Der rationale Abschluss von F [G; η, σ] in End(F ((G))), den wir Dubrovinring von F [G; η, σ] nennen, ist somit ein potentieller Quotientenschiefkörper von F [G; η, σ]. Neben der Existenz von Quotientenschiefkörpern ist deren Eindeutigkeit (bis auf Isomorphie) von Interesse. Im Gegensatz zum kommutativen Fall sind Quotientenschiefkörper im Allgemeinen nicht eindeutig. So lässt sich beispielsweise die freie Gruppe mit zwei Erzeugenden in nicht–isomorphe Quotientenschiefkörper einbetten. Eine große Klasse an Ringen, die eindeutige Quotientenschiefkörper besitzen, sind Ore–Bereiche. Vermutlich lässt sich diese Klasse nicht erweitern, ohne zusätzliche Eigenschaften der Quotientenschiefkörper zu verlangen. Eine solche Eigenschaft, im Folgenden Hughes–frei genannt, wurde von I. Hughes vorgeschlagen. Er konnte beweisen, dass Hughes–freie Quotientenschiefkörper eindeutig sind, wenn sie existieren. In dieser Arbeit verbinden wir die Ideen von I. Hughes und N. I. Dubrovin. Wir zeigen, dass die Elemente von Hughes–freien Quotientenschiefkörpern als formale schiefe Laurent–Reihen dargestellt werden können und dass diese Darstellungen in gewisser Weise eindeutig sind. Dieses Ergebnis nutzen wir um zu beweisen, dass die Aussagen “F [G; η, σ] besitzt einen Hughes–freien Quotientenschiefkörper” und “Der Dubrovinring von F [G; η, σ] ist ein Schiefkörper” äquivalent sind, wenn G eine links–geordnete Gruppe von Conrad–Typ mit maximalem Rang ist. Wir stellen den nötigen Begriffsapparat zur Verfügung. Dieser basiert vorwiegend auf den Arbeiten von N. I. Dubrovin und umfasst spezielle Eigenschaften der Endomorphismen von F ((G)) sowie die Komplexität von Elementen in rationalen Abschlüssen. Des Weiteren gehen wir auf links–geordnete Gruppen von Conrad–Typ ein und untersuchen ihren Zusammenhang mit lokal indizierbaren Gruppen, die eine grundlegende Rolle für Hughes–freie Quotientenschiefkörper spielen. Wir werden zeigen können, dass Dubrovinringe, die Schiefkörper sind, stark Hughes–freie Quotientenschiefkörper sind, was die offene Frage beantwortet, ob Hughes–freie Quotientenschiefkörper stark Hughes–frei sind. Außerdem werden wir einen alternativen Beweis der Eindeutigkeit von Hughes–freien Quotientenschiefkörpern präsentieren und die Fortsetzbarkeit von Automorphismen eines verschränkten Produkts auf Hughes–freie Quotientenschiefkörper untersuchen.
KW  - Hughes-free
KW  - Dubrovinring
KW  - left ordered groups
KW  - Conradian ordered groups
KW  - skew field of fraction
KW  - locally indicable
KW  - series representation
KW  - strongly Hughes-free
KW  - Hughes-frei
KW  - Dubrovinring
KW  - linksgeordnete Gruppen
KW  - geordnete Gruppen von Conrad-Typ
KW  - Quotientenschiefkörper
KW  - lokal indizierbar
KW  - Reihendarstellungen
KW  - stark Hughes-frei
Y1  - 2019
U6  - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-435561
ER  - 
TY  - JOUR
A1  - Clavier, Pierre J.
A1  - Guo, Li
A1  - Paycha, Sylvie
A1  - Zhang, Bin
T1  - An algebraic formulation of the locality principle in renormalisation
JF  - European Journal of Mathematics
N2  - We study the mathematical structure underlying the concept of locality which lies at the heart of classical and quantum field theory, and develop a machinery used to preserve locality during the renormalisation procedure. Viewing renormalisation in the framework of Connes and Kreimer as the algebraic Birkhoff factorisation of characters on a Hopf algebra with values in a Rota-Baxter algebra, we build locality variants of these algebraic structures, leading to a locality variant of the algebraic Birkhoff factorisation. This provides an algebraic formulation of the conservation of locality while renormalising. As an application in the context of the Euler-Maclaurin formula on lattice cones, we renormalise the exponential generating function which sums over the lattice points in a lattice cone. As a consequence, for a suitable multivariate regularisation, renormalisation from the algebraic Birkhoff factorisation amounts to composition by a projection onto holomorphic multivariate germs.
KW  - Locality
KW  - Renormalisation
KW  - Algebraic Birkhoff factorisation
KW  - Partial algebra
KW  - Hopf algebra
KW  - Rota-Baxter algebra
KW  - Multivariate meromorphic functions
KW  - Lattice cones
Y1  - 2019
U6  - https://doi.org/10.1007/s40879-018-0255-8
SN  - 2199-675X
SN  - 2199-6768
VL  - 5
IS  - 2
SP  - 356
EP  - 394
PB  - Springer
CY  - Cham
ER  - 
TY  - JOUR
A1  - Koltai, Peter
A1  - Lie, Han Cheng
A1  - Plonka, Martin
T1  - Frechet differentiable drift dependence of Perron-Frobenius and Koopman operators for non-deterministic dynamics
JF  - Nonlinearity
N2  - We prove the Fréchet differentiability with respect to the drift of Perron–Frobenius and Koopman operators associated to time-inhomogeneous ordinary stochastic differential equations. This result relies on a similar differentiability result for pathwise expectations of path functionals of the solution of the stochastic differential equation, which we establish using Girsanov's formula. We demonstrate the significance of our result in the context of dynamical systems and operator theory, by proving continuously differentiable drift dependence of the simple eigen- and singular values and the corresponding eigen- and singular functions of the stochastic Perron–Frobenius and Koopman operators.
KW  - stochastic differential equations
KW  - transfer operator
KW  - Koopman operator
KW  - Perron-Frobenius operator
KW  - smooth drift dependence
KW  - linear response
KW  - pathwise expectations
Y1  - 2019
U6  - https://doi.org/10.1088/1361-6544/ab1f2a
SN  - 0951-7715
SN  - 1361-6544
VL  - 32
IS  - 11
SP  - 4232
EP  - 4257
PB  - IOP Publ. Ltd.
CY  - Bristol
ER  - 
TY  - JOUR
A1  - Gerlach, Moritz Reinhardt
A1  - Glück, Jochen
T1  - Convergence of positive operator semigroups
JF  - Transactions of the American Mathematical Society
N2  - We present new conditions for semigroups of positive operators to converge strongly as time tends to infinity. Our proofs are based on a novel approach combining the well-known splitting theorem by Jacobs, de Leeuw, and Glicksberg with a purely algebraic result about positive group representations. Thus, we obtain convergence theorems not only for one-parameter semigroups but also for a much larger class of semigroup representations. Our results allow for a unified treatment of various theorems from the literature that, under technical assumptions, a bounded positive C-0-semigroup containing or dominating a kernel operator converges strongly as t ->infinity. We gain new insights into the structure theoretical background of those theorems and generalize them in several respects; especially we drop any kind of continuity or regularity assumption with respect to the time parameter.
KW  - Positive semigroups
KW  - semigroup representations
KW  - asymptotic behavior
KW  - kernel operator
Y1  - 2019
U6  - https://doi.org/10.1090/tran/7836
SN  - 0002-9947
SN  - 1088-6850
VL  - 372
IS  - 9
SP  - 6603
EP  - 6627
PB  - American Mathematical Soc.
CY  - Providence
ER  - 
TY  - JOUR
A1  - Sanchez, S.
A1  - Wicht, J.
A1  - Baerenzung, Julien
A1  - Holschneider, Matthias
T1  - Sequential assimilation of geomagnetic observations
BT  - perspectives for the reconstruction and prediction of core dynamics
JF  - Geophysical journal international
N2  - High-precision observations of the present-day geomagnetic field by ground-based observatories and satellites provide unprecedented conditions for unveiling the dynamics of the Earth’s core. Combining geomagnetic observations with dynamo simulations in a data assimilation (DA) framework allows the reconstruction of past and present states of the internal core dynamics. The essential information that couples the internal state to the observations is provided by the statistical correlations from a numerical dynamo model in the form of a model covariance matrix. Here we test a sequential DA framework, working through a succession of forecast and analysis steps, that extracts the correlations from an ensemble of dynamo models. The primary correlations couple variables of the same azimuthal wave number, reflecting the predominant axial symmetry of the magnetic field. Synthetic tests show that the scheme becomes unstable when confronted with high-precision geomagnetic observations. Our study has identified spurious secondary correlations as the origin of the problem. Keeping only the primary correlations by localizing the covariance matrix with respect to the azimuthal wave number suffices to stabilize the assimilation. While the first analysis step is fundamental in constraining the large-scale interior state, further assimilation steps refine the smaller and more dynamical scales. This refinement turns out to be critical for long-term geomagnetic predictions. Increasing the assimilation steps from one to 18 roughly doubles the prediction horizon for the dipole from about  tree to six centuries, and from 30 to about  60 yr for smaller observable scales. This improvement is also reflected on the predictability of surface intensity features such as the South Atlantic Anomaly. Intensity prediction errors are decreased roughly by a half when assimilating long observation sequences.
KW  - Magnetic field variations through time
KW  - Core dynamics
KW  - Dynamo: theories and simulations
KW  - Inverse theory
KW  - Probabilistic forecasting
Y1  - 2019
U6  - https://doi.org/10.1093/gji/ggz090
SN  - 0956-540X
SN  - 1365-246X
VL  - 217
IS  - 2
SP  - 1434
EP  - 1450
PB  - Oxford Univ. Press
CY  - Oxford
ER  - 
TY  - JOUR
A1  - van Leeuwen, Peter Jan
A1  - Kunsch, Hans R.
A1  - Nerger, Lars
A1  - Potthast, Roland
A1  - Reich, Sebastian
T1  - Particle filters for high-dimensional geoscience applications: A review
JF  - Quarterly journal of the Royal Meteorological Society
N2  - Particle filters contain the promise of fully nonlinear data assimilation. They have been applied in numerous science areas, including the geosciences, but their application to high-dimensional geoscience systems has been limited due to their inefficiency in high-dimensional systems in standard settings. However, huge progress has been made, and this limitation is disappearing fast due to recent developments in proposal densities, the use of ideas from (optimal) transportation, the use of localization and intelligent adaptive resampling strategies. Furthermore, powerful hybrids between particle filters and ensemble Kalman filters and variational methods have been developed. We present a state-of-the-art discussion of present efforts of developing particle filters for high-dimensional nonlinear geoscience state-estimation problems, with an emphasis on atmospheric and oceanic applications, including many new ideas, derivations and unifications, highlighting hidden connections, including pseudo-code, and generating a valuable tool and guide for the community. Initial experiments show that particle filters can be competitive with present-day methods for numerical weather prediction, suggesting that they will become mainstream soon.
KW  - hybrids
KW  - localization
KW  - nonlinear data assimilation
KW  - particle filters
KW  - proposal densities
Y1  - 2019
U6  - https://doi.org/10.1002/qj.3551
SN  - 0035-9009
SN  - 1477-870X
VL  - 145
IS  - 723
SP  - 2335
EP  - 2365
PB  - Wiley
CY  - Hoboken
ER  - 
TY  - JOUR
A1  - Bandara, Menaka Lashitha
A1  - Rosen, Andreas
T1  - Riesz continuity of the Atiyah-Singer Dirac operator under perturbations of local boundary conditions
JF  - Communications in partial differential equations
N2  - On a smooth complete Riemannian spin manifold with smooth compact boundary, we demonstrate that Atiyah-Singer Dirac operator in depends Riesz continuously on perturbations of local boundary conditions The Lipschitz bound for the map depends on Lipschitz smoothness and ellipticity of and bounds on Ricci curvature and its first derivatives as well as a lower bound on injectivity radius away from a compact neighbourhood of the boundary. More generally, we prove perturbation estimates for functional calculi of elliptic operators on manifolds with local boundary conditions.
KW  - Boundary value problems
KW  - Dirac operator
KW  - functional calculus
KW  - real-variable harmonic analysis
KW  - Riesz continuity
KW  - spectral flow
Y1  - 2019
U6  - https://doi.org/10.1080/03605302.2019.1611847
SN  - 0360-5302
SN  - 1532-4133
VL  - 44
IS  - 12
SP  - 1253
EP  - 1284
PB  - Taylor & Francis Group
CY  - Philadelphia
ER  - 
TY  - JOUR
A1  - Ringel, Lisa Maria
A1  - Somogyvári, Márk
A1  - Jalali, Mohammadreza
A1  - Bayer, Peter
T1  - Comparison of hydraulic and tracer tomography for discrete fracture network inversion
JF  - Geosciences
N2  - Fractures serve as highly conductive preferential flow paths for fluids in rocks, which are difficult to exactly reconstruct in numerical models. Especially, in low-conductive rocks, fractures are often the only pathways for advection of solutes and heat. The presented study compares the results from hydraulic and tracer tomography applied to invert a theoretical discrete fracture network (DFN) that is based on data from synthetic cross-well testing. For hydraulic tomography, pressure pulses in various injection intervals are induced and the pressure responses in the monitoring intervals of a nearby observation well are recorded. For tracer tomography, a conservative tracer is injected in different well levels and the depth-dependent breakthrough of the tracer is monitored. A recently introduced transdimensional Bayesian inversion procedure is applied for both tomographical methods, which adjusts the fracture positions, orientations, and numbers based on given geometrical fracture statistics. The used Metropolis-Hastings-Green algorithm is refined by the simultaneous estimation of the measurement error’s variance, that is, the measurement noise. Based on the presented application to invert the two-dimensional cross-section between source and the receiver well, the hydraulic tomography reveals itself to be more suitable for reconstructing the original DFN. This is based on a probabilistic representation of the inverted results by means of fracture probabilities.
KW  - hydraulic tomography
KW  - tracer tomography
KW  - DFN
KW  - Bayesian inversion
KW  - heterogeneity
KW  - fracture
KW  - hydrogeophysics
Y1  - 2019
U6  - https://doi.org/10.3390/geosciences9060274
SN  - 2076-3263
VL  - 9
IS  - 6
PB  - MDPI
CY  - Basel
ER  - 
TY  - JOUR
A1  - Beckus, Siegfried
A1  - Bellissard, Jean
A1  - Cornean, Horia
T1  - Holder Continuity of the Spectra for Aperiodic Hamiltonians
JF  - Annales de l'Institut Henri Poincaré
N2  - We study the spectral location of a strongly pattern equivariant Hamiltonians arising through configurations on a colored lattice. Roughly speaking, two configurations are "close to each other" if, up to a translation, they "almost coincide" on a large fixed ball. The larger this ball, the more similar they are, and this induces a metric on the space of the corresponding dynamical systems. Our main result states that the map which sends a given configuration into the spectrum of its associated Hamiltonian, is Holder (even Lipschitz) continuous in the usual Hausdorff metric. Specifically, the spectral distance of two Hamiltonians is estimated by the distance of the corresponding dynamical systems.
Y1  - 2019
U6  - https://doi.org/10.1007/s00023-019-00848-6
SN  - 1424-0637
SN  - 1424-0661
VL  - 20
IS  - 11
SP  - 3603
EP  - 3631
PB  - Springer
CY  - Cham
ER  - 
TY  - JOUR
A1  - Blanchard, Gilles
A1  - Zadorozhnyi, Oleksandr
T1  - Concentration of weakly dependent Banach-valued sums and applications to statistical learning methods
JF  - Bernoulli : official journal of the Bernoulli Society for Mathematical Statistics and Probability
N2  - We obtain a Bernstein-type inequality for sums of Banach-valued random variables satisfying a weak dependence assumption of general type and under certain smoothness assumptions of the underlying Banach norm. We use this inequality in order to investigate in the asymptotical regime the error upper bounds for the broad family of spectral regularization methods for reproducing kernel decision rules, when trained on a sample coming from a tau-mixing process.
KW  - Banach-valued process
KW  - Bernstein inequality
KW  - concentration
KW  - spectral regularization
KW  - weak dependence
Y1  - 2019
U6  - https://doi.org/10.3150/18-BEJ1095
SN  - 1350-7265
SN  - 1573-9759
VL  - 25
IS  - 4B
SP  - 3421
EP  - 3458
PB  - International Statistical Institute
CY  - Voorburg
ER  -