TY - JOUR A1 - Acevedo, Walter A1 - De Wiljes, Jana A1 - Reich, Sebastian T1 - Second-order accurate ensemble transform particle filters JF - SIAM journal on scientific computing N2 - Particle filters (also called sequential Monte Carlo methods) are widely used for state and parameter estimation problems in the context of nonlinear evolution equations. The recently proposed ensemble transform particle filter (ETPF) [S. Reich, SIAM T. Sci. Comput., 35, (2013), pp. A2013-A2014[ replaces the resampling step of a standard particle filter by a linear transformation which allows for a hybridization of particle filters with ensemble Kalman filters and renders the resulting hybrid filters applicable to spatially extended systems. However, the linear transformation step is computationally expensive and leads to an underestimation of the ensemble spread for small and moderate ensemble sizes. Here we address both of these shortcomings by developing second order accurate extensions of the ETPF. These extensions allow one in particular to replace the exact solution of a linear transport problem by its Sinkhorn approximation. It is also demonstrated that the nonlinear ensemble transform filter arises as a special case of our general framework. We illustrate the performance of the second-order accurate filters for the chaotic Lorenz-63 and Lorenz-96 models and a dynamic scene-viewing model. The numerical results for the Lorenz-63 and Lorenz-96 models demonstrate that significant accuracy improvements can be achieved in comparison to a standard ensemble Kalman filter and the ETPF for small to moderate ensemble sizes. The numerical results for the scene-viewing model reveal, on the other hand, that second-order corrections can lead to statistically inconsistent samples from the posterior parameter distribution. KW - Bayesian inference KW - data assimilation KW - particle filter KW - ensemble Kalman filter KW - Sinkhorn approximation Y1 - 2017 U6 - https://doi.org/10.1137/16M1095184 SN - 1064-8275 SN - 1095-7197 SN - 2168-3417 VL - 39 IS - 5 SP - A1834 EP - A1850 PB - Society for Industrial and Applied Mathematics CY - Philadelphia ER - TY - JOUR A1 - Acevedo, Walter A1 - Fallah, Bijan A1 - Reich, Sebastian A1 - Cubasch, Ulrich T1 - Assimilation of pseudo-tree-ring-width observations into an atmospheric general circulation model JF - Climate of the past : an interactive open access journal of the European Geosciences Union N2 - Paleoclimate data assimilation (DA) is a promising technique to systematically combine the information from climate model simulations and proxy records. Here, we investigate the assimilation of tree-ring-width (TRW) chronologies into an atmospheric global climate model using ensemble Kalman filter (EnKF) techniques and a process-based tree-growth forward model as an observation operator. Our results, within a perfect-model experiment setting, indicate that the "online DA" approach did not outperform the "off-line" one, despite its considerable additional implementation complexity. On the other hand, it was observed that the nonlinear response of tree growth to surface temperature and soil moisture does deteriorate the operation of the time-averaged EnKF methodology. Moreover, for the first time we show that this skill loss appears significantly sensitive to the structure of the growth rate function, used to represent the principle of limiting factors (PLF) within the forward model. In general, our experiments showed that the error reduction achieved by assimilating pseudo-TRW chronologies is modulated by the magnitude of the yearly internal variability in themodel. This result might help the dendrochronology community to optimize their sampling efforts. Y1 - 2017 U6 - https://doi.org/10.5194/cp-13-545-2017 SN - 1814-9324 SN - 1814-9332 VL - 13 SP - 545 EP - 557 PB - Copernicus CY - Göttingen ER - TY - JOUR A1 - Bandara, Lashi A1 - McIntosh, Alan A1 - Rosen, Andreas T1 - Riesz continuity of the Atiyah BT - singer dirac operator under perturbations of the metric JF - Mathematische Annalen N2 - We prove that the Atiyah–Singer Dirac operator in L2 depends Riesz continuously on L∞ perturbations of complete metrics g on a smooth manifold. The Lipschitz bound for the map depends on bounds on Ricci curvature and its first derivatives as well as a lower bound on injectivity radius. Our proof uses harmonic analysis techniques related to Calderón’s first commutator and the Kato square root problem. We also show perturbation results for more general functions of general Dirac-type operators on vector bundles. Y1 - 2017 U6 - https://doi.org/10.1007/s00208-017-1610-7 SN - 0025-5831 SN - 1432-1807 VL - 370 IS - 1-2 SP - 863 EP - 915 PB - Springer CY - Heidelberg ER - TY - JOUR A1 - Benini, Marco A1 - Capoferri, Matteo A1 - Dappiaggi, Claudio T1 - Hadamard States for Quantum Abelian Duality JF - Annales de l'Institut Henri Poincaré N2 - Abelian duality is realized naturally by combining differential cohomology and locally covariant quantum field theory. This leads to a -algebra of observables, which encompasses the simultaneous discretization of both magnetic and electric fluxes. We discuss the assignment of physically well-behaved states on this algebra and the properties of the associated GNS triple. We show that the algebra of observables factorizes as a suitable tensor product of three -algebras: the first factor encodes dynamical information, while the other two capture topological data corresponding to electric and magnetic fluxes. On the former factor and in the case of ultra-static globally hyperbolic spacetimes with compact Cauchy surfaces, we exhibit a state whose two-point correlation function has the same singular structure of a Hadamard state. Specifying suitable counterparts also on the topological factors, we obtain a state for the full theory, ultimately implementing Abelian duality transformations as Hilbert space isomorphisms. Y1 - 2017 U6 - https://doi.org/10.1007/s00023-017-0593-y SN - 1424-0637 SN - 1424-0661 VL - 18 SP - 3325 EP - 3370 PB - Springer CY - Basel ER - TY - JOUR A1 - Benini, Marco A1 - Schenkel, Alexander T1 - Quantum Field Theories on Categories Fibered in Groupoids JF - Communications in mathematical physics N2 - We introduce an abstract concept of quantum field theory on categories fibered in groupoids over the category of spacetimes. This provides us with a general and flexible framework to study quantum field theories defined on spacetimes with extra geometric structures such as bundles, connections and spin structures. Using right Kan extensions, we can assign to any such theory an ordinary quantum field theory defined on the category of spacetimes and we shall clarify under which conditions it satisfies the axioms of locally covariant quantum field theory. The same constructions can be performed in a homotopy theoretic framework by using homotopy right Kan extensions, which allows us to obtain first toy-models of homotopical quantum field theories resembling some aspects of gauge theories. Y1 - 2017 U6 - https://doi.org/10.1007/s00220-017-2986-7 SN - 0010-3616 SN - 1432-0916 VL - 356 SP - 19 EP - 64 PB - Springer CY - New York ER - TY - JOUR A1 - Benini, Marco A1 - Schenkel, Alexander T1 - Poisson Algebras for Non-Linear Field Theories in the Cahiers Topos JF - Annales de l'Institut Henri Poincaré Y1 - 2017 U6 - https://doi.org/10.1007/s00023-016-0533-2 SN - 1424-0637 SN - 1424-0661 VL - 18 SP - 1435 EP - 1464 PB - Springer CY - Basel ER - TY - JOUR A1 - Brungs, Hans H. A1 - Gräter, Joachim T1 - On central extensions of SL(2, F) admitting left-orderings JF - Journal of Algebra N2 - For an arbitrary euclidean field F we introduce a central extension (G(F), Phi) of SL(2, F) admitting a left-ordering and study its algebraic properties. The elements of G(F) are order preserving bijections of the convex hull of Q in F. If F = R then G(F) is isomorphic to the classical universal covering group of the Lie group SL(2, R). Among other results we show that G(F) is a perfect group which possesses a rank 1 cone of exceptional type. We also prove that its centre is an infinite cyclic group and investigate its normal subgroups. KW - Universal covering group KW - Central extensions of groups KW - Perfect groups KW - Ordered fields KW - Left-ordered groups KW - Order-preserving bijections KW - Euclidean fields Y1 - 2017 U6 - https://doi.org/10.1016/j.jalgebra.2017.05.025 SN - 0021-8693 SN - 1090-266X VL - 486 SP - 288 EP - 327 PB - Elsevier CY - San Diego ER - TY - JOUR A1 - Chang, D. -C. A1 - Schulze, Bert-Wolfgang T1 - Calculus on spaces with higher singularities JF - Journal of pseudo-differential operators and applications N2 - We establish extensions of the standard pseudo-differential calculus to specific classes of operators with operator-valued symbols occurring in symbolic hierarchies motivated by manifolds with higher singularities or stratified spaces. KW - Pseudo-differential operators KW - Operator-valued symbols KW - Fourier and Mellin transform Y1 - 2017 U6 - https://doi.org/10.1007/s11868-016-0180-x SN - 1662-9981 SN - 1662-999X VL - 8 SP - 585 EP - 622 PB - Springer CY - Basel ER - TY - JOUR A1 - Chang, Der-Chen A1 - Hedayat Mahmoudi, Mahdi A1 - Schulze, Bert-Wolfgang T1 - Singular degenerate operators JF - Applicable analysis : an international journal N2 - We outline some simplified and more general method for constructing parametrices on higher singular spaces. We also outline basic ideas on operators on manifolds with conical or edge singularities. KW - Operators on singular cones KW - Mellin symbols with values in the edge calculus KW - parametrices of elliptic operators Y1 - 2017 U6 - https://doi.org/10.1080/00036811.2017.1336546 SN - 0003-6811 SN - 1563-504X VL - 96 IS - 14 SP - 2434 EP - 2456 PB - Routledge, Taylor & Francis Group CY - Abingdon ER - TY - JOUR A1 - Chang, Der-Chen A1 - Schulze, Bert-Wolfgang T1 - Ellipticity on spaces with higher singularities JF - Science China Mathematics N2 - We study corner-degenerate pseudo-differential operators of any singularity order and develop ellipticity based on the principal symbolic hierarchy, associated with the stratification of the underlying space. We construct parametrices within the calculus and discuss the aspect of additional trace and potential conditions along lower-dimensional strata. KW - pseudo-differential operators KW - operator-valued symbols KW - Fourier and Mellin transforms Y1 - 2017 U6 - https://doi.org/10.1007/s11425-016-0519-9 SN - 1674-7283 SN - 1869-1862 VL - 60 IS - 11 SP - 2053 EP - 2076 PB - Science China Press CY - Beijing ER -