5,872 research outputs found
The Complexity of Combinations of Qualitative Constraint Satisfaction Problems
The CSP of a first-order theory is the problem of deciding for a given
finite set of atomic formulas whether is satisfiable. Let
and be two theories with countably infinite models and disjoint
signatures. Nelson and Oppen presented conditions that imply decidability (or
polynomial-time decidability) of under the
assumption that and are decidable (or
polynomial-time decidable). We show that for a large class of
-categorical theories the Nelson-Oppen conditions are not
only sufficient, but also necessary for polynomial-time tractability of
(unless P=NP)
Bidirectional Text Compression in External Memory
Bidirectional compression algorithms work by substituting repeated substrings by references that, unlike in the famous LZ77-scheme, can point to either direction. We present such an algorithm that is particularly suited for an external memory implementation. We evaluate it experimentally on large data sets of size up to 128 GiB (using only 16 GiB of RAM) and show that it is significantly faster than all known LZ77 compressors, while producing a roughly similar number of factors. We also introduce an external memory decompressor for texts compressed with any uni- or bidirectional compression scheme
Three-band Hubbard model for NaIrO: Topological insulator, zigzag antiferromagnet, and Kitaev-Heisenberg material
NaIrO was one of the first materials proposed to feature the
Kane-Mele type topological insulator phase. Contemporaneously it was claimed
that the very same material is in a Mott insulating phase which is described by
the Kitaev-Heisenberg (KH) model. First experiments indeed revealed Mott
insulating behavior in conjunction with antiferromagnetic long-range order.
Further refined experiments established antiferromagnetic order of zigzag type
which is not captured by the KH model. Since then several extensions and
modifications of the KH model were proposed in order to describe the
experimental findings. Here we suggest that adding charge fluctuations to the
KH model represents an alternative explanation of zigzag antiferromagnetism.
Moreover, a phenomenological three-band Hubbard model unifies all the pieces of
the puzzle: topological insulator physics for weak and KH model for strong
electron-electron interactions as well as a zigzag antiferromagnet at
intermediate interaction strength.Comment: 5 pages, 3 figures; v2 (as published): added discussion about kinetic
energy scale C; more realistic values of C shift the zigzag AFM phase to
larger values of
Catalytic Asymmetric Pictet-Spengler Reactions toward Tetrahydroisoquinolines
The Pictet-Spengler reaction of 2-arylethylamines with aldehydes is a powerful methodology for the redox-neutral synthesis of partly hydrogenated nitrogen heterocycles. Especially tetrahydro-β-carbolines and tetrahydroisoquinolines represent valuable synthetic targets due to the prevalence of the respective molecular frameworks in naturally occurring alkaloids. The work presented within this thesis describes the development of a catalytic asymmetric Pictet-Spengler reaction toward tetrahydroisoquinolines – a product class that had been largely inaccessible via this route. Furthermore, the utilization of the available products in the biomimetic formal or total synthesis of eleven distinct natural products from diverse alkaloid classes was accomplished. Key to the development of a general methodology was the design and synthesis of bespoke imidodiphosphorimidate catalysts featuring electron-rich substituents that offered unprecedentedly high reactivity and selectivity in the system under study. The mechanistic nuances of the reaction were studied experimentally through in-depth kinetic analyses. Our investigation thus provides insights into the overall reaction mechanism as well as specific interactions offered by the optimal catalysts. Finally, further studies were directed toward the development of a catalytic asymmetric Pictet-Spengler reaction of electronically unbiased phenethylamines
N-loop running should be combined with N-loop matching
We investigate the high-scale behaviour of Higgs sectors beyond the Standard
Model, pointing out that the proper matching of the quartic couplings before
applying the renormalisation group equations (RGEs) is of crucial importance
for reliable predictions at larger energy scales. In particular, the common
practice of leading-order parameters in the RGE evolution is insufficient to
make precise statements on a given model's UV behaviour, typically resulting in
uncertainties of many orders of magnitude. We argue that, before applying
N-loop RGEs, a matching should even be performed at N-loop order in contrast to
common lore. We show both analytical and numerical results where the impact is
sizeable for three minimal extensions of the Standard Model: a singlet
extension, a second Higgs doublet and finally vector-like quarks. We highlight
that the known two-loop RGEs tend to moderate the running of their one-loop
counterparts, typically delaying the appearance of Landau poles. For the
addition of vector-like quarks we show that the complete two-loop matching and
RGE evolution hints at a stabilisation of the electroweak vacuum at high
energies, in contrast to results in the literature.Comment: 16 pages, 11 figures; v2: title changed, accepted for publication in
PR
Quantum disordered insulating phase in the frustrated cubic-lattice Hubbard model
In the quest for quantum spin liquids in three spatial dimensions (3D), we
study the half-filled Hubbard model on the simple cubic lattice with hopping
processes up to third neighbors. Employing the variational cluster approach
(VCA), we determine the zero-temperature phase diagram: In addition to a
paramagnetic metal at small interaction strength and various
antiferromagnetic insulators at large , we find an intermediate-
antiferromagnetic metal. Most interestingly, we also identify a non-magnetic
insulating region, extending from intermediate to strong . Using VCA results
in the large- limit, we establish the phase diagram of the corresponding
-- Heisenberg model. This is qualitatively confirmed - including
the non-magnetic region - using spin-wave theory. Further analysis reveals a
striking similarity to the behavior of the - square-lattice
Heisenberg model, suggesting that the non-magnetic region hosts a 3D
spin-liquid phase.Comment: 5 pages, 4 figures; final version incl. discussion about material
Visual Analysis of Spatio-Temporal Event Predictions: Investigating the Spread Dynamics of Invasive Species
Invasive species are a major cause of ecological damage and commercial
losses. A current problem spreading in North America and Europe is the vinegar
fly Drosophila suzukii. Unlike other Drosophila, it infests non-rotting and
healthy fruits and is therefore of concern to fruit growers, such as vintners.
Consequently, large amounts of data about infestations have been collected in
recent years. However, there is a lack of interactive methods to investigate
this data. We employ ensemble-based classification to predict areas susceptible
to infestation by D. suzukii and bring them into a spatio-temporal context
using maps and glyph-based visualizations. Following the information-seeking
mantra, we provide a visual analysis system Drosophigator for spatio-temporal
event prediction, enabling the investigation of the spread dynamics of invasive
species. We demonstrate the usefulness of this approach in two use cases
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