1,354,134 research outputs found
Almost complex structures on spheres
No full textIn this paper we review the well-known fact that the only spheres admitting an almost complex structure are S2 and S6. The proof described here uses characteristic classes and the Bott periodicity theorem in topological K-theory. This paper originates from the talk “Almost Complex Structures on Spheres” given by the second author at the MAM1 workshop “(Non)-existence of complex structures on S6”, held in Marburg from March 27th to March 30th, 2017. It is a review paper, and as such no result is intended to be original. We tried to produce a clear, motivated and as much as possible self-contained exposition. © 2017 Elsevier B.V
Connectivity and Topology Invariance in Self-Assembled and Halogen-Bonded Anionic (6,3)-Networks
No full textWe report here that the halogen bond driven self-assembly of 1,3,5-trifluorotriiodobenzene with tetraethylammonium and -phosphonium bromides affords 1:1 co-crystals, wherein the mutual induced fit of the triiodobenzene derivative and the bromide anions (halogen bond donor and acceptors, respectively) elicits the potential of these two tectons to function as tritopic modules (6,3). Supramolecular anionic networks are present in the two co-crystals wherein the donor and the acceptor alternate at the vertexes of the hexagonal frames and cations are accommodated in the potential empty space encircled by the frames. The change of one component in a self-assembled multi-component co-crystal often results in a change in its supramolecular connectivity and topology. Our systems have the same supramolecular features of corresponding iodide analogues as the metric aspects seem to prevail over other aspects in controlling the self-assembly process
On the history of the Hopf problem
No full textThis short note serves as a historical introduction to the Hopf problem: "Does there exist a complex structure on S6?"
This unsolved mathematical question was the subject of the Conference "MAM 1 − (Non-)Existence of Complex Structures on S6", which took place at Philipps-Universit"at Marburg, Germany, between March 27th and March 30th, 2017
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Using Imidazolium in the Construction of Hybrid 2D and 3D Lead Bromide Pseudoperovskites
No full textThe field of hybrid organic–inorganic perovskite materials continues to attract the interest of the scientific community due to their fascinating properties and the plethora of promising applications in photovoltaic and optoelectronic devices. To enhance the efficiency and stability of perovskite-based devices, it is essential to discover novel compounds but also to investigate their various physicochemical, structural, and thermal properties. In this work, we report the synthesis and structural characterization of two novel hybrid lead bromide perovskites, combining the imidazolium cation (IMI) with methylammonium (MA) or formamidinium (FA) cations. The isolated polycrystalline powders were studied with X-ray powder diffraction (XPRD) and were formulated as (IMI)(MA)Pb2Br6, a 3D structure consisting of dimers of face-sharing octahedra linked in corner-sharing mode, and (IMI)(FA)PbBr4, a 2D (110) oriented layer structure with zig-zag corner-sharing octahedra. The thermal stability of (IMI)(MA)Pb2Br6 and (IMI)(FA)PbBr4 was investigated with thermogravimetric (TG) and differential scanning calorimetry (DSC) experiments which showed that both compounds are chemically stable (at least) up to 250 °C. Variable-temperature X-ray diffractometric (VT-XRD) studies of (IMI)(FA)PbBr4 highlighted a structural modification occurring above 100 °C, that is a phase transformation from triclinic to orthorhombic, via an elusive monoclinic phase
Variations on the Author
No full text“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
No full textWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Spin Crossover via Supramolecular Halogen⋅⋅⋅Halogen Interactions
No full textWe have added the {ReIVX5}− (X=Br, Cl) synthon to a pocket-based ligand to provide supramolecular design using halogen⋅⋅⋅halogen interactions within an FeIII system that has the potential to undergo spin crossover (SCO). By removing the solvent from the crystal lattice, we “switch on” halogen⋅⋅⋅halogen interactions between neighboring molecules, providing a supramolecular cooperative pathway for SCO. Furthermore, changes to the halogen-based interaction allow us to modify the temperature and nature of the SCO event.</p
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