74 research outputs found
Nouveaux specimens du genre Leclercqia Banks, H.P., Bonamo, P.M. et Grierson, J.D., 1972, du Givetien ( ? ) Du Queensland (Australie)
Axis fragments with « protolepidodendroid » surface pattern from the Middle Devonian (Givetian ?) of the Burdekin Basin (Queensland, Australia)are assigned, following their preparation to the genus Leclercqia,BANKS, H. P., BONAMO, P. M. et GRIERSON, J . D., 1972, formerly restricted to North America
Trigonometric real form of the spin RS model of Krichever and Zabrodin
We investigate the trigonometric real form of the spin Ruijsenaars-Schneider
system introduced, at the level of equations of motion, by Krichever and
Zabrodin in 1995. This pioneering work and all earlier studies of the
Hamiltonian interpretation of the system were performed in complex holomorphic
settings; understanding the real forms is a non-trivial problem. We explain
that the trigonometric real form emerges from Hamiltonian reduction of an
obviously integrable 'free' system carried by a spin extension of the
Heisenberg double of the Poisson-Lie group. The Poisson structure
on the unreduced real phase space is the direct product of that of the Heisenberg double and
copies of a covariant Poisson structure on found by Zakrzewski, also in 1995. We reduce by fixing
a group valued moment map to a multiple of the identity, and analyze the
resulting reduced system in detail. In particular, we derive on the reduced
phase space the Hamiltonian structure of the trigonometric spin
Ruijsenaars-Schneider system and we prove its degenerate integrability.Comment: 52 pages, 1 figure. v2: typos removed, final versio
Multiplicative quiver varieties and generalised RuijsenaarsâSchneider models
We study some classical integrable systems naturally associated with multiplicative quiver varieties for the (extended) cyclic quiver with m vertices. The phase space of our integrable systems is obtained by quasi-Hamiltonian reduction from the space of representations of the quiver. Three families of Poisson-commuting functions are constructed and written explicitly in suitable Darboux coordinates. The case m = 1 corresponds to the tadpole quiver and the RuijsenaarsâSchneider system and its variants, while for m > 1 we obtain new integrable systems that generalise the RuijsenaarsâSchneider system. These systems and their quantum versions also appeared recently in the context of supersymmetric gauge theory and cyclotomic DAHAs (Braverman et al. [32,34,35] and Kodera and Nakajima [36]), as well as in the context of the Macdonald theory (Chalykh and Etingof, 2013)
Automatic Item Text Generation in Educational Assessment
We present an automatic text generation system (ATG) developed for the generation of natural language text for automatically produced test items. This ATG has been developed to work with an automatic item generation system for analytical reasoning items for use in tests with high-stakes outcomes (such as college admissions decisions). As such, the development and implementation of this ATG is couched in the context and goals of automated item generation for educational assessment
-Algebras, the BV Formalism, and Classical Fields
We summarise some of our recent works on -algebras and quasi-groups
with regard to higher principal bundles and their applications in twistor
theory and gauge theory. In particular, after a lightning review of
-algebras, we discuss their Maurer-Cartan theory and explain that any
classical field theory admitting an action can be reformulated in this context
with the help of the Batalin-Vilkovisky formalism. As examples, we explore
higher Chern-Simons theory and Yang-Mills theory. We also explain how these
ideas can be combined with those of twistor theory to formulate maximally
superconformal gauge theories in four and six dimensions by means of
-quasi-isomorphisms, and we propose a twistor space action.Comment: 19 pages, Contribution to Proceedings of LMS/EPSRC Durham Symposium
Higher Structures in M-Theory, August 201
Earliest land plants created modern levels of atmospheric oxygen
The progressive oxygenation of the Earthâs atmosphere was pivotal
to the evolution of life, but the puzzle of when and how
atmospheric oxygen (O2) first approached modern levels (~21%)
remains unresolved. Redox proxy data indicate the deep oceans
were oxygenated during 435-392 Ma, and the appearance of
fossil charcoal indicates O2>15-17% by 420-400 Ma. However,
existing models have failed to predict oxygenation at this time.
Here we show that the earliest plants, which colonized the land
surface from ~470 Ma onwards, were responsible for this mid-
Paleozoic oxygenation event, through greatly increasing global
organic carbon burial â the net long-term source of O2. We use
a trait-based ecophysiological model to predict that cryptogamic
vegetation cover could have achieved ~30% of todayâs global
terrestrial net primary productivity by~445 Ma. Data from modern
bryophytes suggests this plentiful early plant material had a much
higher molar C:P ratio (~2000) than marine biomass (~100), such
that a given weathering flux of phosphorus could support more
organic carbon burial. Furthermore, recent experiments suggest
that early plants selectively increased the flux of phosphorus (relative
to alkalinity) weathered from rocks. Combining these effects
in a model of long-term biogeochemical cycling, we reproduce a
sustained +2â° increase in the carbonate carbon isotope (ÎŽ13C)
record by ~445 Ma, and predict a corresponding rise in O2 to
present levels by 420-400 Ma, consistent with geochemical data.
This oxygen rise represents a permanent shift in regulatory regime
to one where fire-mediated negative feedbacks on organic carbon
burial stabilise high O2 levels
On the Hamiltonian formulation of the trigonometric spin Ruijsenaars-Schneider system
We suggest a Hamiltonian formulation for the spin RuijsenaarsâSchneider system in the trigonometric case. Within this interpretation, the phase space is obtained by a quasi-Hamiltonian reduction performed on (the cotangent bundle to) a representation space of a framed Jordan quiver. For arbitrary quivers, analogous varieties were introduced by Crawley-Boevey and Shaw, and their interpretation as quasi-Hamiltonian quotients was given by Van den Bergh. Using Van den Berghâs formalism, we construct commuting Hamiltonian functions on the phase space and identify one of the flows with the spin RuijsenaarsâSchneider system. We then calculate all the Poisson brackets between local coordinates, thus answering an old question of Arutyunov and Frolov. We also construct a complete set of commuting Hamiltonians and integrate all the flows explicitly
Estinnophyton gracile gen. et sp. nov., a new name for specimens previously determined Protolepidodendron wahnbachense KrÀusel & Weyland, from the Siegenian of Belgium
Estinnophyton gracile gen. et sp. nov. is established for plant remains from the Lower Siegenian of Estinnes-au-Mont (Belgium) which have been long confused with Protolepidodendron wahnbachense KrÀusel & Weyland 1932, from the Siegenian of the Wahnbachtal (Germany). The distinctive features of the new genus are discussed as well as its possible relationship with Lycophytes and Sphenophyllales.Estinnophyton gracile gen. et sp. nov. est fondé pour désigner des restes végétaux du Siegenien Inférieur d'Estinnes-au-Mont (Belgique) qui furent longtemps confondus avec Protolepidodendron wahnbachense KrÀusel & Weyland 1932, du Siegenien de la Wahnbachtal (Allemagne). Les caractÚres distinctifs du nouveau genre sont discutés ainsi que ses affinités éventuelles avec les Lycophytes et les Sphénophyllales.Fairon-Demaret M. Estinnophyton gracile gen. et sp. nov., a new name for specimens previously determined Protolepidodendron wahnbachense KrÀusel & Weyland, from the Siegenian of Belgium. In: Bulletin de la Classe des sciences, tome 64, 1978. pp. 597-610
à propos de certains spécimens de Drepanophycus gaspianus (Dawson) Stockmans, F., 1939, du Dévonien inférieur de Belgique
Some specimens amongst those described by F. CrĂ©pin in 1875 and the same year, by A. Gilkinet (1875) are included in, or excluded from the synonymy of Drepanophycus gaspianus (Dawson) Stockmans, F., 1939. Thus their re-examination was required. It allowed to assert the assignation of F. CrĂ©pin's specimens to D. gaspianus and to set aside A. Gilkinet's specimen. On the other hand special attention is drawn on D. gaspianus leaves morphology.Certains spĂ©cimens, parmi ceux dĂ©crits par F. CrĂ©pin en 1875 et, la mĂȘme annĂ©e, par A. Gilkinet (1875) sont inclus, ou exclus de la synonymie de Drepanophycus gaspianus (Dawson) Stockmans, F., 1939. Leur rĂ©examen s'imposait donc. Il a permis de justifier l'attribution des spĂ©cimens de F. CrĂ©pin au D. gaspianus et d'Ă©carter celui Ă©tudiĂ© par A. Gilkinet. Par ailleurs, la morphologie des feuilles de D. gaspianus fait l'objet d'une attention particuliĂšre.Fairon-Demaret M. Ă propos de certains spĂ©cimens de Drepanophycus gaspianus (Dawson) Stockmans, F., 1939, du DĂ©vonien infĂ©rieur de Belgique. In: Bulletin de la Classe des sciences, tome 63, 1977. pp. 781-790
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