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We investigate how the concepts of intersection and sums of subobjects carry to exact categories. We obtain a new characterisation of quasi-abelian categories in terms of admitting admissible intersections in the sense of Hassoun and Roy. There are also many alternative characterisations of abelian categories as those that additionally admit admissible sums and in terms of properties of admissible morphisms. We then define a generalised notion of intersection and sum which every exact category admits. Using these new notions, we define and study classes of exact categories that satisfy the Jordan-H��lder property for exact categories, namely the Diamond exact categories and Artin-Wedderburn exact categories. By explicitly describing all exact structures on $\mathcal{A}= \mbox{rep}\, ��$ for a Nakayama algebra $��$ we characterise all Artin-Wedderburn exact structures on $\mathcal{A}$ and show that these are precisely the exact structures with the Jordan-H��lder property., revised version following Phd defense, lemma 7.5 and theorem 7.6 added

Extending the notion of maximal green sequences to an abelian category, we characterize the stability functions, as defined by Rudakov, that induce a maximal green sequence in an abelian length category. Furthermore, we use $��$-tilting theory to give a description of the wall and chamber structure of any finite dimensional algebra. Finally we introduce the notion of green paths in the wall and chamber structure of an algebra and show that green paths serve as geometrical generalization of maximal green sequences in this context.

Passenger car and heavy-duty diesel engine manufacturers recognize the need to ensure that engine oils maintain adequate low-temperature pumpability performance throughout their working life. For over two decades, the Mini Rotary Viscometer (MRV TP-1) bench test has been used to develop oils that have protected engines from oil starvation failure during cold weather startup. The test was created to measure the low temperature flow properties of fresh oils. During engine operation, a number of changes take place in the lubricant—both chemical and physical—that can influence oil pumpability. This paper presents a systematic evaluation of the role that various common lubricant additives play in affecting MRV viscosity and pour point and reinforces the notion that new oil MRV is not necessarily a good predictor of used oil pumpability. Presented at the STLE Annual Meeting in Calgary, Alberta, Canada May 7-11, 2006 Review led by Lois Gschwender

Diese Arbeit behandelt die Entwicklung eines Prototyps f��r einen kosteng��nstigen Wohnbau in der Ashanti Region Ghanas. Das Geb��ude bietet einer 4-5 k��pfigen Familie mit niedrigem Einkommen eine Unterkunft und ist mit einem Budget von 6000 USD realisierbar. Im Zuge der Arbeit wird die traditionelle Architektur der Region vorgestellt und bildet gemeinsam mit der Analyse der derzeitgen Wohnsituation den Kontext in dem sich dieser Entwurf bewegt. Das bauen im feucht-hei��en Klima ist ebenso ein Thema, wie die Vorstellung und Analyse der Eignung verschiedener Baumaterialien und Bautechniken. Das Wichtigeste Thema ist jedoch die effiziente Verwaltung des gegebenen Budgets. Letzendlich werden Vorschl��ge zur Erweiterung des Geb��udes gegeben, ebenso wie M��glichkeiten zur Adaptierung., This thesis talks about the developement of a protoype for affordable housing in the Ashanti region of Ghana. The building, which is realised with a budget of 6000 US$, provides a living space for a low-income family consisting of 4-5 members. In the course of this thesis traditional architecture of the region is introduced and forms, together with an analysis of the current housing situation, the context in which this design takes place. Building in the hot-humid tropics as well as the introduction and analsys of various building materials and techniques are also discussed. The most important topic however, is an efficient management of the given budget. Lastly ways to expand and adapt the design are suggested.

Babbitt temperatures in pivoted shoe bearings can be significantly reduced by offsetting the pad pivots in the direction of rotation. However, reverse rotation can occur in certain types of machinery under temporary or adverse conditions. The offset is then in the wrong direction and it becomes important to know if the bearing can withstand the reverse rotation operating conditions without damage. This paper compares pad temperature data from tests of a 152.4-mm, 60 percent offset bearing in forward and reverse rotation. The data indicate that the offset pivot bearing can run in reverse without damage for the operating conditions tested. Reverse rotation pad temperatures are hotter. Discussions towards the end of the paper address assessment of the data.

The exchange graph of a cluster algebra encodes the combinatorics of mutations of clusters. Through the recent "categorifications" of cluster algebras using representation theory one obtains a whole variety of exchange graphs associated with objects such as a finite-dimensional algebra or a differential graded algebra concentrated in non-positive degrees. These constructions often come from variations of the concept of tilting, the vertices of the exchange graph being torsion pairs, t-structures, silting objects, support $\tau$-tilting modules and so on. All these exchange graphs stemming from representation theory have the additional feature that they are the Hasse quiver of a partial order which is naturally defined for the objects. In this sense, the exchange graphs studied in this article can be considered as a generalization or as a completion of the poset of tilting modules which has been studied by Happel and Unger. The goal of this article is to axiomatize the thus obtained structure of an ordered exchange graph, to present the various constructions of ordered exchange graphs and to relate them among each other., Comment: References updated, and Theorem A.7 added

A four step mechanism for combustion of methane in perfectly stirred reactors with special emphasis on formation and destruction of hydrocarbon radicals has been developed using steady state and partial equilibrium assumptions for minor species. The reduced mechanism has been extended to include the nitrogen chemistry with NO and HCN as independent reactive species. The reduced nitrogen scheme includes thermal NO and prompt NO formation, as well as the NO to HCN recycle reactions, and conversion of HCN to NO and N2. We have tested the reduced mechanism by comparing perfectly stirred reactor calculations performed with full and reduced chemistry over a wide range of stoichiometries, temperatures and residence times. The reduced model generally provides a good description of the methane oxidation process as well as formation and destruction of nitrogen oxides. However, at low temperatures or very fuel-rich conditions reduced model predictions deteriorate, partly due to neglection of the C2-chemistry and partly because the OH partial equilibrium assumption becomes less accurate.

Acyclic cluster algebras have an interpretation in terms of tilting objects in a Calabi-Yau category defined by some hereditary algebra. For a given quiver $Q$ it is thus desirable to decide if the cluster algebra defined by $Q$ is acyclic. We call $Q$ cluster-acyclic in this case, otherwise cluster-cyclic. In this note we classify the cluster-cyclic quivers with three vertices using a Diophantine equation studied by Markov., 11 pages, with an appendix by Otto Kerner

The evolution of the Sicily Channel Rift Zone (SCRZ) is thought to accommodate the regional tectonic stresses of the Calabrian subduction system. Much of the observations we have today are either limited to the surface or to the upper crust or deeper from regional seismic tomography, missing important details about the lithospheric structure and dynamics. It is unclear whether the rifting is passive from far-field extensional stresses or active from mantle upwelling beneath. We measure Rayleigh-and Love-wave phase velocities from ambient seismic noise and invert for 3-D shear-velocity and radial anisotropic models. Variations in crustal S-velocities coincide with topographic and tectonic features. The Tyrrhenian Sea has a similar to 10 km thin crust, followed by the SCRZ (similar to 20 km). The thickest crust is beneath the Apennine-Maghrebian Mountains (similar to 55 km). Areas experiencing extension and intraplate volcanism have positive crustal radial anisotropy (V-SH > V-SV); areas experiencing compression and subduction-related volcanism have negative anisotropy. The crustal anisotropy across the Channel shows the extent of the extension. Beneath the Tyrrhenian Sea, we find very low sub-Moho S-velocities. In contrast, the SCRZ has a thin mantle lithosphere underlain by a low-velocity zone. The lithosphere-asthenosphere boundary rises from 60 km depth beneath Tunisia to similar to 33 km beneath the SCRZ. Negative radial anisotropy in the upper mantle beneath the SCRZ is consistent with vertical mantle flow. We hypothesize a more active mantle upwelling beneath the rift than previously thought from an interplay between poloidal and toroidal fluxes related to the Calabrian slab, which in turn produces uplift at the surface and induces volcanism.

International audience

International audience

International audience

International audience

International audience

International audience