4,972 research outputs found
Structure peculiarities of cementite and their influence on the magnetic characteristics
The iron carbide is studied by the first-principle density functional
theory. It is shown that the crystal structure with the carbon disposition in a
prismatic environment has the lowest total energy and the highest energy of
magnetic anisotropy as compared to the structure with carbon in an octahedron
environment. This fact explains the behavior of the coercive force upon
annealing of the plastically deformed samples. The appearance of carbon atoms
in the octahedron environment can be revealed by Mossbauer experiment.Comment: 10 pages, 3 figures, 3 tables. submitted to Phys.Rev.
A compiler approach to scalable concurrent program design
The programmer's most powerful tool for controlling complexity in program design is abstraction. We seek to use abstraction in the design of concurrent programs, so as to
separate design decisions concerned with decomposition, communication, synchronization, mapping, granularity, and load balancing. This paper describes programming and compiler techniques intended to facilitate this design strategy. The programming techniques are based on a core programming notation with two important properties: the ability to separate concurrent programming concerns, and extensibility with reusable programmer-defined
abstractions. The compiler techniques are based on a simple transformation system together with a set of compilation transformations and portable run-time support. The
transformation system allows programmer-defined abstractions to be defined as source-to-source transformations that convert abstractions into the core notation. The same
transformation system is used to apply compilation transformations that incrementally transform the core notation toward an abstract concurrent machine. This machine can be implemented on a variety of concurrent architectures using simple run-time support.
The transformation, compilation, and run-time system techniques have been implemented and are incorporated in a public-domain program development toolkit. This
toolkit operates on a wide variety of networked workstations, multicomputers, and shared-memory
multiprocessors. It includes a program transformer, concurrent compiler, syntax checker, debugger, performance analyzer, and execution animator. A variety of substantial
applications have been developed using the toolkit, in areas such as climate modeling and fluid dynamics
Some hyperbolic 4-manifolds with low volume and number of cusps
We construct here two new examples of non-orientable, non-compact, hyperbolic
4-manifolds. The first has minimal volume and two cusps.
This example has the lowest number of cusps among known minimal volume
hyperbolic 4-manifolds. The second has volume and one cusp. It has
lowest volume among known one-cusped hyperbolic 4-manifolds.Comment: 12 pages, 11 figure
Arctic curves of the octahedron equation
We study the octahedron relation (also known as the -system),
obeyed in particular by the partition function for dimer coverings of the Aztec
Diamond graph. For a suitable class of doubly periodic initial conditions, we
find exact solutions with a particularly simple factorized form. For these, we
show that the density function that measures the average dimer occupation of a
face of the Aztec graph, obeys a system of linear recursion relations with
periodic coefficients. This allows us to explore the thermodynamic limit of the
corresponding dimer models and to derive exact "arctic" curves separating the
various phases of the system.Comment: 39 pages, 21 figures; typos fixed, four references and an appendix
adde
Discrete Folding
Models of folding of a triangular lattice embedded in a discrete space are
studied as simple models of the crumpling transition of fixed-connectivity
membranes. Both the case of planar folding and three-dimensional folding on a
face-centered-cubic lattice are treated. The 3d-folding problem corresponds to
a 96-vertex model and exhibits a first-order folding transition from a crumpled
phase to a completely flat phase as the bending rigidity increases.Comment: LaTeX, 13 pages, 11 eps/ps figures: To appear in the Proceedings of
the 4th Chia Meeting on "Condensed Matter and High-Energy Physics" (World
Scientific, Singapore
Algebra versus analysis in the theory of flexible polyhedra
Two basic theorems of the theory of flexible polyhedra were proven by
completely different methods: R. Alexander used analysis, namely, the Stokes
theorem, to prove that the total mean curvature remains constant during the
flex, while I.Kh. Sabitov used algebra, namely, the theory of resultants, to
prove that the oriented volume remains constant during the flex. We show that
none of these methods can be used to prove the both theorems. As a by-product,
we prove that the total mean curvature of any polyhedron in the Euclidean
3-space is not an algebraic function of its edge lengths.Comment: 5 pages, 5 figures; condition (iii) in Theorem 5 is correcte
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