4,089 research outputs found
Two-dimensional anisotropic Heisenberg antiferromagnet in a field
The classical, square lattice, uniaxially anisotropic Heisenberg
antiferromagnet in a magnetic field parallel to the easy axis is studied using
Monte Carlo techniques. The model displays a long-range ordered
antiferromagnetic, an algebraically ordered spin-flop, and a paramagnetic
phase. The simulations indicate that a narrow disordered phase intervenes
between the ordered phases down to quite low temperatures. Results are compared
to previous, partially conflicting findings on related classical models as well
as the quantum variant with spin S=1/2.Comment: 8 pages, 9 figure
A Resolved Molecular Gas Disk around the Nearby A Star 49 Ceti
The A star 49 Ceti, at a distance of 61 pc, is unusual in retaining a
substantial quantity of molecular gas while exhibiting dust properties similar
to those of a debris disk. We present resolved observations of the disk around
49 Ceti from the Submillimeter Array in the J=2-1 rotational transition of CO
with a resolution of 1.0x1.2 arcsec. The observed emission reveals an extended
rotating structure viewed approximately edge-on and clear of detectable CO
emission out to a distance of ~90 AU from the star. No 1.3 millimeter continuum
emission is detected at a 3-sigma sensitivity of 2.1 mJy/beam. Models of disk
structure and chemistry indicate that the inner disk is devoid of molecular
gas, while the outer gas disk between 40 and 200 AU from the star is dominated
by photochemistry from stellar and interstellar radiation. We determine
parameters for a model that reproduces the basic features of the spatially
resolved CO J=2-1 emission, the spectral energy distribution, and the
unresolved CO J=3-2 spectrum. We investigate variations in disk chemistry and
observable properties for a range of structural parameters. 49 Ceti appears to
be a rare example of a system in a late stage of transition between a gas-rich
protoplanetary disk and a tenuous, virtually gas-free debris disk.Comment: 11 pages, 6 figures, accepted for publication in Ap
Automated Termination Proofs for Logic Programs by Term Rewriting
There are two kinds of approaches for termination analysis of logic programs:
"transformational" and "direct" ones. Direct approaches prove termination
directly on the basis of the logic program. Transformational approaches
transform a logic program into a term rewrite system (TRS) and then analyze
termination of the resulting TRS instead. Thus, transformational approaches
make all methods previously developed for TRSs available for logic programs as
well. However, the applicability of most existing transformations is quite
restricted, as they can only be used for certain subclasses of logic programs.
(Most of them are restricted to well-moded programs.) In this paper we improve
these transformations such that they become applicable for any definite logic
program. To simulate the behavior of logic programs by TRSs, we slightly modify
the notion of rewriting by permitting infinite terms. We show that our
transformation results in TRSs which are indeed suitable for automated
termination analysis. In contrast to most other methods for termination of
logic programs, our technique is also sound for logic programming without occur
check, which is typically used in practice. We implemented our approach in the
termination prover AProVE and successfully evaluated it on a large collection
of examples.Comment: 49 page
TRENDS AND DEVELOPMENTS IN UNITED STATES AGRICULTURAL POLICY: 1993-1995
A number of factors including budget pressures, emphasis on environmentally sensitive agriculture, emphasis on finding agricultural export markets, and anti-agricultural program sentiment have fueled a climate for change in United States agricultural policy. Whether significant changes will occur depends on the political strength of agricultural interest groups and on the compromises which can be reached between them. Several notable achievements have been accomplished in recent domestic agricultural policy legislation. The 1995 farm bill will define the commodity and conservation programs for the next five years. In addition to domestic developments, there have been history-setting accomplishments in reducing barriers to international agricultural trade. The tri-partite North American Free Trade Agreement became effective January 1, 1994. The Uruguay Round Agreement of the General Agreement on Tariffs and Trade was completed in late 1994 and will become effective in 1995.Agricultural and Food Policy,
The Shrinking Forum: The Supreme Court\u27s Limitation of Jurisdiction - An Argument for a Federal Forum in Multi-Party, Multi-State Litigation
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