329 research outputs found
Equations of parametric surfaces with base points via syzygies
Suppose is a parametrized surface in complex projective 3-space given as the image of . The implicitization problem is to compute an implicit equation of using the parametrization . An algorithm using syzygies exists for computing if has no base points, i.e. is everywhere defined. This work is an extension of this algorithm to the case of a surface with multiple base points of total multiplicity k. We accomplish this in three chapters. In Chapter 2, we develop the concept and properties of Castelnuovo-Mumford regularity in biprojective spaces. In Chapter 3, we give a criterion for regularity in biprojective spaces. These results are applied to the implicitization problem in Chapter 4
Multigraded regularity: coarsenings and resolutions
Let S = k[x_1,...,x_n] be a Z^r-graded ring with deg (x_i) = a_i \in Z^r for
each i and suppose that M is a finitely generated Z^r-graded S-module. In this
paper we describe how to find finite subsets of Z^r containing the multidegrees
of the minimal multigraded syzygies of M. To find such a set, we first coarsen
the grading of M so that we can view M as a Z-graded S-module. We use a
generalized notion of Castelnuovo-Mumford regularity, which was introduced by
D. Maclagan and G. Smith, to associate to M a number which we call the
regularity number of M. The minimal degrees of the multigraded minimal syzygies
are bounded in terms of this invariant.Comment: 20 pages, 1 figure; small corrections made; final version; to appear
in J. of Algebr
Ferroelectric metals in 1T/1T'-phase transition metal dichalcogenides bilayers
Ferroelectricity and metallicity cannot coexist due to the screening effect
of conducting electrons, and a large number of stable monolayers with
1T/1T phase lack spontaneous polarization due to inversion symmetry
(IS). In this work, we have constructed the -bilayer structures for
transition metal dichalcogenides (TMD; Te Pt, Pd, and Ni) with van
der Waals stacking, where two monolayers are related by rotation, and
have demonstrated that these -bilayers are typical ferroelectric metals
(FEMs). The -bilayer structure widely exist in nature, such as
1T/T-TMD, -BiBr. The computed vertical
polarization of PtTe and MoTe -bilayers are 0.46 and 0.25 pC/m,
respectively. We show that the switching of polarization can be realized
through interlayer sliding, which only requires crossing a low energy barrier.
The interlayer vdW charge transfer is the source of both vertical polarization
and metallicity, and these properties are closely related to the spatially
extended Te- orbital. Finally, we reveal that electron doping can
significantly adjust the vertical polarization of these FEMs in both magnitude
and direction. Our findings introduce a new class of FEMs, which have potential
applications in functional nanodevices such as ferroelectric tunneling junction
and nonvolatile ferroelectric memory
Stress Monitoring for Anchor Rods System in Subway Tunnel Using FBG Technology
This paper presents a model test, used on the tunnels on Xi’an Metro Line 2, as the prototype for evaluating the reinforcing effect of the anchor rod in tunnel construction in loess areas. An independently designed fiber Bragg grating (FBG) sensor was used to monitor the seven strain conditions of the rock bolts during the construction. The result shows that the axial stress of the rock bolt changes after the excavation and increases steadily with the growing pressure in the wall rock. Results additionally show that the anchor rods at the tunnel vault are subjected to a compressive stress that remains relatively constant after the primary and the secondary lining, while those at the spandrel and the corner of the tunnel are subjected to increased tensile stress. This paper demonstrates the feasibility and the superiority of FBG technology for tunnel model tests
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