4,446 research outputs found
Nonparametric Bayesian Lomax delegate racing for survival analysis with competing risks
We propose Lomax delegate racing (LDR) to explicitly model the mechanism of
survival under competing risks and to interpret how the covariates accelerate
or decelerate the time to event. LDR explains non-monotonic covariate effects
by racing a potentially infinite number of sub-risks, and consequently relaxes
the ubiquitous proportional-hazards assumption which may be too restrictive.
Moreover, LDR is naturally able to model not only censoring, but also missing
event times or event types. For inference, we develop a Gibbs sampler under
data augmentation for moderately sized data, along with a stochastic gradient
descent maximum a posteriori inference algorithm for big data applications.
Illustrative experiments are provided on both synthetic and real datasets, and
comparison with various benchmark algorithms for survival analysis with
competing risks demonstrates distinguished performance of LDR.Comment: NeurIPS 201
Multiple types of topological fermions in transition metal silicides
Exotic massless fermionic excitations with non-zero Berry flux, other than
Dirac and Weyl fermions, could exist in condensed matter systems under the
protection of crystalline symmetries, such as spin-1 excitations with 3-fold
degeneracy and spin-3/2 Rarita-Schwinger-Weyl fermions. Herein, by using ab
initio density functional theory, we show that these unconventional
quasiparticles coexist with type-I and type-II Weyl fermions in a family of
transition metal silicides, including CoSi, RhSi, RhGe and CoGe, when the
spin-orbit coupling (SOC) is considered. Their non-trivial topology results in
a series of extensive Fermi arcs connecting projections of these bulk
excitations on side surface, which is confirmed by (010) surface electronic
spectra of CoSi. In addition, these stable arc states exist within a wide
energy window around the Fermi level, which makes them readily accessible in
angle-resolved photoemission spectroscopy measurements.Comment: 5 pages, 4 figures, Comments are welcom
Cubic vertex-transitive non-Cayley graphs of order 12p
A graph is said to be {\em vertex-transitive non-Cayley} if its full
automorphism group acts transitively on its vertices and contains no subgroups
acting regularly on its vertices. In this paper, a complete classification of
cubic vertex-transitive non-Cayley graphs of order , where is a prime,
is given. As a result, there are sporadic and one infinite family of such
graphs, of which the sporadic ones occur when , or , and the
infinite family exists if and only if , and in this family
there is a unique graph for a given order.Comment: This paper has been accepted for publication in SCIENCE CHINA
Mathematic
Analysis on Transmit Antenna Selection for Spatial Multiplexing Systems: A Geometrical Approach
Recently, the remarkable potential of a multiple-input multiple-output (MIMO)
wireless communication system was unveiled for its ability to provide spatial
diversity or multiplexing gains. For MIMO diversity schemes, it is already
known that. by the optimal antenna selection maximizing the post-processing
signal-to-noise ratio, the diversity order of the full system can be
maintained. On the other hand, the diversity order achieved by antenna
selection in spatial multiplexing systems, especially those exploiting
practical coding and decoding schemes, has not been rigorously analyzed thus
far. In this paper, from a geometric standpoint, we propose a new framework for
theoretically analyzing the diversity order achieved by transmit antenna
selection for separately encoded spatial multiplexing systems with linear and
decision-feedback receivers. We rigorously show that a diversity order of
(Nt-1)(Nr-1) can be achieved for an Nr by Nt SM system when L=2 antennas are
selected from the transmit side; while for L>2 scenarios, we give bounds for
the achievable diversity order and show that the optimal diversity order is at
least (Nt-L+1)(Nr-L+1) . Furthermore, the same geometrical approach can be used
to evaluate the diversity-multiplexing tradeoff curves for the considered
spatial multiplexing systems with transmit antenna selection.Comment: 28 Pages, Submitted IEEE Trans. Info. Theor
Dirac Fermions in Antiferromagnetic Semimetal
The analogues of elementary particles have been extensively searched for in
condensed matter systems because of both scientific interests and technological
applications. Recently massless Dirac fermions were found to emerge as low
energy excitations in the materials named Dirac semimetals. All the currently
known Dirac semimetals are nonmagnetic with both time-reversal symmetry
and inversion symmetry . Here we show that Dirac
fermions can exist in one type of antiferromagnetic systems, where
and are broken but their combination
is respected. We propose orthorhombic antiferromagnet CuMnAs as a candidate,
analyze the robustness of the Dirac points with symmetry protections, and
demonstrate its distinctive bulk dispersions as well as the corresponding
surface states by \emph{ab initio} calculations. Our results give a new route
towards the realization of Dirac materials, and provide a possible platform to
study the interplay of Dirac fermion physics and magnetism
Chiral topological superconductor and half-integer conductance plateau from quantum anomalous Hall plateau transition
We propose to realize a two-dimensional chiral topological superconducting
(TSC) state from the quantum anomalous Hall plateau transition in a magnetic
topological insulator thin film through the proximity effect to a conventional
-wave superconductor. This state has a full pairing gap in the bulk and a
single chiral Majorana mode at the edge. The optimal condition for realizing
such chiral TSC is to have inequivalent superconducting pairing amplitudes on
top and bottom surfaces of the doped magnetic topological insulator. We further
propose several transport experiments to detect the chiral TSC. One unique
signature is that the conductance will be quantized into a half-integer plateau
at the coercive field in this hybrid system. In particular, with the point
contact formed by a superconducting junction, the conductance oscillates
between and with the frequency determined by the voltage
across the junction. We close by discussing the feasibility of these
experimental proposals.Comment: 9 pages, 5 figure
Pressure and strain effects on the optical properties of K4 phosphorus
An investigation of the mechanical, electronic, and optical properties of the
recently reported material K4 phosphorus was made in this work. The K4
phosphorus has been proved to be mechanically and dynamically stable up to 7
GPa under hydrostatic pressure. We compared the elastic anisotropy, average
acoustic velocity and Debye temperature of K4 phosphorus at 0 and 7 GPa. The
ideal tensile at large strains of K4 phosphorus was also examined, with the
results showing that it would cleave under the tensile strength of 8.5 GPa with
the strain of 0.3. In addition, the effect of tensile strain and pressure on
optical properties and band gap were studied
Edge-transitive bi-Cayley graphs
A graph \G admitting a group of automorphisms acting semi-regularly on
the vertices with exactly two orbits is called a {\em bi-Cayley graph\/} over
. Such a graph \G is called {\em normal\/} if is normal in the full
automorphism group of \G, and {\em normal edge-transitive\/} if the
normaliser of in the full automorphism group of \G is transitive on the
edges of \G. % In this paper, we give a characterisation of normal
edge-transitive bi-Cayley graphs, %which form an important subfamily of
bi-Cayley graphs, and in particular, we give a detailed description of
-arc-transitive normal bi-Cayley graphs. Using this, we investigate three
classes of bi-Cayley graphs, namely those over abelian groups, dihedral groups
and metacyclic -groups. We find that under certain conditions, `normal
edge-transitive' is the same as `normal' for graphs in these three classes. As
a by-product, we obtain a complete classification of all connected trivalent
edge-transitive graphs of girth at most , and answer some open questions
from the literature about -arc-transitive, half-arc-transitive and
semisymmetric graphs
Impacts of Opinion Leaders on Social Contagions
Opinion leaders are ubiquitous in both online and offline social networks,
but the impacts of opinion leaders on social behavior contagions are still not
fully understood, especially by using a mathematical model. Here we generalize
the classical Watts threshold model and address the influences of the opinion
leaders, where an individual adopts a new behavior if one of his/her opinion
leaders adopts the behavior. First, we choose the opinion leaders randomly from
all individuals in the network and find the impacts of opinion leaders make
other individuals adopt the behavior more easily. Specifically, the existence
of opinion leaders reduces the lowest mean degree of the network required for
the global behavior adoption, and increases the highest mean degree of the
network that the global behavior adoption can occur. Besides, the introduction
of opinion leaders accelerates the behavior adoption, but does not change the
adoption order of individuals. The developed theoretical predictions agree with
the simulation results. Second, we randomly choose the opinion leaders from the
top h% of the highest degree individuals, and find an optimal h% for the
network with the lowest mean degree that the global behavior adoption can
occur. Meanwhile, the influences of opinion leaders on accelerating the
adoption of behaviors become less significant and can even be ignored when
reducing the value of h%.Comment: Accepted by Chao
Automatic Differentiation for Complex Valued SVD
In this note, we report the back propagation formula for complex valued
singular value decompositions (SVD). This formula is an important ingredient
for a complete automatic differentiation(AD) infrastructure in terms of complex
numbers, and it is also the key to understand and utilize AD in tensor
networks.Comment: 4.2 page
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