2,562 research outputs found
The nullity of the net Laplacian matrix of a signed graph
Let be a signed graph, where is an
(unsigned) graph, called the underlying graph. The net Laplacian matrix of
is defined as , where
and are the diagonal matrix of net-degrees and
the adjacency matrix of , respectively.
The nullity of , written as , is
the multiplicity of 0 as an eigenvalue of . In this paper, we
focus our attention on the nullity of the net Laplacian matrix of a connected
signed graph and prove that , where is the cyclomatic number of . The connected signed
graphs with nullity are completely determined. Moreover, we
characterize the signed cactus graphs with nullity or Comment: 11 pages, 1 figure
Navigation in a small world with local information
It is commonly known that there exist short paths between vertices in a
network showing the small-world effect. Yet vertices, for example, the
individuals living in society, usually are not able to find the shortest paths,
due to the very serious limit of information. To theoretically study this
issue, here the navigation process of launching messages toward designated
targets is investigated on a variant of the one-dimensional small-world network
(SWN). In the network structure considered, the probability of a shortcut
falling between a pair of nodes is proportional to , where is
the lattice distance between the nodes. When , it reduces to the SWN
model with random shortcuts. The system shows the dynamic small-world (SW)
effect, which is different from the well-studied static SW effect. We study the
effective network diameter, the path length as a function of the lattice
distance, and the dynamics. They are controlled by multiple parameters, and we
use data collapse to show that the parameters are correlated. The central
finding is that, in the one-dimensional network studied, the dynamic SW effect
exists for . For each given value of in this
region, the point that the dynamic SW effect arises is ,
where is the number of useful shortcuts and is the average
reduced (effective) length of them.Comment: 10 pages, 5 figures, accepted for publication in Physical Review
On the eigenvalues and Seidel eigenvalues of chain graphs
In this paper we consider the eigenvalues and the Seidel eigenvalues of a
chain graph. An\dbareli\'{c}, da Fonseca, Simi\'{c}, and Du
\cite{andelic2020tridiagonal} conjectured that there do not exist
non-isomorphic cospectral chain graphs with respect to the adjacency spectrum.
Here we disprove this conjecture. Furthermore, by considering the relation
between the Seidel matrix and the adjacency matrix of a graph, we solve two
problems on the number of distinct Seidel eigenvalues of a chain graph, which
was posed by Mandal, Mehatari, and Das \cite{mandal2022spectrum}.Comment: 13 pages, 2 figure
Affine Transformation Edited and Refined Deep Neural Network for Quantitative Susceptibility Mapping
Deep neural networks have demonstrated great potential in solving dipole
inversion for Quantitative Susceptibility Mapping (QSM). However, the
performances of most existing deep learning methods drastically degrade with
mismatched sequence parameters such as acquisition orientation and spatial
resolution. We propose an end-to-end AFfine Transformation Edited and Refined
(AFTER) deep neural network for QSM, which is robust against arbitrary
acquisition orientation and spatial resolution up to 0.6 mm isotropic at the
finest. The AFTER-QSM neural network starts with a forward affine
transformation layer, followed by an Unet for dipole inversion, then an inverse
affine transformation layer, followed by a Residual Dense Network (RDN) for QSM
refinement. Simulation and in-vivo experiments demonstrated that the proposed
AFTER-QSM network architecture had excellent generalizability. It can
successfully reconstruct susceptibility maps from highly oblique and
anisotropic scans, leading to the best image quality assessments in simulation
tests and suppressed streaking artifacts and noise levels for in-vivo
experiments compared with other methods. Furthermore, ablation studies showed
that the RDN refinement network significantly reduced image blurring and
susceptibility underestimation due to affine transformations. In addition, the
AFTER-QSM network substantially shortened the reconstruction time from minutes
using conventional methods to only a few seconds
DPR: An Algorithm Mitigate Bias Accumulation in Recommendation feedback loops
Recommendation models trained on the user feedback collected from deployed
recommendation systems are commonly biased. User feedback is considerably
affected by the exposure mechanism, as users only provide feedback on the items
exposed to them and passively ignore the unexposed items, thus producing
numerous false negative samples. Inevitably, biases caused by such user
feedback are inherited by new models and amplified via feedback loops.
Moreover, the presence of false negative samples makes negative sampling
difficult and introduces spurious information in the user preference modeling
process of the model. Recent work has investigated the negative impact of
feedback loops and unknown exposure mechanisms on recommendation quality and
user experience, essentially treating them as independent factors and ignoring
their cross-effects. To address these issues, we deeply analyze the data
exposure mechanism from the perspective of data iteration and feedback loops
with the Missing Not At Random (\textbf{MNAR}) assumption, theoretically
demonstrating the existence of an available stabilization factor in the
transformation of the exposure mechanism under the feedback loops. We further
propose Dynamic Personalized Ranking (\textbf{DPR}), an unbiased algorithm that
uses dynamic re-weighting to mitigate the cross-effects of exposure mechanisms
and feedback loops without additional information. Furthermore, we design a
plugin named Universal Anti-False Negative (\textbf{UFN}) to mitigate the
negative impact of the false negative problem. We demonstrate theoretically
that our approach mitigates the negative effects of feedback loops and unknown
exposure mechanisms. Experimental results on real-world datasets demonstrate
that models using DPR can better handle bias accumulation and the universality
of UFN in mainstream loss methods
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