903 research outputs found
Equivariant Moduli Theory on Surfaces
We study the orbifold Hirzebruch-Riemann-Roch (HRR) theorem for quotient
Deligne-Mumford stacks, explore its relation with representation theory of
finite groups, and derive a new orbifold HRR formula via an orbifold Mukai
pairing. As a first application, we use this formula to compute the dimensions
of -equivariant moduli spaces of stable sheaves on a surface
under the action of a finite subgroup of its symplectic automorphism
group. We then apply the orbifold HRR formula to reproduce the number of fixed
points on when is cyclic without using the Lefschetz fixed point
formula. We prove that under some mild conditions, equivariant moduli spaces of
stable sheaves on are irreducible symplectic manifolds deformation
equivalent to Hilbert schemes of points on via a connection between
Gieseker and Bridgeland moduli spaces, as well as the derived McKay
correspondence. As a corollary, these moduli spaces are also deformation
equivalent to equivariant Hilbert schemes of points on Comment: Made many minor changes, added more materials, and corrected some
mistake
Downside Risk and the Momentum Effect
Stocks with greater downside risk, which is measured by higher correlations conditional on downside moves of the market, have higher returns. After controlling for the market beta, the size effect and the book-to-market effect, the average rate of return on stocks with the greatest downside risk exceeds the average rate of return on stocks with the least downside risk by 6.55% per annum. Downside risk is important for explaining the cross-section of expected returns. In particular of the profitability of investing in momentum strategies can be explained as compensation for bearing high exposure to downside risk.
Downside Risk
Economists have long recognized that investors care differently about downside losses versus upside gains. Agents who place greater weight on downside risk demand additional compensation for holding stocks with high sensitivities to downside market movements. We show that the cross-section of stock returns reflects a premium for downside risk. Specifically, stocks that covary strongly with the market when the market declines have high average returns. We estimate that the downside risk premium is approximately 6% per annum. The reward for bearing downside risk is not simply compensation for regular market beta, nor is it explained by coskewness or liquidity risk, or size, book-to-market, and momentum characteristics.
Game among Interdependent Networks: The Impact of Rationality on System Robustness
Many real-world systems are composed of interdependent networks that rely on
one another. Such networks are typically designed and operated by different
entities, who aim at maximizing their own payoffs. There exists a game among
these entities when designing their own networks. In this paper, we study the
game investigating how the rational behaviors of entities impact the system
robustness. We first introduce a mathematical model to quantify the interacting
payoffs among varying entities. Then we study the Nash equilibrium of the game
and compare it with the optimal social welfare. We reveal that the cooperation
among different entities can be reached to maximize the social welfare in
continuous game only when the average degree of each network is constant.
Therefore, the huge gap between Nash equilibrium and optimal social welfare
generally exists. The rationality of entities makes the system inherently
deficient and even renders it extremely vulnerable in some cases. We analyze
our model for two concrete systems with continuous strategy space and discrete
strategy space, respectively. Furthermore, we uncover some factors (such as
weakening coupled strength of interdependent networks, designing suitable
topology dependency of the system) that help reduce the gap and the system
vulnerability
Semi-Supervised and Long-Tailed Object Detection with CascadeMatch
This paper focuses on long-tailed object detection in the semi-supervised
learning setting, which poses realistic challenges, but has rarely been studied
in the literature. We propose a novel pseudo-labeling-based detector called
CascadeMatch. Our detector features a cascade network architecture, which has
multi-stage detection heads with progressive confidence thresholds. To avoid
manually tuning the thresholds, we design a new adaptive pseudo-label mining
mechanism to automatically identify suitable values from data. To mitigate
confirmation bias, where a model is negatively reinforced by incorrect
pseudo-labels produced by itself, each detection head is trained by the
ensemble pseudo-labels of all detection heads. Experiments on two long-tailed
datasets, i.e., LVIS and COCO-LT, demonstrate that CascadeMatch surpasses
existing state-of-the-art semi-supervised approaches -- across a wide range of
detection architectures -- in handling long-tailed object detection. For
instance, CascadeMatch outperforms Unbiased Teacher by 1.9 AP Fix on LVIS when
using a ResNet50-based Cascade R-CNN structure, and by 1.7 AP Fix when using
Sparse R-CNN with a Transformer encoder. We also show that CascadeMatch can
even handle the challenging sparsely annotated object detection problem.Comment: International Journal of Computer Vision (IJCV), 202
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