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Stochastic Doubly Robust Gradient [article]

Kanghoon Lee, Jihye Choi, Moonsu Cha, Jung-Kwon Lee, Taeyoon Kim
2018 arXiv   pre-print
When training a machine learning model with observational data, it is often encountered that some values are systemically missing. Learning from the incomplete data in which the missingness depends on some covariates may lead to biased estimation of parameters and even harm the fairness of decision outcome. This paper proposes how to adjust the causal effect of covariates on the missingness when training models using stochastic gradient descent (SGD). Inspired by the design of doubly robust
more » ... mator and its theoretical property of double robustness, we introduce stochastic doubly robust gradient (SDRG) consisting of two models: weight-corrected gradients for inverse propensity score weighting and per-covariate control variates for regression adjustment. Also, we identify the connection between double robustness and variance reduction in SGD by demonstrating the SDRG algorithm with a unifying framework for variance reduced SGD. The performance of our approach is empirically tested by showing the convergence in training image classifiers with several examples of missing data.
arXiv:1812.08997v1 fatcat:lrisiqakcfhp3bcu4xupazhpvm

Double copying Exceptional Field theories [article]

David S. Berman, Kwangeon Kim, Kanghoon Lee
2022 arXiv   pre-print
We examine exceptional field theory through the lens of the generalised double copy formalism. This allows us to construct classical solutions in M-theory using a generalised Kerr-Schild ansatz and along the way indicates hints towards a single copy of M-theory. Based on a talk at the Nankai Symposium given by DSB.
arXiv:2201.10854v1 fatcat:655qmoxvtfgg5i44bbtr2kteky

New gaugings and non-geometry [article]

Kanghoon Lee, Charles Strickland-Constable, Daniel Waldram
2015 arXiv   pre-print
We discuss the possible realisation in string/M theory of the recently discovered family of four-dimensional maximal SO(8) gauged supergravities, and of an analogous family of seven-dimensional half-maximal SO(4) gauged supergravities. We first prove a no-go theorem that neither class of gaugings can be realised via a compactification that is locally described by ten- or eleven-dimensional supergravity. In the language of Double Field Theory and its M theory analogue, this implies that the
more » ... on condition must be violated. Introducing the minimal number of additional coordinates possible, we then show that the standard S^3 and S^7 compactifications of ten- and eleven-dimensional supergravity admit a new class of section-violating generalised frames with a generalised Lie derivative algebra that reproduces the embedding tensor of the SO(4) and SO(8) gaugings respectively. The physical meaning, if any, of these constructions is unclear. They highlight a number of the issues that arise when attempting to apply the formalism of Double Field Theory to non-toroidal backgrounds. Using a naive brane charge quantisation to determine the periodicities of the additional coordinates restricts the SO(4) gaugings to an infinite discrete set and excludes all the SO(8) gaugings other than the standard one.
arXiv:1506.03457v1 fatcat:guzznndohbfijllsqdywq5jcwu

Quantum Off-Shell Recursion Relation [article]

Kanghoon Lee
2022 arXiv   pre-print
We construct off-shell recursion relations for arbitrary loop-level scattering amplitudes beyond the conventional tree-level recursion relations for ϕ^4-theory and the Yang-Mills theory. We define a quantum perturbiner expansion that includes loop corrections from the quantum effective action formalism by identifying the external source. Our method clearly shows how the perturbiner expansion becomes an off-shell current generating function. Instead of using the classical equations of motion in
more » ... he conventional perturbiner method, we exploit the Dyson-Schwinger equation to derive the quantum off-shell recursion relation to arbitrary order of loop-level scattering amplitudes. We solve the recursion relation and reproduce the results which agree up to one-loop six-point scattering amplitudes for ϕ^4-theory. Furthermore, we construct the recursions for computing loop-level correlation functions by replacing the choice of the external source.
arXiv:2202.08133v2 fatcat:iayj7ofgxjcfvjcky52c66lf2i

Kerr-Schild Double Field Theory and Classical Double Copy [article]

Kanghoon Lee
2018 arXiv   pre-print
The Kerr-Schild (KS) formalism is a powerful tool for constructing exact solutions in general relativity. In this paper, we present a generalization of the conventional KS formalism to double field theory (DFT) and supergravities. We introduce a generalized KS ansatz for the generalized metric in terms of a pair of null vectors. Applying this ansatz to the equations of motion of DFT, we construct the generalized KS field equation. While the generalized KS equations are quadratic in the fields,
more » ... e show that it is possible to find solutions by considering linear equations only. Furthermore, we construct a Killing spinor equation under the generalized KS ansatz. Based on this formalism, we show that the classical double copy structure, which represents solutions of the Einstein equation in terms of solutions of the Maxwell equation, can be extended to the entire massless string NS-NS sector. We propose a supersymmetric classical double copy which shows that solutions of the Killing spinor equation can be realized in terms of solutions of the BPS equation of the supersymmetric Maxwell theory.
arXiv:1807.08443v1 fatcat:2ri7y4mqbbew3abrmhywo6mwbq

Bayesian optimistic Kullback–Leibler exploration

Kanghoon Lee, Geon-Hyeong Kim, Pedro Ortega, Daniel D. Lee, Kee-Eung Kim
2018 Machine Learning  
We consider a Bayesian approach to model-based reinforcement learning, where the agent uses a distribution of environment models to find the action that optimally trades off exploration and exploitation. Unfortunately, it is intractable to find the Bayes-optimal solution to the problem except for restricted cases. In this paper, we present BOKLE, a simple algorithm that uses Kullback-Leibler divergence to constrain the set of plausible models for guiding the exploration. We provide a formal
more » ... ysis that this algorithm is near Bayes-optimal with high probability. We also show an asymptotic relation between the solution pursued by BOKLE and a well-known algorithm called Bayesian exploration bonus. Finally, we show experimental results that clearly demonstrate the exploration efficiency of the algorithm.
doi:10.1007/s10994-018-5767-4 fatcat:wmph4v5jkrdjji5g4f7b6ugzre

Vortices and 3 dimensional dualities [article]

Hee-Cheol Kim, Jungmin Kim, Seok Kim, Kanghoon Lee
2012 arXiv   pre-print
Acknowledgements We are grateful to Tudor Dimofte, Dongmin Gang, Choonkyu Lee, Kimyeong Lee, Sungjay Lee, This work is supported by the BK21 program of the Ministry of Education, Science and Technology  ... 
arXiv:1204.3895v1 fatcat:sxi2gcvu3zfdrcoiix5skbhlji

Development of a genetically-encoded oxytocin sensor [article]

Neymi Layne Mignocchi, Sarah Kruessel, Kanghoon Jung, Dongmin Lee, Hyung-Bae Kwon
2020 bioRxiv   pre-print
Lee et al., 2017) were both 239 digested with the ClaI restriction enzyme.  ...  Lee & Dan, 2012; Yagishita et al., 2014). Especially, neuropeptides are small molecules 44 synthesized and released from neurons and affect activity and signaling of neurons.  ... 
doi:10.1101/2020.07.14.202598 fatcat:mlvmhan7mzblrbagi563uipzee

Design of Imperceptible Metamer Markers and Application Systems

Kanghoon Lee, Kyudong Sim, Taeyoung Uhm, Sang Hwa Lee, Jong-Il Park
2020 IEEE Access  
For more information, see https://creativecommons.org/licenses/by/4.0/ KANGHOON LEE received the B.S. degree in multimedia engineering from Hansung University, Seoul, South Korea, in 2008.  ...  SANG HWA LEE received the B.S. and M.S. degrees in electronics and the Ph.D. degree in electrical engineering from Seoul National University, Seoul, South Korea, in 1994, 1996, and 2000, respectively.  ... 
doi:10.1109/access.2020.2980869 fatcat:sjrlp4mr5bf5bpb7cokj2nwlu4

Towards weakly constrained double field theory

Kanghoon Lee
2016 Nuclear Physics B  
We show that it is possible to construct a well-defined effective field theory incorporating string winding modes without using strong constraint in double field theory. We show that X-ray (Radon) transform on a torus is well-suited for describing weakly constrained double fields, and any weakly constrained fields are represented as a sum of strongly constrained fields. Using inverse X- ray transform we define a novel binary operation which is compatible with the level matching constraint.
more » ... on this formalism, we construct a consistent gauge transform and gauge invariant action without using strong constraint. We then discuss the relation of our result to the closed string field theory. Our construction suggests that there exists an effective field theory description for massless sector of closed string field theory on a torus in an associative truncation.
doi:10.1016/j.nuclphysb.2016.05.015 fatcat:7tuyqmomtnfongabdw7433ym5i

Property database for single-element doping in ZnO obtained by automated first-principles calculations

Kanghoon Yim, Joohee Lee, Dongheon Lee, Miso Lee, Eunae Cho, Hyo Sug Lee, Ho-Hyun Nahm, Seungwu Han
2017 Scientific Reports  
Throughout the past decades, doped-ZnO has been widely used in various optical, electrical, magnetic, and energy devices. While almost every element in the Periodic Table was doped in ZnO, the systematic computational study is still limited to a small number of dopants, which may hinder a firm understanding of experimental observations. In this report, we systematically calculate the single-element doping property of ZnO using first-principles calculations. We develop an automation code that
more » ... bles efficient and reliable high-throughput calculations on thousands of possible dopant configurations. As a result, we obtain formation-energy diagrams for total 61 dopants, ranging from Li to Bi. Furthermore, we evaluate each dopant in terms of n-type/p-type behaviors by identifying the major dopant configurations and calculating carrier concentrations at a specific dopant density. The existence of localized magnetic moment is also examined for spintronic applications. The property database obtained here for doped ZnO will serve as a useful reference in engineering the material property of ZnO through doping. As a representative semiconducting oxide, zinc oxide (ZnO) is arguably one of the most popular material in science and technology. Its application areas encompass essentially every engineering field; electronic and optoelectronic devices 1-3 , sensors and catalysts 4-6 , and piezoelectric devices for energy harvesting 7 . Such a wide popularity of ZnO stems from several intrinsic/extrinsic advantages as a functional material; low cost, high electron mobility, transparency, and easy fabrication into various nanostructures such as nanorods, nanowires, nanobelts, and nanotubes. Another important advantage of ZnO is facile external doping 8 . As summarized in Fig. 1 , the dopants in ZnO cover most of atoms in the Periodic Table, and they vary depending on the application target. Specifically, for transparent electrodes, elements such as Al, Ga, and In (P and As) atoms are incorporated to increase electron (hole) carrier densities 9-12 . To enhance absorption of visible lights, various dopants are chosen that create defect levels within the band gap or shift band-edge positions, narrowing the optical gap 13,14 . In sensor and catalyst applications, transition and novel metals are doped to increase active sites that accelerate chemical reactions 5,6 . Lastly, the magnetic impurities are incorporated to use ZnO as the spin-transport layer in spintronic devices 15 . Above examples show that doping is a powerful tool to tailor the optical, electrical, chemical, and magnetic properties of ZnO. However, the detailed mechanism underlying the property tuning is not easily revealed by experimental analysis alone because defects or dopants are difficult to investigate with spectroscopic tools. Theoretical study based on the first-principles calculations is very useful in this respect, and several studies successfully enlightened the atomistic origin of property changes induced by dopants 16, 17 . However, systematic and thorough studies have been limited to representative elements 12,16,18-26 . In Fig. 1 , elements that have been fully studied are marked in red outlines while those in blue indicate partial studies. Here, the 'fully studied' means that i) possible dopant configurations have been compared with respect to the defect formation energy and ii)
doi:10.1038/srep40907 pmid:28112188 pmcid:PMC5256097 fatcat:ugz5mfklqvbo5fg3nogkuxvon4

Emergent dual holographic description as a non-perturbative generalization of the Wilsonian renormalization group [article]

Ki-Seok Kim, Shinsei Ryu, Kanghoon Lee
2021 arXiv   pre-print
In holographic duality, dynamics along the emergent extra-dimensional space describes a renormalization group (RG) flow of the corresponding quantum field theory (QFT). Following this idea, we develop an emergent holographic description of a QFT, where not only the information of the RG flow is introduced into an IR holographic dual effective field theory (HDEFT), but also the UV information of the QFT is encoded in the HDEFT through the IR boundary condition. In particular, we argue that this
more » ... ual holographic construction is self-consistent within the assumption of bulk locality, showing the following two aspects: The solution of the Hamilton-Jacobi equation is given by the IR boundary effective action, and the Ward identity involving the QFT energy-momentum tensor current is satisfied naturally. We discuss the role of the RG β-function in the bulk effective dynamics of the metric tensor near a conformally invariant fixed point. We claim that this emergent dual gravity theory generalizes the perturbative Wilsonian RG framework into a non-perturbative way.
arXiv:2112.06237v1 fatcat:cci4mupfdffo3kezf2w7epi4aa

Computational discovery of p-type transparent oxide semiconductors using hydrogen descriptor

Kanghoon Yim, Yong Youn, Miso Lee, Dongsun Yoo, Joohee Lee, Sung Haeng Cho, Seungwu Han
2018 npj Computational Materials  
The ultimate transparent electronic devices require complementary and symmetrical pairs of n-type and p-type transparent semiconductors. While several n-type transparent oxide semiconductors like InGaZnO and ZnO are available and being used in consumer electronics, there are practically no p-type oxides that are comparable to the n-type counterpart in spite of tremendous efforts to discover them. Recently, high-throughput screening with the density functional theory calculations attempted to
more » ... ntify candidate p-type transparent oxides, but none of suggested materials was verified experimentally, implying need for a better theoretical predictor. Here, we propose a highly reliable and computationally efficient descriptor for p-type dopability-the hydrogen impurity energy. We show that the hydrogen descriptor can distinguish well-known p-type and n-type oxides. Using the hydrogen descriptor, we screen most binary oxides and a selected pool of ternary compounds that covers Sn 2+ -bearing and Cu 1+ -bearing oxides as well as oxychalcogenides. As a result, we suggest La 2 O 2 Te and CuLiO as promising p-type oxides.
doi:10.1038/s41524-018-0073-z fatcat:kjgmk3gbq5dw7nq6gui2tl74kq

Heterotic Kerr-Schild Double Field Theory and Classical Double Copy [article]

Wonyong Cho, Kanghoon Lee
2019 arXiv   pre-print
We discuss the generalization of the Kerr-Schild (KS) formalism for general relativity and double field theory (DFT) to the heterotic DFT and supergravity. We first introduce a heterotic KS ansatz by introducing a pair of null O(d,d+G) generalized tangent vectors. The pair of null vectors are represented by a pair of d-dimensional vector fields, and one of the vector fields is not a null vector. This implies that the null property of the usual KS formalism, which plays a crucial role in
more » ... ing the field equations, can be partially relaxed in a consistent way. We show that the equations of motion under the heterotic KS ansatz in a flat background can be reduced to linear equations. Using the heterotic KS equations, we establish the single and zeroth copy for heterotic supergravity and derive the Maxwell and Maxwell-scalar equations. This agrees with the KLT relation for heterotic string theory.
arXiv:1904.11650v1 fatcat:gorm5ffoojhqzbz5qra3rcobdq

Popliteal Artery Entrapment Syndrome: A Case with Bilateral Different Types

Eun Joo Lee, Jae Seung Jung, Kanghoon Lee, Seung Hun Lee, Ho Sung Son, Kyung Sun
2014 Korean Journal of Thoracic and Cardiovascular Surgery  
Popliteal artery entrapment syndrome (PAES) is a non-artherosclerotic cause of claudication and acute ischemia of the legs in young athletic individuals. It is classified in terms of the abnormal anatomical relationship between the popliteal artery and surrounding structures. All types of PAES have the same pathophysiology. Repetitive arterial compression by surrounding structures causes progressive vascular injury. Bilateral PAES is reported in about 30% of cases. Bilateral PAES is usually of
more » ... he same type in each artery; exceptions are rare. We report a case of a young athletic patient who suffered bilateral PAES of two different types. A previously healthy 22-year-old male had subacute claudication in both legs 3 months prior. He was a non-smoker and
doi:10.5090/kjtcs.2014.47.3.302 pmid:25207233 pmcid:PMC4157486 fatcat:rcwicvs4ybfnjn465o2l6clbpy
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