Seung-Woo Son

Finding communities in directed networks

To identify communities in directed networks, we propose a generalized form of modularity in directed networks by presenting the quantity LinkRank, which can be considered as the PageRank of links. This generalization is consistent with the original modularity in undirected networks and the modularity optimization methods developed for undirected networks can be directly applied to directed networks by optimizing our modified modularity. Also, a model network, which can be used as a benchmark network in further community studies, is proposed to verify our method. Our method is supposed to find communities effectively in citation- or reference-based directed networks.

A protein interaction network associated with asthma.

Identifying candidate genes related to complex diseases or traits and mapping their relationships require a system-level analysis at a cellular scale. The objective of the present study is to systematically analyze the complex effects of interrelated genes and provide a framework for revealing their relationships in association with a specific disease (asthma in this case). We observed that protein-protein interaction (PPI) networks associated with asthma have a power-law connectivity distribution as many other biological networks have. The hub nodes and skeleton substructure of the result network are consistent with the prior knowledge about asthma pathways, and also suggest unknown candidate target genes associated with asthma, including GNB2L1, BRCA1, CBL, and VAV1. In particular, GNB2L1 appears to play a very important role in the asthma network through frequent interactions with key proteins in cellular signaling. This network-based approach represents an alternative method for analyzing the complex effects of candidate genes associated with complex diseases and suggesting a list of gene drug targets. The full list of genes and the analysis details are available in the following online supplementary materials: http://biosoft.kaist.ac.kr:8080/resources/asthma_ppi.

Random field Ising model and community structure in complex networks

Abstract. We propose a method to determine the communitynstructure of a complex network. In this method the ground statenproblem of a ferromagnetic random field Ising model is considerednon the network with the magnetic field Bs = +∞, Bt =n-∞, and Bi≠s,t=0 for a node pair s and t. Thenground state problem is equivalent to the so-called maximum flownproblem, which can be solved exactly numerically with the help ofna combinatorial optimization algorithm. The community structure isnthen identified from the ground state Ising spin domains for allnpairs of s and t. Our method provides a criterion for thenexistence of the community structure, and is applicable equallynwell to unweighted and weighted networks. We demonstrate thenperformance of the method by applying it to the Barabási-Albertnnetwork, Zachary karate club network, the scientific collaborationnnetwork, and the stock price correlation network.n (Ising, Potts, etc.) n

Design of an integrated tactile display system

Tactile sensation is essential for many exploration and manipulation tasks not only in a real environment but also in a virtual environment. We suggest the design of an integrated tactile display system that provides kinesthetic force, pressure distribution, vibration and slip/stretch. The system consists of two parts: a 2 DOF force feedback device for kinesthetic display and a tactile feedback device for displaying the normal stimulation to the skin and the skin slip/stretch. Psychophysical experiments measure the effects of fingerpad selection, the direction of finger movements and the texture width on tactile sensitivity. We also investigate the characteristics of lateral finger movement while subjects perceive different textures. From the experimental results, the principal parameters for designing a tactile display are suggested. A tactile display device, using eight piezoelectric bimorphs and a linear actuator, is implemented and attached to a 2 DOF translational force feedback device to simultaneously simulate the texture and stiffness of the object. As a result, we find out that the capability of the suggested device is sufficient to display physical quantities for tactile sensing.

Shape retaining chain linked model for real-time volume haptic rendering

Haptic rendering is the process of computing and generating forces in response to user interactions with virtual objects. While we speak of real-time volume rendering for visualization, we are still very much limited to Surface models for manipulation due to overwhelming computational requirements for volumetric models. In this paper, we propose a new volumetric deformable model that is suitable for volume haptic interactions. The volume elements of our proposed model are linked to their nearest neighbors and their displacements are transformed into potential energy of the virtual object. The original 3D ChainMail algorithm does not account the fact that the residual energy left in the object after some interactions becomes a critical problem in haptic rendering. We present the shape-retaining chain linked model, which allows for fast and realistic deformation of elastic objects. Furthermore, we incorporate force-voltage analogy (duality) concepts into the proposed shape-retaining chain linked representation in order to develop a fast volumetric haptic model that is Suitable for realtime applications. We experimented with homogenous and non-homogenous virtual objects of size 75/spl times/75/spl times/75 volume elements, and we were able to verify real-time and realistic haptic interaction with a 3DOF PHANToM/spl trade/ haptic device.

Interactive Mouse Systems Providing Haptic Feedback During the Exploration in Virtual Environment

This paper presents two kinds of novel haptic mouse systems as new human computer interfaces, which have a force and tactile feedback capability. The first one can reflect 1 dof grabbing force as well as 2 dof translation force. Five-bar mechanism has been adapted to realize the 2 dof translation force feedback, and double prismatic joint mechanism has been used to implement the grabbing force feedback. This system helps the user to feel grabbing force, contact force and weight while picking up and moving an object in virtual environment. The second system can simulate the surface roughness as well as contact force. This system consists of two parts: a 2 DOF force fee dback device for kinesthetic display and a tactile feedback unit for displaying the normal stimulation to the skin and the skin stretch. The proposed systems are expected to be used as new human computer interfaces by presenting rea listic haptic interaction in ecommerce or VR environment.

How to Effectively Display Surface Properties Using an Integrated Tactile Display System

In this paper, we suggest a mouse-type integrated tactile display system that provides kinesthetic force, distributed pressure, vibration and skin stretch. The system consists of two parts: a 2-DOF force feedback mechanism for kinesthetic force display and a tactile display part for providing normal stimulation to the skin and the lateral skin stretch. A tactile display device, using eight piezoelectric bimorphs and a linear actuator, is fabricated and attached to a 2-DOF translational force feedback device to simultaneously simulate the texture and stiffness of the object. The developed system was adopted as a test bed to study the factors affecting perception of surface properties. We investigate three things: effective stimulating methods, limitation of surface discrimination based on kinesthetic force and the effectiveness of combining kinesthetic force feedback with tactile feedback. Seven kinds of stimulating methods are investigated to study effective methods in simulating texture. In addition, in order to find out the role of skin stretch during perceiving texture, the discernment abilities of active touch (rubbing), laterally-active-display and moving-wave-form have been compared.

Anomalous scaling behavior in polymer thin film growth by vapor deposition

As a first step towards understanding the anomalous kinetic roughening with multifractality found in recent experiments on vapor deposition polymerization (VDP) growth, we study a simple toy model of the VDP growth in a (1+1)-dimensional lattice, along with monomer diffusion, polymer nucleation, limited active end bonding, and shadowing effects. Using extensive numerical simulations, we observe that the global roughness exponent is different from the local one. It is argued that such anomalies in VDP growth are attributable to the instability induced by the non-local shadowing effects on active ends of polymers. Varying the ratio of the diffusion coefficient to the deposition rate by means of a cosine flux, we also consider the role of diffusion in kinetic roughening of polymer thin film growth, which is quite different from that for metal or semiconductor film growth. Finally, we suggest a (2+1)-dimensional version, which can be directly compared with experimental results.

Underlying Scale-Free Trees in Complex Networks

We investigate the properties of two relatively different spanning trees of complex networks, so-called communication kernel and response network. First, for the communication kernel, we construct spanning trees carrying a maximum total weight of edges that is given by average traffic, which is defined as edge betweenness centrality. It is found that the resulting spanning tree plays an important role in communication between vertices. We also find that the degree distribution of spanning trees shows scale-free behavior for many model and real-world networks and the degree of the spanning trees has strong correlation with their original network topology. For the response network, we launch an attack on a single vertex which can drastically change the communication pattern between vertices of networks. By using minimum spanning tree technique, we construct the response network based on the measurement of the betweenness centrality changes due to a vertex removal. We find that the degree distribution of the response network indicates the scale-free behavior as well as that of the communication kernel. Interestingly, these two minimum spanning trees from different methods not only have same scale-free behavior but overlap each other in their structures. This fact indicates that the complex network has a concrete skeleton, scale-free tree, as a basic structure.

Phase Transition of Active Rotators in Complex Networks

We study the nonequilibrium phenomena of a coupled active rotator model in complex networks. From a numerical Langevin simulation, we find the peculiar phase transition not only on globally connected network but also on other complex networks and reveal the corresponding phase diagram. In this model, two phases — stationary and quasi-periodic moving phases — are observed, in which microscopic dynamics are thoroughly investigated. We extend our study to the non-identical oscillators and the more heterogeneous degree distribution of complex networks.