• The Bulletin of The Korean Astronomical Society
• /
• v.34 no.1
• /
• pp.151.1-151.1
• /
• 2009

• Bulletin of the Korean Mathematical Society
• /
• v.49 no.4
• /
• pp.863-874
• /
• 2012

We provide a study of lightlike hypersurfaces of a generalized Robertson-Walker (GRW) space-time. In particular, we investigate lightlike hypersurfaces with curvature invariance, parallel second fundamental forms, totally umbilical second fundamental forms, null sectional curvatures and null Ricci curvatures, respectively.

• Bulletin of the Korean Mathematical Society
• /
• v.38 no.2
• /
• pp.285-292
• /
• 2001

The authors compute the Ricci curvature of the GRW space-time to obtain two conditions for the conjugate points which appear as the Timelike Convergence Condition(TCG) and the Jacobi inequality. Moreover, under such two conditions, we obtain a lower bound of the length of a unit timelike geodesic for focal points emanating form the immersed spacelike hypersurface, the graph over the fiber in the GRW space-time.

• 제어로봇시스템학회:학술대회논문집
• /
• 1986.10a
• /
• pp.321-325
• /
• 1986

A new control scheme for the state space model of the robot system using the theory of optimal multi-variable structure is presented in this paper. It is proposed to optimize multi-dimensional variable structure systems for obtaining the required stabilizing signal by minimizing a performance index with respect to the state vector in the sliding mode. It is concluded the proposed variable structure controller yields better system dynamic performance than that obtained by using the only linear optimal controller inthat responses for a step disturbance have a shorter setting time, no matter what overshoot values and rising time.

• 제어로봇시스템학회:학술대회논문집
• /
• 1990.10a
• /
• pp.388-393
• /
• 1990

A robot manipulator and an obstacle are described mathematically in joint space, with the mathematical representation for the collision between the robot manipulator and the obstacle. Using these descriptions, the robot motion planning problem is formulated which can be used to avoide a time varying obstacle. To solve the problem, the constraints on motion planning are discretized in joint space. An analytical method is proposed for planning the motion in joint space from a given starting point to the goal point. It is found that solving the inverse kinematics problem is not necessary to get the control input to the joint motion controller for collision avoidance.

• Communications of the Korean Mathematical Society
• /
• v.11 no.1
• /
• pp.215-224
• /
• 1996

In this paper, we classify hypersurfaces M in $S^{n+1}_{1}$ or in $H^{n+1}$ satisfying $\Delta x = Rx + b$.

• The Journal of Korean Institute of Communications and Information Sciences
• /
• v.32 no.1B
• /
• pp.11-16
• /
• 2007

This paper describes the design and performance of a secure routing protocol with time-space cryptography for mobile ad-hoc networks. The proposed time-space scheme works in the time domain for key distribution between source and destination as well as in the space domain for intrusion detection along the route between them. For data authentication, it relies on the symmetric key cryptography due to high efficiency and a secret key is distributed using a time difference from the source to the destination. Also, a one-way hash chain is formed on a hop-by-hop basis to prevent a compromised node or an intruder from manipulating the routing information. In order to evaluate the performance of our routing protocol, we compare it with the existing AODV protocol by simulation under the same conditions. The proposed protocol has been validated using the ns-2 network simulator with wireless and mobility extensions.

• Journal of Korea Water Resources Association
• /
• v.47 no.10
• /
• pp.959-972
• /
• 2014

In this study, a space-time rainfall grid field generation model based on multifractal theory was verified using nine flood events in the upstream watershed of Chungju dam in South Korea. For this purpose, KMA radar rainfall data sets were analyzed for the space-time multifractal characteristics. Simulated rainfall fields that represent the multifractal characteristics of observed rainfall field were reproduced using the space-time rainfall grid field generation model with log-Poisson distribution and three-dimension wavelet function. Simulated rainfall fields were applied to the S-RAT model as input data and compared with both observed rainfall fields and low-resolution rainfall field runoff. Error analyses using RMSE, RRMSE, MAE, SS, NPE and PTE indicated that the peak discharge increases about 20.03% and the time to peak decreases about 0.81%.

• The KIPS Transactions:PartB
• /
• v.11B no.3
• /
• pp.337-344
• /
• 2004

Most of the virtual space constructed sufficiently realistic need a lot of memory space to navigate smoothly. And this kind of virtual space also requires real-time responsibility for the navigation as well as realism. In the off-line virtual system, real-time responsibility can be resolved by using large scale if secondary memory. In the web-based online virtual system, on the other hand, real-time responsibility is highly related to the latency time of network data communication. This induces the necessity of the algorithm for fast data loading. In this paper, we propose and verify the validity of the two methodology for cell leading algorithm. According to the results of computer simulation, the algorithm using hexagonal type cell promotes the real-time responsibility over 30％ than that of the rectangular type.

• The Transactions of the Korea Information Processing Society
• /
• v.2 no.1
• /
• pp.119-129
• /
• 1995

In this paper, we propose a schedulatility analysis method for real-time programs. Several approaches to anlayzing schedulability have been developed, but since these approaches use a fixed priority scheduling method and/or traverse all possible state spaces, there take place exponential time and space complexity of these methods, Therefore it is necessary to reduce the state space and detect schedulability at earlier time. Our schedulability analysis method uses a minimum unit time taken to terminate synchronization action, a minimum unit time taken to terminate actions after synchronization, and a deadline of processes to detect unschedulability at earlier time and dynamic scheduling scheme to reduce state space. We conclude that our method can detected unschedulability earlier 50 percent unit time than Fredette's method.