Friendster is a social networking service which used to be popular all over the world at the beginning of the 21st century and declined thereafter. Some researchers have examined the process of decline and explained that the social network on Friendster collapsed from the outside of the core structure. In order to verify if their assertion is true, we analyze the time evolution of the network structure more carefully. The result implies that the collapse of the Friendster networks actually started from the center of the core structure. We also attempt to explain its mechanism by a model based on the SIR model, which is a model in epidemiology. The findings imply that the tendency of “non-users who have many friends on Friendster are likely to register for Friendster” and “users who have many friends that are already tired of Friendster are likely to leave Friendster” played an important role in the time evolution of the core structure of the network. The tendency of “users who have few friends on Friendster are likely to leave soon” would have also played a key role in the process.

A cycling wheel chair (CWC) is a pedal-driven wheelchair developed as a personal transportation device for patients with hemiplegia after cerebral stroke. The CWC enables the patient to travel much faster than an ordinary wheel chair and is effective for prevention of disuse syndrome in lower limbs. However, the patients often have impairment of cognitive function as well as motor function. It is dangerous for such patients to ride on the CWC on outdoor roads. To cope with this problem, a virtual reality system for the CWC named “Virtual CWC” was developed. The Virtual CWC can provide safe and space-saving training and test environment for the patients. In the present study, a new scenario of the Virtual CWC has been developed to test the patient's cognitive function with respect to navigation as well as driving skills. The normal subjects' “homing vectors” obtained after riding on the Virtual CWC in three kinds of routes were analyzed, and compared between a young group (26 normal subjects; age 23.9 ± 2.5) and an elderly group (14 normal subjects; age 69.9 ± 4.1). As a result, the effect of cooperation between pedaling and steering could be found in the angular error of homing vectors in the case of the elderly group.

Attention to CSCW has been increasing, resulting in greater needs for secure group communication. In order to realize secure group communication, data which is sent from a member to other members of a group should be encrypted by the cryptographic communication key of the group. We propose an identity-based non-interactive group communication key sharing scheme using smart cards based on the modified copy key (MCK) method. We assume that the smart cards contain the key generators and are secure for tampering. Each user has a smart card and a key generator. A user can generate a group communication key non-interactively with his key generator and the IDs of the other group members using his smart card.>