Contents 1 Introduction
As the World Wide Web (WWW) has become popular, web resources have been increasing rapidly. For this reason, it is difficult for users to obtain useful information efficiently. To cope with this problem, some tools have been developed. Those tools can be categorized into the following two categories: one is for “open community,” like web search engines, net-news systems, and bulletin-board sys-tems; the other is for “closed community,” like mailing-list and bulletin-board systems in closed groups. However, such tools have some problems. In the former case, users have to access to information in the community actively and fre-quently. They cannot know the update until they access to it even if the information has been updated, In the latter case, users are responsible for actively accessing and responding to the information, otherwise the activity of the community would be declined. For instance, in a mailing-list community, its activity is kept high while a lot of messages are interchanged. However, once the number of messages go down, it become inactive.
