tags, to it. We call the collection of all his assignments his personomy, and the collection of all personomies is called folksonomy. The user can also explore the personomies of other users in all dimensions: for a given user he can see the resources that user has uploaded, together with the tags he has assigned to them(see Fig. 1);when clicking on a resource he sees which other users have uploaded this resource and how they tagged it; and when clicking on a tag he sees who assigned it to which resources The systems allow for additional functionality. For instance, one can copy a resource from another user, and label it with one owns tags. Overall, these systems provide a very intuitive navigation through the data 2. 1 State of the art web collaboration systems. Among the rare exceptions are Hammond et al (2005)and Lund et al.(2005) who provide good overviews of social book marking tools with special emphasis on folksonomies, and Mathes(2004)who discusses strengths and limitations of folksonomies. The main discussion on folksonomies and related topics is currently only going on mailing lists, e. g. Con- note(2005). To the best of our knowledge, the ideas presented in this paper have not been explored before, but there is a lot of recent work dealing with folksonomies Mika(2005) defines a model of semantic-social networks for extracting light eight ontologies from del icio us besides calculating measures like the cluster ing coefficient, (local)betweenness centrality or the network constraint on the extracted one-mode network, Mika uses co-occurence techniques for clustering e conce eton There are several systems working on top of del icio us to explore the un- derlying folksonomy. Collaborative Rank provides ranked search results on top of del icio us bookmarks. The ranking takes into account, how early someone bookmarked an URL and how many people followed him or her. Other sys- tems show popular sites(pop )of statistics about delicio. ulicious0)or focus on graphical representations 2.2 a Formal model for folksonomies A folksonomy basically describes users, resources, tags, and allows users to assign(arbitrary) tags to resources. We present here a formal definition of folksonomies, which is also underlying our BibSonomy system Definition 1. A folksonomy is a tuple F:=(U,T, R,Y, < )where .U, T, and R are finite sets, whose elements are called users, tags and resources, resp. °htt/ ollabrank org/ http://populicio.us http://cloudalicio.us/ http://www.neuroticweb.com/recursos/del.icio.us-graphs/ 3tags, to it. We call the collection of all his assignments his personomy, and the collection of all personomies is called folksonomy. The user can also explore the personomies of other users in all dimensions: for a given user he can see the resources that user has uploaded, together with the tags he has assigned to them (see Fig. 1); when clicking on a resource he sees which other users have uploaded this resource and how they tagged it; and when clicking on a tag he sees who assigned it to which resources. The systems allow for additional functionality. For instance, one can copy a resource from another user, and label it with one owns tags. Overall, these systems provide a very intuitive navigation through the data. 2.1 State of the Art There are currently virtually no scientific publications about folksonomy-based web collaboration systems. Among the rare exceptions are Hammond et al. (2005) and Lund et al. (2005) who provide good overviews of social book￾marking tools with special emphasis on folksonomies, and Mathes (2004) who discusses strengths and limitations of folksonomies. The main discussion on folksonomies and related topics is currently only going on mailing lists, e.g. Con￾notea (2005). To the best of our knowledge, the ideas presented in this paper have not been explored before, but there is a lot of recent work dealing with folksonomies. Mika (2005) defines a model of semantic-social networks for extracting light￾weight ontologies from del.icio.us. Besides calculating measures like the cluster￾ing coefficient, (local) betweenness centrality or the network constraint on the extracted one-mode network, Mika uses co-occurence techniques for clustering the concept network. There are several systems working on top of del.icio.us to explore the un￾derlying folksonomy. CollaborativeRank9 provides ranked search results on top of del.icio.us bookmarks. The ranking takes into account, how early someone bookmarked an URL and how many people followed him or her. Other sys￾tems show popular sites (Populicious10) or focus on graphical representations (Cloudalicious11, Grafolicious12) of statistics about del.icio.us. 2.2 A Formal Model for Folksonomies A folksonomy basically describes users, resources, tags, and allows users to assign (arbitrary) tags to resources. We present here a formal definition of folksonomies, which is also underlying our BibSonomy system. Definition 1. A folksonomy is a tuple F := (U, T, R, Y, ≺) where • U, T , and R are finite sets, whose elements are called users, tags and resources, resp., 9 http://collabrank.org/ 10 http://populicio.us/ 11 http://cloudalicio.us/ 12 http://www.neuroticweb.com/recursos/del.icio.us-graphs/ 3