Nesting and Defoliation of Index Expressions for Information Retrieval

In th is article, a form alisation of ind ex expressions is presented . Index expressions are m ore expressive th an keyw ords w hile m aintaining a com ­ prehensible com plexity. Index expressions are w ell-know n in Inform ation R etrieval (IR ), where th ey are used for characterising docum ent contents, form ulation of user in terests, and m atch ing m echanism s. In addition , in­ dex expressions have found b o th practical and theoretica l applicability in 2-level hyperm edia system s for IR. In these app lications, properties of (the structure of) ind ex expressions are heavily relied upon. H owever, a correct m athem atica l form alisation of index expressions and their proper­ ties still lacks. Our form alism is based on the structural n ota tio n of index expressions. D efoliation, w hich plays an im portant role in defining prop­ erties o f index expressions, is provided as a recursively defined operator. Finally, tw o other representational form alism s for index expressions are com pared to ours.