Formalizing and Using Persistency

In many formalizations of a changing world things do not change all the t ime but are persistent thoughout a time interval. Often this persistency is represented by facts refering to intervals int which are still valid if int is replaced by any subinterval i n t ' Approaches l ike episode propagat ion or Penberthy's temporal unification try to employ this property for efficient reasoning. However these approaches lack formality. In this paper their way of reasoning about persistency is reconstructed as inference rules that combine appropriate t imeboxes wi th standard resolution. In many cases Burckert's Constrained Resolution may be used. More complex examples may be handled by a new inference rule, called Persistency Resolution. An analysis of this rule leads to a more general notion of persistency.