Bisimilarity in Term Graph Rewriting

We present a survey of con uence properties of (acyclic) term graph rewriting. Results and counterexamples are given for di erent kinds of term graph rewriting: besides plain applications of rewrite rules, extensions with the operations of collapsing and copying, and with both operations together are considered. Collapsing and copying together constitute bisimilarity of term graphs. We establish su cient conditions for|and counterexamples to|con uence, con uence modulo bisimilarity and the Church-Rosser property modulo bisimilarity. Moreover, we address rewriting modulo bisimilarity, that is, rewriting of bisimilarity classes of term graphs. 3 List of Symbols \Sigma \Pi \sigma \tau \lambda \rho \pi R (calligraphic) X,V,E (roman) arity (roman) lab (roman) att (roman) res (roman) root (roman) term (roman) tr (roman) Cpl (roman) h,i \langle,\rangle \circ ; \emptyset 2 \in [ \cup \subseteq = \cong \sim \approx \succ \succeq \prec \preceq M \vartriangle O \triangledown \lozenge ! \to ) \Rightarrow ( \Leftarrow V \Rrightarrow W \Lleftarrow WV \Lleftarrow\!\!\!\!\Rrightarrow $ \leftrightarrow , \Leftrightarrow 4