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Problem #16

Originator: Yoshihito Toyama
Date: April 1991

Summary: Under what conditions does confluence of a normal semi-equational conditional term rewriting system imply confluence of the associated oriented system?

For a normal conditional term-rewriting system R = { s! tlr }, where t must be a ground normal from of s, we can consider the corresponding semi-equational conditional rewrite system Rse = { s* tlr }. Under what conditions does confluence of Rse imply confluence of R? In general, this is not the case, as can be seen from the following non-confluent system R (due to Aart Middeldorp):

a → b
a → c
b →! c ⇒    b → c


Solutions have been provided by [YASM00]. They show that confluence of R follows from confluence of Rse if any of the two following conditions is satisfied:

See [YASM00] for definitions of these properties.


Toshiyuki Yamada, Jürgen Avenhaus, Carlos Loría Sáenz, and Aart Middeldorp. Logicality of conditional rewrite systems. Theoretical Computer Science, 236(1–2):209–232, April 2000.

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