Newman's lemma

In mathematics, in the theory of rewriting systems, Newman's lemma, also commonly called the diamond lemma, states that a terminating (or strongly normalizing) abstract rewriting system (ARS), that is, one in which there are no infinite reduction sequences, is confluent if it is locally confluent. In fact a terminating ARS is confluent precisely when it is locally confluent.[1]

Equivalently, for every binary relation with no decreasing infinite chains and satisfying a weak version of the diamond property, there is a unique minimal element in every connected component of the relation considered as a graph.

Today, this is seen as a purely combinatorial result based on well-foundedness due to a proof of Gérard Huet in 1980.[2] Newman's original proof was considerably more complicated.[3]

Diamond lemma
Proof idea (straight and wavy lines denoting → and ∗→, respectively):
Given t1 ∗←t∗→t2, perform an induction on the derivation length. Obtain t′ from local confluence, and t′′ from the induction hypothesis; similar for t′′′.

In general, Newman's lemma can be seen as a combinatorial result about binary relations → on a set A (written backwards, so that a → b means that b is below a) with the following two properties:

  • → is a well-founded relation: every non-empty subset X of A has a minimal element (an element a of X such that a → b for no b in X). Equivalently, there is no infinite chain a0 → a1 → a2 → a3 → .... In the terminology of rewriting systems, → is terminating.
  • Every covering is bounded below. That is, if an element a in A covers elements b and c in A in the sense that a → b and a → c, then there is an element d in A such that b ∗→ d and c ∗→ d, where ∗→ denotes the reflexive transitive closure of →. In the terminology of rewriting systems, → is locally confluent.

If the above two conditions hold, then the lemma states that → is confluent: whenever a ∗→ b and a ∗→ c, there is an element d such that b ∗→ d and c ∗→ d. In view of the termination of →, this implies that every connected component of → as a graph contains a unique minimal element a, moreover b ∗→ a for every element b of the component.[4]

Notes
  1. Franz Baader, Tobias Nipkow, (1998) Term Rewriting and All That, Cambridge University Press ISBN 0-521-77920-0
  2. Gérard Huet, "Confluent Reductions: Abstract Properties and Applications to Term Rewriting Systems", Journal of the ACM (JACM), October 1980, Volume 27, Issue 4, pp. 797 - 821.
  3. Harrison, p. 260, Paterson(1990), p. 354.
  4. Paul M. Cohn, (1980) Universal Algebra, D. Reidel Publishing, ISBN 90-277-1254-9 (See pp. 25-26)

References

Textbooks
  • Term Rewriting Systems, Terese, Cambridge Tracts in Theoretical Computer Science, 2003. (book weblink)
  • Term Rewriting and All That, Franz Baader and Tobias Nipkow, Cambridge University Press, 1998 (book weblink)
  • John Harrison, Handbook of Practical Logic and Automated Reasoning, Cambridge University Press, 2009, ISBN 978-0-521-89957-4, chapter 4 "Equality".
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