A sequent calculus for a semi-associative law

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Abstract

We introduce a sequent calculus with a simple restriction of Lambek's product rules that precisely captures the classical Tamari order, i.e., the partial order on fully-bracketed words (equivalently, binary trees) induced by a semi-associative law (equivalently, right rotation). We establish a focusing property for this sequent calculus (a strengthening of cut-elimination), which yields the following coherence theorem: every valid entailment in the Tamari order has exactly one focused derivation. We then describe two main applications of the coherence theorem, including: 1. A new proof of the lattice property for the Tamari order, and 2. A new proof of the Tutte-Chapoton formula for the number of intervals in the Tamari lattice Yn .

Details

Original languageEnglish
Article number9
Pages (from-to)9:1-9:23
Number of pages23
JournalLogical Methods in Computer Science
Volume15
Issue number1
Publication statusPublished - 5 Feb 2019

Keywords

  • Tamari lattice, associativity, coherence theorem, combinatorics, proof theory