,
Arturo De Faveri
,
Giulio Manzonetto
,
Antonino Salibra
Creative Commons Attribution 4.0 International license
We study invertibility of λ-terms modulo λ-theories. Here a fundamental role is played by a class of λ-terms called finite hereditary permutations (FHP) and by their infinite generalisations (HP). More precisely, FHPs are the invertible elements in the least extensional λ-theory λ η and HPs are those in the greatest sensible λ-theory H^*. Our approach is based on inverse semigroups, algebraic structures that generalise groups and semilattices. We show that FHP modulo a λ-theory T is always an inverse semigroup and that HP modulo T is an inverse semigroup whenever T contains the theory of Böhm trees. An inverse semigroup comes equipped with a natural order. We prove that the natural order corresponds to η-expansion in FHP/T, and to infinite η-expansion in HP/T. Building on these correspondences we obtain the two main contributions of this work: firstly, we recast in a broader framework the results cited at the beginning; secondly, we prove that the FHPs are the invertible λ-terms in all the λ-theories lying between λ η and H^+. The latter is Morris' observational λ-theory, defined by using the β-normal forms as observables.
@InProceedings{bucciarelli_et_al:LIPIcs.FSCD.2026.9,
author = {Bucciarelli, Antonio and De Faveri, Arturo and Manzonetto, Giulio and Salibra, Antonino},
title = {{Groups and Inverse Semigroups in Lambda Calculus}},
booktitle = {11th International Conference on Formal Structures for Computation and Deduction (FSCD 2026)},
pages = {9:1--9:22},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-433-8},
ISSN = {1868-8969},
year = {2026},
volume = {378},
editor = {Pfenning, Frank},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.FSCD.2026.9},
URN = {urn:nbn:de:0030-drops-263596},
doi = {10.4230/LIPIcs.FSCD.2026.9},
annote = {Keywords: Lambda Calculus, Invertibility, Groups, Inverse Semigroups}
}