182 research outputs found
Extensional proofs in a propositional logic modulo isomorphisms
System I is a proof language for a fragment of propositional logic where
isomorphic propositions, such as and , or
and are made
equal. System I enjoys the strong normalisation property. This is sufficient to
prove the existence of empty types, but not to prove the introduction property
(every closed term in normal form is an introduction). Moreover, a severe
restriction had to be made on the types of the variables in order to obtain the
existence of empty types. We show here that adding -expansion rules to
System I permits to drop this restriction, and yields a strongly normalising
calculus with enjoying the full introduction property.Comment: 15 pages plus references and appendi
An Intuitionistic Formula Hierarchy Based on High-School Identities
We revisit the notion of intuitionistic equivalence and formal proof
representations by adopting the view of formulas as exponential polynomials.
After observing that most of the invertible proof rules of intuitionistic
(minimal) propositional sequent calculi are formula (i.e. sequent) isomorphisms
corresponding to the high-school identities, we show that one can obtain a more
compact variant of a proof system, consisting of non-invertible proof rules
only, and where the invertible proof rules have been replaced by a formula
normalisation procedure.
Moreover, for certain proof systems such as the G4ip sequent calculus of
Vorob'ev, Hudelmaier, and Dyckhoff, it is even possible to see all of the
non-invertible proof rules as strict inequalities between exponential
polynomials; a careful combinatorial treatment is given in order to establish
this fact.
Finally, we extend the exponential polynomial analogy to the first-order
quantifiers, showing that it gives rise to an intuitionistic hierarchy of
formulas, resembling the classical arithmetical hierarchy, and the first one
that classifies formulas while preserving isomorphism
Automatic and Transparent Transfer of Theorems along Isomorphisms in the Coq Proof Assistant
In mathematics, it is common practice to have several constructions for the
same objects. Mathematicians will identify them modulo isomorphism and will not
worry later on which construction they use, as theorems proved for one
construction will be valid for all.
When working with proof assistants, it is also common to see several
data-types representing the same objects. This work aims at making the use of
several isomorphic constructions as simple and as transparent as it can be done
informally in mathematics. This requires inferring automatically the missing
proof-steps.
We are designing an algorithm which finds and fills these missing proof-steps
and we are implementing it as a plugin for Coq
Non-wellfounded trees in Homotopy Type Theory
We prove a conjecture about the constructibility of coinductive types - in
the principled form of indexed M-types - in Homotopy Type Theory. The
conjecture says that in the presence of inductive types, coinductive types are
derivable. Indeed, in this work, we construct coinductive types in a subsystem
of Homotopy Type Theory; this subsystem is given by Intensional Martin-L\"of
type theory with natural numbers and Voevodsky's Univalence Axiom. Our results
are mechanized in the computer proof assistant Agda.Comment: 14 pages, to be published in proceedings of TLCA 2015; ancillary
files contain Agda files with formalized proof
Heterogeneous substitution systems revisited
Matthes and Uustalu (TCS 327(1-2):155-174, 2004) presented a categorical
description of substitution systems capable of capturing syntax involving
binding which is independent of whether the syntax is made up from least or
greatest fixed points. We extend this work in two directions: we continue the
analysis by creating more categorical structure, in particular by organizing
substitution systems into a category and studying its properties, and we
develop the proofs of the results of the cited paper and our new ones in
UniMath, a recent library of univalent mathematics formalized in the Coq
theorem prover.Comment: 24 page
Large and Infinitary Quotient Inductive-Inductive Types
Quotient inductive-inductive types (QIITs) are generalized inductive types
which allow sorts to be indexed over previously declared sorts, and allow usage
of equality constructors. QIITs are especially useful for algebraic
descriptions of type theories and constructive definitions of real, ordinal and
surreal numbers. We develop new metatheory for large QIITs, large elimination,
recursive equations and infinitary constructors. As in prior work, we describe
QIITs using a type theory where each context represents a QIIT signature.
However, in our case the theory of signatures can also describe its own
signature, modulo universe sizes. We bootstrap the model theory of signatures
using self-description and a Church-coded notion of signature, without using
complicated raw syntax or assuming an existing internal QIIT of signatures. We
give semantics to described QIITs by modeling each signature as a finitely
complete CwF (category with families) of algebras. Compared to the case of
finitary QIITs, we additionally need to show invariance under algebra
isomorphisms in the semantics. We do this by modeling signature types as
isofibrations. Finally, we show by a term model construction that every QIIT is
constructible from the syntax of the theory of signatures
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