1,540 research outputs found
Kleene Algebra with Observations
Kleene algebra with tests (KAT) is an algebraic framework for reasoning about the control flow of sequential programs. Generalising KAT to reason about concurrent programs is not straightforward, because axioms native to KAT in conjunction with expected axioms for concurrency lead to an anomalous equation. In this paper, we propose Kleene algebra with observations (KAO), a variant of KAT, as an alternative foundation for extending KAT to a concurrent setting. We characterise the free model of KAO, and establish a decision procedure w.r.t. its equational theory
Kleene algebra with observations
Kleene algebra with tests (KAT) is an algebraic framework for reasoning about the control flow of sequential programs. Generalising KAT to reason about concurrent programs is not straightforward, because axioms native to KAT in conjunction with expected axioms for concurrency lead to an anomalous equation. In this paper, we propose Kleene algebra with observations (KAO), a variant of KAT, as an alternative foundation for extending KAT to a concurrent setting. We characterise the free model of KAO, and establish a decision procedure w.r.t. its equational theory
On Tools for Completeness of Kleene Algebra with Hypotheses
In the literature on Kleene algebra, a number of variants have been proposed
which impose additional structure specified by a theory, such as Kleene algebra
with tests (KAT) and the recent Kleene algebra with observations (KAO), or make
specific assumptions about certain constants, as for instance in NetKAT. Many
of these variants fit within the unifying perspective offered by Kleene algebra
with hypotheses, which comes with a canonical language model constructed from a
given set of hypotheses. For the case of KAT, this model corresponds to the
familiar interpretation of expressions as languages of guarded strings. A
relevant question therefore is whether Kleene algebra together with a given set
of hypotheses is complete with respect to its canonical language model. In this
paper, we revisit, combine and extend existing results on this question to
obtain tools for proving completeness in a modular way. We showcase these tools
by giving new and modular proofs of completeness for KAT, KAO and NetKAT, and
we prove completeness for new variants of KAT: KAT extended with a constant for
the full relation, KAT extended with a converse operation, and a version of KAT
where the collection of tests only forms a distributive lattice
Concurrent Kleene Algebra with Observations: from Hypotheses to Completeness
Concurrent Kleene Algebra (CKA) extends basic Kleene algebra with a parallel
composition operator, which enables reasoning about concurrent programs.
However, CKA fundamentally misses tests, which are needed to model standard
programming constructs such as conditionals and -loops. It
turns out that integrating tests in CKA is subtle, due to their interaction
with parallelism. In this paper we provide a solution in the form of Concurrent
Kleene Algebra with Observations (CKAO). Our main contribution is a
completeness theorem for CKAO. Our result resorts on a more general study of
CKA "with hypotheses", of which CKAO turns out to be an instance: this analysis
is of independent interest, as it can be applied to extensions of CKA other
than CKAO
Concurrent Kleene Algebra: Free Model and Completeness
Concurrent Kleene Algebra (CKA) was introduced by Hoare, Moeller, Struth and
Wehrman in 2009 as a framework to reason about concurrent programs. We prove
that the axioms for CKA with bounded parallelism are complete for the semantics
proposed in the original paper; consequently, these semantics are the free
model for this fragment. This result settles a conjecture of Hoare and
collaborators. Moreover, the techniques developed along the way are reusable;
in particular, they allow us to establish pomset automata as an operational
model for CKA.Comment: Version 2 includes an overview section that outlines the completeness
proof, as well as some extra discussion of the interpolation lemma. It also
includes better typography and a number of minor fixes. Version 3
incorporates the changes by comments from the anonymous referees at ESOP.
Among other things, these include a worked example of computing the syntactic
closure by han
Multiorder, Kleene stars and cyclic projectors in the geometry of max cones
This paper summarizes results on some topics in the max-plus convex geometry,
mainly concerning the role of multiorder, Kleene stars and cyclic projectors,
and relates them to some topics in max algebra. The multiorder principle leads
to max-plus analogues of some statements in the finite-dimensional convex
geometry and is related to the set covering conditions in max algebra. Kleene
stars are fundamental for max algebra, as they accumulate the weights of
optimal paths and describe the eigenspace of a matrix. On the other hand, the
approach of tropical convexity decomposes a finitely generated semimodule into
a number of convex regions, and these regions are column spans of uniquely
defined Kleene stars. Another recent geometric result, that several semimodules
with zero intersection can be separated from each other by max-plus halfspaces,
leads to investigation of specific nonlinear operators called cyclic
projectors. These nonlinear operators can be used to find a solution to
homogeneous multi-sided systems of max-linear equations. The results are
presented in the setting of max cones, i.e., semimodules over the max-times
semiring.Comment: 26 pages, a minor revisio
Completeness and Incompleteness of Synchronous Kleene Algebra
Synchronous Kleene algebra (SKA), an extension of Kleene algebra (KA), was
proposed by Prisacariu as a tool for reasoning about programs that may execute
synchronously, i.e., in lock-step. We provide a countermodel witnessing that
the axioms of SKA are incomplete w.r.t. its language semantics, by exploiting a
lack of interaction between the synchronous product operator and the Kleene
star. We then propose an alternative set of axioms for SKA, based on Salomaa's
axiomatisation of regular languages, and show that these provide a sound and
complete characterisation w.r.t. the original language semantics.Comment: Accepted at MPC 201
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