117 research outputs found
Approximately multiplicative maps from weighted semilattice algebras
We investigate which weighted convolution algebras , where
is a semilattice, are AMNM in the sense of Johnson (JLMS, 1986). We give an
explicit example where this is not the case. We show that the unweighted
examples are all AMNM, as are all where has either
finite width or finite height. Some of these finite-width examples are
isomorphic to function algebras studied by Feinstein (IJMMS, 1999).
We also investigate when is an AMNM pair in
the sense of Johnson (JLMS, 1988), where denotes the algebra of
2-by-2 complex matrices. In particular, we obtain the following two contrasting
results: (i) for many non-trivial weights on the totally ordered semilattice
, the pair is not
AMNM; (ii) for any semilattice , the pair is AMNM.
The latter result requires a detailed analysis of approximately commuting,
approximately idempotent matrices.Comment: AMS-LaTeX. v3: 31 pages, additional minor corrections to v2. Final
version, to appear in J. Austral. Math. Soc. v4: small correction of
mis-statement at start of Section 4 (this should also be fixed in the journal
version
A Generic Undo Support for State-Based CRDTs
CRDTs (Conflict-free Replicated Data Types) have properties desirable for large-scale distributed systems with variable network latency or transient partitions. With CRDT, data are always available for local updates and data states converge when the replicas have incorporated the same updates. Undo is useful for correcting human mistakes and for restoring system-wide invariant violated due to long delays or network partitions.
There is currently no generally applicable undo support for CRDTs. There are at least two reasons for this. First, there is currently no abstraction that we can practically use to capture the relations between undo and normal operations with respect to concurrency and causality. Second, using inverse operations as the existing partial solutions, the CRDT designer has to hard-code certain rules and design a new CRDT for almost every operation that needs undo support.
In this paper, we present an approach to generic support of undo for CRDTs. The approach consists of two major parts. We first work out an abstraction that captures the semantics of concurrent undo and redo operations through equivalence classes. The abstraction is a natural extension of undo and redo in sequential applications and is straightforward to implement in practice. By using this abstraction, we then device a mechanism to augment existing CRDTs. The mechanism provides an "out of the box" support for undo without the involvement of the CRDT designers. We also present a practical application of the approach in collaborative editing
Tabling with Sound Answer Subsumption
Tabling is a powerful resolution mechanism for logic programs that captures
their least fixed point semantics more faithfully than plain Prolog. In many
tabling applications, we are not interested in the set of all answers to a
goal, but only require an aggregation of those answers. Several works have
studied efficient techniques, such as lattice-based answer subsumption and
mode-directed tabling, to do so for various forms of aggregation.
While much attention has been paid to expressivity and efficient
implementation of the different approaches, soundness has not been considered.
This paper shows that the different implementations indeed fail to produce
least fixed points for some programs. As a remedy, we provide a formal
framework that generalises the existing approaches and we establish a soundness
criterion that explains for which programs the approach is sound.
This article is under consideration for acceptance in TPLP.Comment: Paper presented at the 32nd International Conference on Logic
Programming (ICLP 2016), New York City, USA, 16-21 October 2016, 15 pages,
LaTeX, 0 PDF figure
Categoricity and multidimensional diagrams
We study multidimensional diagrams in independent amalgamation in the
framework of abstract elementary classes (AECs). We use them to prove the
eventual categoricity conjecture for AECs, assuming a large cardinal axiom.
More precisely, we show assuming the existence of a proper class of strongly
compact cardinals that an AEC which has a single model of some high-enough
cardinality will have a single model in any high-enough cardinal. Assuming a
weak version of the generalized continuum hypothesis, we also establish the
eventual categoricity conjecture for AECs with amalgamation.Comment: 63 page
- …