Games, axiomatisations
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R. Hirsch and I. Hodkinson
Proc. 11th Amsterdam Colloquium, P Dekker, M Stokhof, Y Venema (eds),
ILLC/Department
of Philosophy, Universiteit van Amsterdam, 1997, pp. 7-12
A gentle survey of some of our work in this area.
R. Hirsch and I. Hodkinson
In JFAK - a CD-ROM
of essays dedicated to Johan van Benthem on the occasion of his 50th
birthday.
This is a short account of (some of) the history of the axiomatisation
problem in algebraic logic, together with an outline of a general
method of axiomatising pseudo-elementary classes. This covers probably
all the axiomatisations given in the papers below, using games to obtain
explicit axioms. This is a discursive essay not intended as a technical
paper; there are no formal proofs.
R. Hirsch and I. Hodkinson
J. Symbolic Logic 62 (1997) 225-279
We consider the problem of finding and classifying representations in
algebraic
logic. This is approached by letting two players build a representation
using a game. Homogeneous and universal representations are
characterised
according to the outcome of certain games. The 'Lyndon conditions'
defining
representable relation algebras (for the finite case) and a similar
schema
for cylindric algebras are derived. Finite relation algebras with
homogeneous
representations are characterised by first order formulas. Equivalence
games are defined, and are used to establish whether an algebra is
$\omega$-categorical.
We have a simple proof that the perfect extension of a representable
relation
algebra is completely representable. Another two-player game is used to
derive equational axiomatisations for the classes of all representable
relation algebras and representable cylindric algebras.
R. Hirsch and I. Hodkinson
Logic Journal of the IGPL vol. 5
no. 2 (1997) pp 209--229
We outline a simple approach to axiomatising the class of representable
relation algebras, using games. We discuss generalisations of the
method to cylindric algebras, homogeneous and complete representations, and
atom structures of relation algebras.
Ian Hodkinson, Szabolcs Mikulás, and Yde Venema
Algebra Universalis, 46 (2001) 455-478
ILLC Prepublication
series
no. PP-2000-01
Given a variety V we provide an axiomatization P(V) of the class SCmV
of
complex algebras of algebras in V. P(V) can be obtained
effectively
from the axiomatization of V; in fact, if this axiomatization is
recursively
enumerable, then P(V) is recursive.
Ian Hodkinson and Yde Venema
Trans. Amer. Math. Soc. 357 (2005), 4579-4605.
We give a simple example of a variety V of modal algebras that is
canonical
but cannot be axiomatised by canonical equations or first-order
sentences.
We then show that the variety RRA of representable relation algebras,
although
canonical, has no canonical axiomatisation. Indeed, we show that
every axiomatisation of these varieties involves infinitely many
non-canonical
sentences.
Using probabilistic methods of Erdös, we construct an infinite
sequence
G0, G1, ... of finite graphs with arbitrarily
large
chromatic number, such that each Gn is a bounded morphic
image
of Gn+1 and has no odd cycles of length at most n. The
inverse
limit of the sequence is a graph with no odd cycles and hence is
2-colourable.
It follows that a modal algebra (respectively, a relation algebra)
obtained
from the Gn satisfies arbitrarily many axioms from a certain
axiomatisation of V (RRA), while its canonical extension satisfies only
a bounded number of them. It now follows by compactness that V (RRA)
has
no canonical axiomatisation. A variant of this argument shows that
there
is no axiomatisation using finitely many non-canonical sentences.
Survey talk on this paper.
Jannis Bulian and Ian Hodkinson
Ann. Pure Appl. Logic 164 (2013) 884-906, doi 10.1016/j.apal.2013.04.002.
We show that for finite n at least 3, every first-order
axiomatisation of the varieties
of representable n-dimensional cylindric algebras,
diagonal-free cylindric algebras, polyadic algebras, and polyadic equality algebras
contains an infinite number of non-canonical formulas.
We also show that the class of structures for each of these varieties is non-elementary.
The proofs employ algebras derived from random graphs.