191 research outputs found
Generalisation : graphs and colourings
The interaction between practice and theory in mathematics is a central theme. Many mathematical structures and theories result from the formalisation of a real problem. Graph Theory is rich with such examples. The graph structure itself was formalised by Leonard Euler in the quest to solve the problem of the Bridges of Königsberg. Once a structure is formalised, and results are proven, the mathematician seeks to generalise. This can be considered as one of the main praxis in mathematics. The idea of generalisation will be illustrated through graph colouring. This idea also results from a classic problem, in which it was well known by topographers that four colours suffice to colour any map such that no countries sharing a border receive the same colour. The proof of this theorem eluded mathematicians for centuries and was proven in 1976. Generalisation of graphs to hypergraphs, and variations on the colouring theme will be discussed, as well as applications in other disciplines.peer-reviewe
Combinatorics of embeddings
We offer the following explanation of the statement of the Kuratowski graph
planarity criterion and of 6/7 of the statement of the Robertson-Seymour-Thomas
intrinsic linking criterion. Let us call a cell complex 'dichotomial' if to
every cell there corresponds a unique cell with the complementary set of
vertices. Then every dichotomial cell complex is PL homeomorphic to a sphere;
there exist precisely two 3-dimensional dichotomial cell complexes, and their
1-skeleta are K_5 and K_{3,3}; and precisely six 4-dimensional ones, and their
1-skeleta all but one graphs of the Petersen family.
In higher dimensions n>2, we observe that in order to characterize those
compact n-polyhedra that embed in S^{2n} in terms of finitely many "prohibited
minors", it suffices to establish finiteness of the list of all (n-1)-connected
n-dimensional finite cell complexes that do not embed in S^{2n} yet all their
proper subcomplexes and proper cell-like combinatorial quotients embed there.
Our main result is that this list contains the n-skeleta of (2n+1)-dimensional
dichotomial cell complexes. The 2-skeleta of 5-dimensional dichotomial cell
complexes include (apart from the three joins of the i-skeleta of
(2i+2)-simplices) at least ten non-simplicial complexes.Comment: 49 pages, 1 figure. Minor improvements in v2 (subsection 4.C on
transforms of dichotomial spheres reworked to include more details;
subsection 2.D "Algorithmic issues" added, etc
Hyperconvexity and Tight Span Theory for Diversities
The tight span, or injective envelope, is an elegant and useful construction
that takes a metric space and returns the smallest hyperconvex space into which
it can be embedded. The concept has stimulated a large body of theory and has
applications to metric classification and data visualisation. Here we introduce
a generalisation of metrics, called diversities, and demonstrate that the rich
theory associated to metric tight spans and hyperconvexity extends to a
seemingly richer theory of diversity tight spans and hyperconvexity.Comment: revised in response to referee comment
Continuity, Discontinuity and Dynamics in Mathematics & Economics - Reconsidering Rosser's Visions
Barkley Rosser has been a pioneer in arguing the case for the mathematics of discontinuity, broadly conceived, to be placed at the foundations of modelling economic dynamics. In this paper we reconsider this vision from the broad perspective of a variety of different kinds of mathematics and suggest a broadening of Rosser’s methodology to the study of economic dynamicsContinuity, Discontinuity, Economic Dynamics, Relaxation Oscillations
Variations on Noetherianness
In constructive mathematics, several nonequivalent notions of finiteness
exist. In this paper, we continue the study of Noetherian sets in the
dependently typed setting of the Agda programming language. We want to say that
a set is Noetherian, if, when we are shown elements from it one after another,
we will sooner or later have seen some element twice. This idea can be made
precise in a number of ways. We explore the properties and connections of some
of the possible encodings. In particular, we show that certain implementations
imply decidable equality while others do not, and we construct counterexamples
in the latter case. Additionally, we explore the relation between
Noetherianness and other notions of finiteness.Comment: In Proceedings MSFP 2016, arXiv:1604.0038
SMT Solving for Functional Programming over Infinite Structures
We develop a simple functional programming language aimed at manipulating
infinite, but first-order definable structures, such as the countably infinite
clique graph or the set of all intervals with rational endpoints. Internally,
such sets are represented by logical formulas that define them, and an external
satisfiability modulo theories (SMT) solver is regularly run by the interpreter
to check their basic properties.
The language is implemented as a Haskell module.Comment: In Proceedings MSFP 2016, arXiv:1604.0038
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