arXiv subjects

Richard Elwes

Publications and source records attributed to Richard Elwes.

7 recordsLinked to original sources

arXiv2021

Preferential Attachment Processes Approaching The Rado Multigraph

We consider a preferential attachment process in which a multigraph is built one node at a time. The number of edges added at stage $t$, emanating from the new node, is given by some prescribed function $f(t)$, generalising a model considered by Kleinberg and Kleinberg in 2005 where $f$ was presumed constant. We show that if $f(t)$ is asymptotically bounded above and below by linear functions in $t$, then with probability $1$ the infinite limit of the process will be isomorphic to the \emph{Rado multigraph}. This structure is the natural multigraph analogue of the Rado graph, which we introduce here.

math.CO↗

arXiv2019

A Preferential Attachment Process Approaching the Rado Graph

We consider a simple Preferential Attachment graph process, which begins with a finite graph, and in which a new $(t+1)$st vertex is added at each subsequent time step $t$, and connected to each previous vertex $u \leq t$ with probability $\frac{d_u(t)}{t}$ where $d_u(t)$ is the degree of $u$ at time $t$. We analyse the graph obtained as the infinite limit of this process, and show that so long as the initial finite graph is neither edgeless nor complete, with probability 1 the outcome will be a copy of the Rado graph augmented with a finite number of either isolated or universal vertices.

math.CO↗

arXiv2019

Evolving Shelah-Spencer Graphs

An \emph{evolving Shelah-Spencer process} is one by which a random graph grows, with at each time $τ\in {\bf N}$ a new node incorporated and attached to each previous node with probability $τ^{-α}$, where $α\in (0,1) \setminus {\bf Q}$ is fixed. We analyse the graphs that result from this process, including the infinite limit, in comparison to Shelah-Spencer sparse random graphs discussed in [Spencer, J., 2013. The strange logic of random graphs (Vol. 22). Springer Science & Business Media.] and throughout the model-theoretic literature. The first order axiomatisation for classical Shelah-Spencer graphs comprises a 'Generic Extension' axiom and a 'No Dense Subgraphs' axiom. We show that in our context 'Generic Extension' continues to hold. While 'No Dense Subgraphs' fails, a weaker 'Few Rigid Subgraphs' property holds.

math.CO↗

arXiv2016

Unperturbed Schelling segregation in two or three dimensions

Schelling's model of segregation, first described in 1969, has become one of the best known models of self-organising behaviour. While Schelling's explicit concern was to understand the mechanisms underlying racial segregation in large cities from a game theoretic perspective, the model should be seen as one of a family, arising in fields as diverse as statistical mechanics, neural networks and the social sciences, and which are concerned with interacting populations situated on network structures. Despite extensive study, however, the (unperturbed) Schelling model has largely resisted rigorous analysis, prior results in the literature generally pertaining to variants of the model in which noise is introduced into the dynamics of the system, the resulting model then being amenable to standard techniques from statistical mechanics or stochastic evolutionary game theory. A series of recent papers (one by Brandt, Immorlica, Kamath, and Kleinberg, and two by the authors), has seen the first rigorous analysis of the one dimensional version of the unperturbed model. Here we provide the first rigorous analysis of the two and three dimensional unperturbed models, establishing most of the phase diagram, and answering a challenge from a recent paper by Brandt, Immorlica, Kamath, and Kleinberg.

cs.MA↗

arXiv2015

Minority population in the one-dimensional Schelling model of segregation

The Schelling model of segregation looks to explain the way in which a population of agents or particles of two types may come to organise itself into large homogeneous clusters, and can be seen as a variant of the Ising model in which the system is subjected to rapid cooling. While the model has been very extensively studied, the unperturbed (noiseless) version has largely resisted rigorous analysis, with most results in the literature pertaining to versions of the model in which noise is introduced into the dynamics so as to make it amenable to standard techniques from statistical mechanics or stochastic evolutionary game theory. We rigorously analyse the one-dimensional version of the model in which one of the two types is in the minority, and establish various forms of threshold behaviour. Our results are in sharp contrast with the case when the distribution of the two types is uniform (i.e. each agent has equal chance of being of each type in the initial configuration), which was studied by Brandt, Immorlica, Kamath, and Kleinberg.

cs.DM↗

arXiv2015

Digital morphogenesis via Schelling segregation

Schelling's model of segregation looks to explain the way in which particles or agents of two types may come to arrange themselves spatially into configurations consisting of large homogeneous clusters, i.e.\ connected regions consisting of only one type. As one of the earliest agent based models studied by economists and perhaps the most famous model of self-organising behaviour, it also has direct links to areas at the interface between computer science and statistical mechanics, such as the Ising model and the study of contagion and cascading phenomena in networks. While the model has been extensively studied it has largely resisted rigorous analysis, prior results from the literature generally pertaining to variants of the model which are tweaked so as to be amenable to standard techniques from statistical mechanics or stochastic evolutionary game theory. In \cite{BK}, Brandt, Immorlica, Kamath and Kleinberg provided the first rigorous analysis of the unperturbed model, for a specific set of input parameters. Here we provide a rigorous analysis of the model's behaviour much more generally and establish some surprising forms of threshold behaviour, notably the existence of situations where an \emph{increased} level of intolerance for neighbouring agents of opposite type leads almost certainly to \emph{decreased} segregation.

cs.GT↗

arXiv2014

Tipping Points in Schelling Segregation

One of the earliest agent-based economical models, Schelling's spacial proximity model illustrated how global segregation can emerge, often unwanted, from the actions of agents of two races acting in accordance with their individual local preferences. Here a 1-dimensional unperturbed variant of the model is studied, which is additionally open in the sense that agents may enter and exit the model. Following the authors' previous work in [1] and that of Brandt, Immorlica, Kamath, and Kleinberg in [2], rigorous results are established, whose statements are asymptotic in both the model and neighbourhood sizes. The current model's openness allows one race or the other to take over almost everywhere in a measure-theoretic sense. Tipping points are identified between the two regions of takeover and the region of staticity, in terms of the parameters of the model. In a significant generalization from previous work, the parameters comprise the initial proportions of the two races, along with independent values of the tolerance for each race.

cs.GT↗