Balance constants for Coxeter groups

The $1/3$-$2/3$ Conjecture, originally formulated in 1968, is one of the best-known open problems in the theory of posets, stating that the balance constant (a quantity determined by the linear extensions) of any non-total order is at least $1/3$. By reinterpreting balance constants of posets in terms of convex subsets of the symmetric group, we extend the study of balance constants to convex subsets $C$ of any Coxeter group. Remarkably, we conjecture that the lower bound of $1/3$ still applies in any finite Weyl group, with new and interesting equality cases appearing. We generalize several of the main results towards the $1/3$-$2/3$ Conjecture to this new setting: we prove our conjecture when $C$ is a weak order interval below a fully commutative element in any acyclic Coxeter group (an generalization of the case of width-two posets), we give a uniform lower bound for balance constants in all finite Weyl groups using a new generalization of order polytopes to this context, and we introduce generalized semiorders for which we resolve the conjecture. We hope this new perspective may shed light on the proper level of generality in which to consider the $1/3$-$2/3$ Conjecture, and therefore on which methods are likely to be successful in resolving it.Comment: 27 page

Risky decision making : testing for violations of transitivity predicted by an editing mechanism

Transitivity is the assumption that if a person prefers A to B and B to C, then that person should prefer A to C. This article explores a paradigm in which Birnbaum, Patton and Lott (1999) thought people might be systematically intransitive. Many undergraduates choose C = ($96, .85;$90, .05; $12, .10) over A = ($96, .9; $14, .05;$12, .05), violating dominance. Perhaps people would detect dominance in simpler choices, such as A versus B = ($96, .9;$12, .10) and B versus C, and yet continue to violate it in the choice between A and C, which would violate transitivity. In this study we apply a true and error model to test intransitive preferences predicted by a partially effective editing mechanism. The results replicated previous findings quite well; however, the true and error model indicated that very few, if any, participants exhibited true intransitive preferences. In addition, violations of stochastic dominance showed a strong and systematic decrease in prevalence over time and viola

Rationality or irrationality of preferences? A quantitative test of intransitive decision heuristics

In this paper, I present a comprehensive analysis of two decision heuristics that permit intransitive preferences: the lexicographic semiorder model and the similarity model. I also compare these two intransitive decision heuristics with transitive linear order models and two simple transitive heuristics. For each decision theory, I use two types of probabilistic specifications: distance-based models (which assume deterministic preferences and probabilistic response processes), and mixture models (which assume probabilistic preferences and deterministic response processes). I test 26 such probabilistic models on datasets from three different experiments using both frequentist and Bayesian order-constrained statistical methods. The frequentist goodness-of-fit tests show that the distance-based models with modal choice and the mixture models for all of the decision heuristics explain the participants' data fairly well for all stimulus sets. The frequentist analysis generates little evidence against transitivity. Model selection using Bayes factors suggests extensive heterogeneity across participants and stimulus sets

Random structures for partially ordered sets

This thesis is presented in two parts. In the first part, we study a family of models of random partial orders, called classical sequential growth models, introduced by Rideout and Sorkin as possible models of discrete space-time. We analyse a particular model, called a random binary growth model, and show that the random partial order produced by this model almost surely has infinite dimension. We also give estimates on the size of the largest vertex incomparable to a particular element of the partial order. We show that there is some positive probability that the random partial order does not contain a particular subposet. This contrasts with other existing models of partial orders. We also study "continuum limits" of sequences of classical sequential growth models. We prove results on the structure of these limits when they exist, highlighting a deficiency of these models as models of space-time. In the second part of the thesis, we prove some correlation inequalities for mappings of rooted trees into complete trees. For T a rooted tree we can define the proportion of the total number of embeddings of T into a complete binary tree that map the root of T to the root of the complete binary tree. A theorem of Kubicki, Lehel and Morayne states that, for two binary trees with one a subposet of the other, this proportion is larger for the larger tree. They conjecture that the same is true for two arbitrary trees with one a subposet of the other. We disprove this conjecture by analysing the asymptotics of this proportion for large complete binary trees. We show that the theorem of Kubicki, Lehel and Morayne can be thought of as a correlation inequality which enables us to generalise their result in other directions

Enumeration in algebra and geometry

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mathematics, 1997.Includes bibliographical references (p. 85-88).by Alexander Postnikov.Ph.D

Sandpiles and Dominos

We consider the subgroup of the abelian sandpile group of the grid graph consisting of configurations of sand that are symmetric with respect to central vertical and horizontal axes. We show that the size of this group is (i) the number of domino tilings of a corresponding weighted rectangular checkerboard; (ii) a product of special values of Chebyshev polynomials; and (iii) a double-product whose factors are sums of squares of values of trigonometric functions. We provide a new derivation of the formula due to Kasteleyn and to Temperley and Fisher for counting the number of domino tilings of a 2m x 2n rectangular checkerboard and a new way of counting the number of domino tilings of a 2m x 2n checkerboard on a M\"obius strip.Comment: 35 pages, 24 figure

Poset limits can be totally ordered

S. Janson [Poset limits and exchangeable random posets, Combinatorica 31 (2011), 529-563] defined limits of finite posets in parallel to the emerging theory of limits of dense graphs. We prove that each poset limit can be represented as a kernel on the unit interval with the standard order, thus answering an open question of Janson. We provide two proofs: real-analytic and combinatorial. The combinatorial proof is based on a Szemerédi-type Regularity Lemma for posets which may be of independent interest. Also, as a by-product of the analytic proof, we show that every atomless ordered probability space admits a measure-preserving and almost order-preserving map to the unit interval

Rationality or irrationality of preferences? Quantitative tests of decision theories

To have transitive preferences, for any options x, y, and z, one who prefers x to y and y to z must prefer x to z. Transitivity of preferences is a very fundamental element of utility and plays an important role in many major contemporary theories of decision making under risk or uncertainty. One has to be very careful about claiming violations of transitivity of preferences. In my thesis, I present a comprehensive analysis of several decision heuristics that permit intransitive preferences: the lexicographic semiorder model (Tversky, 1969), the similarity model (Rubinstein, 1988), and perceived relative argument model (PRAM, Loomes, 2010a), as well as several transitive decision theories: the linear order model and 49 versions of Cumulative Prospect Theory (CPT, Tversky and Kahneman, 1992a). For each decision theory, I use two kinds of probabilistic specifications to explain choice variability: a distance-based probabilistic specification models preferences as deterministic and response processes as probabilistic, and a mixture specification models preferences as probabilistic and response processes as deterministic. I test these probabilistic models on data sets from different experiments, using both frequentist (Davis-Stober, 2009, Iverson and Falmagne, 1985, Silvapulle and Sen, 2005) and Bayesian (Myung et al., 2005) order-constrained, likelihood-based statistical inference methods. This thesis is one of the largest scale projects for a systematic evaluation of both transitive and intransitive decision theories. The quantitative analyses in this paper consumed about 822,000 CPU hours on Pittsburgh Supercomputer Center’s Blacklight, Greenfield, and Bridges supercomputers, as an Extreme Science and Engineering Discovery Environment project (see also, Towns et al., 2014). Individual model selection using Bayes factors shown extensive heterogeneity across participants and stimulus sets. In general, the overall conclusion is that Cumulative Prospective Theory and Perceived Relative Argument Model was systematically violated, and the intransitive heuristics performed reasonable well

The shape of the PCA trajectories and the population neural coding of movement initiation in the basal ganglia

In this work we study the shape of the neural trajectories obtained from Principal Components Analysis (PCA) of neural activity, related to movement initiation, in the basal ganglia of rats. We focus on the relation between global and local aspects of the shape, and on the population code by means of the ensemble structure. We find that the structure of the principal components is intimately related to the ensembles in the population which, in turn, drive the evolution of the neural trajectories. From this point of view, the coding schemes in the input and output stages of the basal ganglia differ, being the output lower dimensional than the input. In the output stage we can identify specific ensembles that explain the main features (sharp points, singular points, etc.) of the shape, revealing novel aspects of the computations performed by these regions during movement. Also, based upon new measures of heterogeneity and sparseness, we conclude that the output stages are homogeneous but dense while the input stages are more heterogeneous. This work also contains novel mathematical results in relation with the restrictions on the shape imposed by the PCA and the by structure of the data. A novel relationship between the principal components and Catalan objects is proved