86 research outputs found
Counting, grafting and evolving binary trees
Binary trees are fundamental objects in models of evolutionary biology and population genetics. Here, we discuss some of their combinatorial and structural properties as they depend on the tree class considered. Furthermore, the process by which trees are generated determines the probability distribution in tree space. Yule trees, for instance, are generated by a pure birth process. When considered as unordered, they have neither a closed-form enumeration nor a simple probability distribution. But their ordered siblings have both. They present the object of choice when studying tree structure in the framework of evolving genealogies
Enumeration of coalescent histories for caterpillar species trees and -pseudocaterpillar gene trees
For a fixed set containing taxon labels, an ordered pair consisting
of a gene tree topology and a species tree bijectively labeled with the
labels of possesses a set of coalescent histories -- mappings from the set
of internal nodes of to the set of edges of describing possible lists
of edges in on which the coalescences in take place. Enumerations of
coalescent histories for gene trees and species trees have produced suggestive
results regarding the pairs that, for a fixed , have the largest
number of coalescent histories. We define a class of 2-cherry binary tree
topologies that we term -pseudocaterpillars, examining coalescent histories
for non-matching pairs , in the case in which has a caterpillar
shape and has a -pseudocaterpillar shape. Using a construction that
associates coalescent histories for with a class of "roadblocked"
monotonic paths, we identify the -pseudocaterpillar labeled gene tree
topology that, for a fixed caterpillar labeled species tree topology, gives
rise to the largest number of coalescent histories. The shape that maximizes
the number of coalescent histories places the "second" cherry of the
-pseudocaterpillar equidistantly from the root of the "first" cherry and
from the tree root. A symmetry in the numbers of coalescent histories for
-pseudocaterpillar gene trees and caterpillar species trees is seen to exist
around the maximizing value of the parameter . The results provide insight
into the factors that influence the number of coalescent histories possible for
a given gene tree and species tree
An Introduction to Wishart Matrix Moments
These lecture notes provide a comprehensive, self-contained introduction to
the analysis of Wishart matrix moments. This study may act as an introduction
to some particular aspects of random matrix theory, or as a self-contained
exposition of Wishart matrix moments. Random matrix theory plays a central role
in statistical physics, computational mathematics and engineering sciences,
including data assimilation, signal processing, combinatorial optimization,
compressed sensing, econometrics and mathematical finance, among numerous
others. The mathematical foundations of the theory of random matrices lies at
the intersection of combinatorics, non-commutative algebra, geometry,
multivariate functional and spectral analysis, and of course statistics and
probability theory. As a result, most of the classical topics in random matrix
theory are technical, and mathematically difficult to penetrate for non-experts
and regular users and practitioners. The technical aim of these notes is to
review and extend some important results in random matrix theory in the
specific context of real random Wishart matrices. This special class of
Gaussian-type sample covariance matrix plays an important role in multivariate
analysis and in statistical theory. We derive non-asymptotic formulae for the
full matrix moments of real valued Wishart random matrices. As a corollary, we
derive and extend a number of spectral and trace-type results for the case of
non-isotropic Wishart random matrices. We also derive the full matrix moment
analogues of some classic spectral and trace-type moment results. For example,
we derive semi-circle and Marchencko-Pastur-type laws in the non-isotropic and
full matrix cases. Laplace matrix transforms and matrix moment estimates are
also studied, along with new spectral and trace concentration-type
inequalities
Strict monotonic trees arising from evolutionary processes: combinatorial and probabilistic study
In this paper we study two models of labelled random trees that generalise the original unlabelled Schröder tree. Our new models can be seen as models for phylogenetic trees in which nodes represent species and labels encode the order of appearance of these species, and thus the chronology of evolution. One important feature of our trees is that they can be generated efficiently thanks to a dynamical, recursive construction. Our first model is an increasing tree in the classical sense (labels increase along each branch of the tree and each label appears only once). To better model phylogenetic trees, we relax the rules of labelling by allowing repetitions in the second model.For each of the two models, we provide asymptotic theorems for different characteristics of the tree (e.g. degree of the root, degree distribution, height, etc.), thus giving extensive information about the typical shapes of these trees. We also provide efficient algorithms to generate large trees efficiently in the two models. The proofs are based on a combination of analytic combinatorics, probabilistic methods, and bijective methods (we exhibit bijections between our models and well-known models of the literature such as permutations and Stirling numbers of both kinds).It turns out that even though our models are labelled, they can be specified simply in the world of ordinary generating functions. However, the resulting generating functions will be formal. Then, by applying Borel transforms the models will be amenable to techniques of analytic combinatorics
Faà di Bruno's formula and inversion of power series
Faà di Bruno's formula gives an expression for the derivatives of the composition of two real-valued functions. In this paper we prove a multivariate and synthesised version of Faà di Bruno's formula in higher dimensions, providing a combinatorial expression for the derivatives of chain compositions of functions in terms of sums over labelled trees. We give several applications of this formula, including a new involution formula for the inversion of multivariate power series. We use this framework to outline a combinatorial approach to studying the invertibility of polynomial mappings, giving a purely combinatorial restatement of the Jacobian conjecture. Our methods extend naturally to the non-commutative case, where we prove a free version of Faà di Bruno's formula for multivariate power series in free indeterminates, and use this formula as a tool for obtaining a new inversion formula for free power series
An introduction to Wishart matrix moments
© 2018 Now Publishers Inc. All rights reserved. These lecture notes provide a comprehensive, self-contained introduction to the analysis of Wishart matrix moments. This study may act as an introduction to some particular aspects of random matrix theory, or as a self-contained exposition of Wishart matrix moments. Random matrix theory plays a central role in statistical physics, computational mathematics and engineering sciences, including data assimilation, signal processing, combinatorial optimization, compressed sensing, econometrics and mathematical finance, among numerous others. The mathematical foundations of the theory of random matrices lies at the intersection of combinatorics, non-commutative algebra, geometry, multivariate functional and spectral analysis, and of course statistics and probability theory. As a result, most of the classical topics in random matrix theory are technical, and mathematically difficult to penetrate for non-experts and regular users and practitioners. The technical aim of these notes is to review and extend some important results in random matrix theory in the specific context of real random Wishart matrices. This special class of Gaussian-type sample covariance matrix plays an important role in multivariate analysis and in statistical theory.We derive non-asymptotic formulae for the full matrix moments of real valued Wishart random matrices. As a corollary, we derive and extend a number of spectral and trace-type results for the case of non-isotropic Wishart random matrices. We also derive the full matrix moment analogues of some classic spectral and trace-type moment results. For example, we derive semi-circle and Marchencko-Pastur-type laws in the non-isotropic and full matrix cases. Laplace matrix transforms and matrix moment estimates are also studied, along with new spectral and trace concentration-type inequalities
Deformations of the braid arrangement and Trees
International audienceWe establish counting formulas and bijections for deformations of the braid arrangement. Precisely, we consider real hyperplane arrangements such that all the hyperplanes are of the form for some integer . Classical examples include the braid, Catalan, Shi, semiorder and Linial arrangements, as well as graphical arrangements. We express the number of regions of any such arrangement as a signed count of decorated plane trees. The characteristic and coboundary polynomials of these arrangements also have simple expressions in terms of these trees. We then focus on certain ``well-behaved'' deformations of the braid arrangement that we call transitive. This includes the Catalan, Shi, semiorder and Linial arrangements, as well as many other arrangements appearing in the literature. For any transitive deformation of the braid arrangement we establish a simple bijection between regions of the arrangement and a set of plane trees defined by local conditions. This answers a question of Gessel
Improved Cardinality Bounds for Rectangle Packing Representations
Axis-aligned rectangle packings can be characterized by the set of spatial relations that hold for pairs of rectangles (west, south, east, north). A representation of a packing consists of one satisfied spatial relation for each pair. We call a set of representations complete for n ∈ ℕ if it contains a representation of every packing of any n rectangles. Both in theory and practice, fastest known algorithms for a large class of rectangle packing problems enumerate a complete set R of representations. The running time of these algorithms is dominated by the (exponential) size of R. In this thesis, we improve the best known lower and upper bounds on the minimum cardinality of complete sets of representations. The new upper bound implies theoretically faster algorithms for many rectangle packing problems, for example in chip design, while the new lower bound imposes a limit on the running time that can be achieved by any algorithm following this approach. The proofs of both results are based on pattern-avoiding permutations. Finally, we empirically compute the minimum cardinality of complete sets of representations for small n. Our computations directly suggest two conjectures, connecting well-known Baxter permutations with the set of permutations avoiding an apparently new pattern, which in turn seem to generate complete sets of representations of minimum cardinality
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