37,182 research outputs found
Intersecting families of discrete structures are typically trivial
The study of intersecting structures is central to extremal combinatorics. A
family of permutations is \emph{-intersecting} if
any two permutations in agree on some indices, and is
\emph{trivial} if all permutations in agree on the same
indices. A -uniform hypergraph is \emph{-intersecting} if any two of its
edges have vertices in common, and \emph{trivial} if all its edges share
the same vertices.
The fundamental problem is to determine how large an intersecting family can
be. Ellis, Friedgut and Pilpel proved that for sufficiently large with
respect to , the largest -intersecting families in are the trivial
ones. The classic Erd\H{o}s--Ko--Rado theorem shows that the largest
-intersecting -uniform hypergraphs are also trivial when is large. We
determine the \emph{typical} structure of -intersecting families, extending
these results to show that almost all intersecting families are trivial. We
also obtain sparse analogues of these extremal results, showing that they hold
in random settings.
Our proofs use the Bollob\'as set-pairs inequality to bound the number of
maximal intersecting families, which can then be combined with known stability
theorems. We also obtain similar results for vector spaces.Comment: 19 pages. Update 1: better citation of the Gauy--H\`an--Oliveira
result. Update 2: corrected statement of the unpublished Hamm--Kahn result,
and slightly modified notation in Theorem 1.6 Update 3: new title, updated
citations, and some minor correction
Discrepancy bounds for low-dimensional point sets
The class of -nets and -sequences, introduced in their most
general form by Niederreiter, are important examples of point sets and
sequences that are commonly used in quasi-Monte Carlo algorithms for
integration and approximation. Low-dimensional versions of -nets and
-sequences, such as Hammersley point sets and van der Corput sequences,
form important sub-classes, as they are interesting mathematical objects from a
theoretical point of view, and simultaneously serve as examples that make it
easier to understand the structural properties of -nets and
-sequences in arbitrary dimension. For these reasons, a considerable
number of papers have been written on the properties of low-dimensional nets
and sequences
High-Dimensional Joint Estimation of Multiple Directed Gaussian Graphical Models
We consider the problem of jointly estimating multiple related directed
acyclic graph (DAG) models based on high-dimensional data from each graph. This
problem is motivated by the task of learning gene regulatory networks based on
gene expression data from different tissues, developmental stages or disease
states. We prove that under certain regularity conditions, the proposed
-penalized maximum likelihood estimator converges in Frobenius norm to
the adjacency matrices consistent with the data-generating distributions and
has the correct sparsity. In particular, we show that this joint estimation
procedure leads to a faster convergence rate than estimating each DAG model
separately. As a corollary, we also obtain high-dimensional consistency results
for causal inference from a mix of observational and interventional data. For
practical purposes, we propose \emph{jointGES} consisting of Greedy Equivalence
Search (GES) to estimate the union of all DAG models followed by variable
selection using lasso to obtain the different DAGs, and we analyze its
consistency guarantees. The proposed method is illustrated through an analysis
of simulated data as well as epithelial ovarian cancer gene expression data
From van der Corput to modern constructions of sequences for quasi-Monte Carlo rules
In 1935 J.G. van der Corput introduced a sequence which has excellent uniform
distribution properties modulo 1. This sequence is based on a very simple
digital construction scheme with respect to the binary digit expansion.
Nowadays the van der Corput sequence, as it was named later, is the prototype
of many uniformly distributed sequences, also in the multi-dimensional case.
Such sequences are required as sample nodes in quasi-Monte Carlo algorithms,
which are deterministic variants of Monte Carlo rules for numerical
integration. Since its introduction many people have studied the van der Corput
sequence and generalizations thereof. This led to a huge number of results.
On the occasion of the 125th birthday of J.G. van der Corput we survey many
interesting results on van der Corput sequences and their generalizations. In
this way we move from van der Corput's ideas to the most modern constructions
of sequences for quasi-Monte Carlo rules, such as, e.g., generalized Halton
sequences or Niederreiter's -sequences
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