72 research outputs found
A central limit theorem for temporally non-homogenous Markov chains with applications to dynamic programming
We prove a central limit theorem for a class of additive processes that arise
naturally in the theory of finite horizon Markov decision problems. The main
theorem generalizes a classic result of Dobrushin (1956) for temporally
non-homogeneous Markov chains, and the principal innovation is that here the
summands are permitted to depend on both the current state and a bounded number
of future states of the chain. We show through several examples that this added
flexibility gives one a direct path to asymptotic normality of the optimal
total reward of finite horizon Markov decision problems. The same examples also
explain why such results are not easily obtained by alternative Markovian
techniques such as enlargement of the state space.Comment: 27 pages, 1 figur
Quickest Online Selection of an Increasing Subsequence of Specified Size
Given a sequence of independent random variables with a common continuous
distribution, we consider the online decision problem where one seeks to
minimize the expected value of the time that is needed to complete the
selection of a monotone increasing subsequence of a prespecified length .
This problem is dual to some online decision problems that have been considered
earlier, and this dual problem has some notable advantages. In particular, the
recursions and equations of optimality lead with relative ease to asymptotic
formulas for mean and variance of the minimal selection time.Comment: 17 page
Optimal Online Selection of a Monotone Subsequence: a Central Limit Theorem
Consider a sequence of independent random variables with a common
continuous distribution , and consider the task of choosing an increasing
subsequence where the observations are revealed sequentially and where an
observation must be accepted or rejected when it is first revealed. There is a
unique selection policy that is optimal in the sense that it
maximizes the expected value of , the number of selected
observations. We investigate the distribution of ; in particular,
we obtain a central limit theorem for and a detailed
understanding of its mean and variance for large . Our results and methods
are complementary to the work of Bruss and Delbaen (2004) where an analogous
central limit theorem is found for monotone increasing selections from a finite
sequence with cardinality where is a Poisson random variable that is
independent of the sequence.Comment: 26 page
Optimal Online Selection of an Alternating Subsequence: A Central Limit Theorem
We analyze the optimal policy for the sequential selection of an alternating subsequence from a sequence of n independent observations from a continuous distribution F, and we prove a central limit theorem for the number of selections made by that policy. The proof exploits the backward recursion of dynamic programming and assembles a detailed understanding of the associated value functions and selection rules
Beardwood-Halton-Hammersly Theorem for Stationary Ergodic Sequences: A Counterexample
We construct a stationary ergodic process X1,X2,…such that each Xt has the uniform distribution on the unit square and the length Ln of the shortest path through the points X1,X2,…,Xn is not asymptotic to a constant times the square root of n. In other words, we show that the Beardwood, Halton, and Hammersley theorem does not extend from the case of independent uniformly distributed random variables to the case of stationary ergodic sequences with uniform marginal distributions
Optimal Sequential Selection of a Unimodal Subsequence of a Random Sequence
We consider the problem of selecting sequentially a unimodal subsequence from
a sequence of independent identically distributed random variables, and we find
that a person doing optimal sequential selection does within a factor of the
square root of two as well as a prophet who knows all of the random
observations in advance of any selections. Our analysis applies in fact to
selections of subsequences that have d+1 monotone blocks, and, by including the
case d=0, our analysis also covers monotone subsequences
Online Selection of Alternating Subsequences from a Random Sample
We consider sequential selection of an alternating subsequence from a sequence of independent, identically distributed, continuous random variables, and we determine the exact asymptotic behavior of an optimal sequentially selected subsequence. Moreover, we find (in a sense we make precise) that a person who is constrained to make sequential selections does only about 12 percent worse than a person who can make selections with full knowledge of the random sequence
Formation et esprit d'entreprendre chez les étudiants
Cette contribution s'attache à étudier entre esprit d'entreprendre et domaine d'études chez les étudiants français. L'esprit d'entreprendre est le substrat dans lequel peut germer l'intention qui pourra donner lieu à une création d'entreprise. Nous choisissons de l'appréhender à partir d'une grille de lecture dérivée du modèle psychosocial du comportement planifié d'Ajzen, enrichie par des variables reflétant l'implication dans la vie associative et les opinions sur les formations à la création d'entreprise.esprit d'entreprendre ; domaine d'étude ; étudiants français
Humoral Responses against BQ.1.1 Elicited after Breakthrough Infection and SARS-CoV-2 mRNA Vaccination.
The Omicron BQ.1.1 variant is now the major SARS-CoV-2 circulating strain in many countries. Because of the many mutations present in its Spike glycoprotein, this variant is resistant to humoral responses elicited by monovalent mRNA vaccines. With the goal to improve immune responses against Omicron subvariants, bivalent mRNA vaccines have recently been approved in several countries. In this study, we measure the capacity of plasma from vaccinated individuals, before and after a fourth dose of mono- or bivalent mRNA vaccine, to recognize and neutralize the ancestral (D614G) and the BQ.1.1 Spikes. Before and after the fourth dose, we observe a significantly better recognition and neutralization of the ancestral Spike. We also observe that fourth-dose vaccinated individuals who have been recently infected better recognize and neutralize the BQ.1.1 Spike, independently of the mRNA vaccine used, than donors who have never been infected or have an older infection. Our study supports that hybrid immunity, generated by vaccination and a recent infection, induces higher humoral responses than vaccination alone, independently of the mRNA vaccine used
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