264,474 research outputs found
Multi-objective model for optimizing railway infrastructure asset renewal
Trabalho inspirado num problema real da empresa Infraestruturas de Portugal, EP.A multi-objective model for managing railway infrastructure asset renewal is presented. The model aims to optimize three objectives, while respecting operational constraints: levelling investment throughout multiple years, minimizing total cost and minimizing work start postponements. Its output is an optimized intervention schedule. The model is based on a case study from a Portuguese infrastructure management company, which specified the objectives and constraints, and reflects management practice on railway infrastructure. The results show that investment levelling greatly influences the other objectives and that total cost fluctuations may range from insignificant to important, depending on the condition of the infrastructure. The results structure is argued to be general and suggests a practical methodology for analysing trade-offs and selecting a solution for implementation.info:eu-repo/semantics/publishedVersio
Recurrence and transience for suspension flows
We study the thermodynamic formalism for suspension flows over countable
Markov shifts with roof functions not necessarily bounded away from zero. We
establish conditions to ensure the existence and uniqueness of equilibrium
measures for regular potentials. We define the notions of recurrence and
transience of a potential in this setting. We define the "renewal flow", which
is a symbolic model for a class of flows with diverse recurrence features. We
study the corresponding thermodynamic formalism, establishing conditions for
the existence of equilibrium measures and phase transitions. Applications are
given to suspension flows defined over interval maps having parabolic fixed
points.Comment: In this version of the paper some typos have been corrected and some
references updated. Note that the former title of this paper was "Parabolic
suspension flows
On the Statistical Modeling and Analysis of Repairable Systems
We review basic modeling approaches for failure and maintenance data from
repairable systems. In particular we consider imperfect repair models, defined
in terms of virtual age processes, and the trend-renewal process which extends
the nonhomogeneous Poisson process and the renewal process. In the case where
several systems of the same kind are observed, we show how observed covariates
and unobserved heterogeneity can be included in the models. We also consider
various approaches to trend testing. Modern reliability data bases usually
contain information on the type of failure, the type of maintenance and so
forth in addition to the failure times themselves. Basing our work on recent
literature we present a framework where the observed events are modeled as
marked point processes, with marks labeling the types of events. Throughout the
paper the emphasis is more on modeling than on statistical inference.Comment: Published at http://dx.doi.org/10.1214/088342306000000448 in the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Stability Analysis of GI/G/c/K Retrial Queue with Constant Retrial Rate
We consider a GI/G/c/K-type retrial queueing system with constant retrial
rate. The system consists of a primary queue and an orbit queue. The primary
queue has identical servers and can accommodate the maximal number of
jobs. If a newly arriving job finds the full primary queue, it joins the orbit.
The original primary jobs arrive to the system according to a renewal process.
The jobs have general i.i.d. service times. A job in front of the orbit queue
retries to enter the primary queue after an exponentially distributed time
independent of the orbit queue length. Telephone exchange systems, Medium
Access Protocols and short TCP transfers are just some applications of the
proposed queueing system. For this system we establish minimal sufficient
stability conditions. Our model is very general. In addition, to the known
particular cases (e.g., M/G/1/1 or M/M/c/c systems), the proposed model covers
as particular cases the deterministic service model and the Erlang model with
constant retrial rate. The latter particular cases have not been considered in
the past. The obtained stability conditions have clear probabilistic
interpretation
Non trivial limit distributions for transient renewal chains
In this work we study the asymptotic of renewal sequences associated with
certain transient renewal Markov chains and enquire about the existence of
limit laws in this set up
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