PROGRAM
Abstracts presentations phd students
BRITO, MARISA DE
Erasmus University Rotterdam
P.O. Box 1738
3000 DR Rotterdam
debrito@few.eur.nl
Supervisor
R. Dekker
Title
Product returns in inventory control: an emperical study
Abstract
Already for a long time, companies take back products as part of
their marketing strategy, where products in good condition go back to inventory.
Furthermore, environmental consciousness, legal and economic forces have
brought more attention to systems with reverse flows, and to its control.
From a modelling perspective, one of the consequences of reverse flows
is the loss of monotonicity of inventory levels between replenishments
of new products. That is, the inventory level does not only decrease because
of demand but it also may increase in case of returns. Since this makes
the analysis much more difficult than traditional inventory control, authors
use simplifying assumptions regarding the return process. These assumptions
typically are 1) the demand is a homogeneous Poisson process; 2) the return
flow is similarly a homogeneous Poisson process, and 3) the return process
is independent of the demand process. However, there is nearly no (scientific)
literature on empirical analysis of data with reverse flows. Thus, we present
a methodology to check empirically the common assumptions on the inventory
modelling literature concerning the demand and return processes. Moreover,
we apply it to real data and we discuss practical implications of our findings,
for instance relatively to information management on inventory systems
with returns. The data to be used concerns inventory transactions, both
purchases and returns, in the warehouse of the European Organization for
Nuclear Research (CERN).
BUMB, ADRIANA
Department of Discrete Mathematics, Operations Research and Statistics
Faculty of Applied Mathematics
University of Twente
P.O. Box 217
7500 AE Enschede
a.f.bumb@math.utwente.nl
http://www.math.utwente.nl/dos/bumb.htm
Supervisors
U. Faigle and W. Kern
Title
An approximation algorithm for the 2level uncapacitated facility
location problem
Abstract
We present an approximation algorithm for the maximization version
of the two level uncapacitated facility location problem.The problem can
be described as follows.There are two types of facilities to be built:one
type of depots and one type of transit stations.Each unit of demand must
be shipped from a depot through a transit station to the demand points.The
goal is to decide simultaneously which facilities to open and how to assign
the clients to the open facilities, such that the total profit is maximized.
The main idea of the algorithm is to reduce the problem to a special case
of MAX SAT, for which an approximation algorithm based on randomized rounding
is presented.
GRIGORIEV, ALEXANDER
Operations Research Group
Department of Quantitative Economics
Faculty of Economics abd Business Administration
University of Maastricht
P.O. Box 616
6200 MD Maastricht
a.grigoriev@ke.unimaas.nl
Supervisors
A.W.J. Kolen/Y. Crama/J. van de Klundert
Title
Throughput Rate Optimization in High Multiplicity Sequencing Problems
Abstract
In mixed model assembly systems products or parts of different types
are being assembled in certain ratios. A minimal part set is a smallest
possible set of product type quantities, to be called the multiplicities,
in which the numbers of assembled products of the various types are in
the desired ratios. It is common practice to repeatedly input the minimal
part set into the assembly system, where the products of each of the minimal
part set are fed into the system in a same sequence. Such a sequencing
strategy is expected to yield a good line balance and low inventories.
Very little is known however regarding the resulting throughput rate, in
particular in comparison to the throughput rates attainable by other input
strategies. In general, other strategies can be described by considering
the same problem with modified multiplicities yielding the same ratios,
as they can be obtained by multiplying the multiplicities by a same number.
In this work, we investigate the relationship between throughput rates
and the multiplicities for two basic sequencing problems, namely, the traveling
salesman problem and the nowait flowshop scheduling problem. We will investigate
and answer questions such as: What is the possible loss of throughput that
results from inputing minimal part sets? What are the smallest multiplicities
for which the minimal throughput rate is the lowest possible? What
are the smallest multiplicities for which an optimal input sequence has
the same structure as the overall optimal, but possibly infinitely long,
input sequence? Our analysis uses well known concepts from scheduling theory
and combinatorial optimization.
HUITZING,
HIDDO
Faculty of Psychological, Pedagogical and Sociological Sciences
Groningen University
Grote Rozenstraat 15
9712 TG Groningen
h.a.huitzing@ppsw.rug.nl
http://www2.ppsw.rug.n/~beukema/persoon/hiddo.html
Supervisors
W.K. Klein Haneveld/Timminga
Title
Detecting, Pinpointing and Analyzing of Infeasibility of Linear
Programming Test Assembly Models using Data Sampling and Set Covering
Abstract
In this paper it shown how Set Covering (SC) methods can be used
in the preanalysis of linear programming (LP) models for test assembly
(LPTA) models. The focus will be on the LPTA models. Use of SC and data
sampling to analyze infeasibility was first suggested by Feng (personal
communication, 2000). While SC can in a first step help to detect
feasibility or infeasibility, its power lies in pinpointing causes of infeasibility
such as Irreducible Infeasible Sets of constraints (IIS), Maximal Feasible
Subsets of constraints (MFS) and Minimal Cardinality IIS Set Covers (MCISC)
of the original infeasible LPTA model. Timminga & Adema (1996), Timminga
(1998), van der Linden (1998) and Huitzing (2000), among others, have presented
means to detect infeasibility in LPTA models and described methods to resolve
infeasibility, actually adjusting the model constraints minimizing “loss”,
where "loss" is to be defined beforehand. First, a short introduction to
the theory and its possible applications to LPTA models is given. Second,
a simulation study, using data sampling, of the power of SC in detecting,
pinpointing and analysis of infeasibility in LPTA models is presented.
Third, the practical advantages and shortcomings of SC are explored.
KOVALEVA, SOFIA
Department of Mathematics
Maastricht University
P.O. Box 616
6200 MD Maastricht
s.kovaleva@math.unimaas.nl
Supervisor
F. Spieksma
Title
Approximation algorithms for an interval packing and covering problem
Abstract
Consider a grid consisting of rows and columns. Intervals are placed
at arbitrary positions on each row, each interval having a nonnegative
integral weight and integral length. A nonnegative integral capacity is
associated to each row and to each column. The packing problem is to specify
a multiplicity for each interval maximizing total weight while the sum
of multiplicities of intervals that share each column or row does not exceed
the given capacity. The covering problem is to specify a multiplicity for
each column and row minimizing total weight while the sum of the
multiplicities that stab each interval is not below its weight. These MAXSNPhard
problems arise in project scheduling, admission control and DNA sequencing.
Their linear relaxations constitute a primaldual pair of problems.
We prove an approximative minmax result for the case of unit weights
and that of unit capacities. We get a 1/2approximation algorithm for the
packing problem and 2approximation algorithm for the covering
problem.
LISTES, OVIDIU
Econometric Institute
Erasmus University Rotterdam
P.O. Box 1738
3000 DR Rotterdam
listes@few.eur.nl
http://www.few.eur.nl/few/people/listes
Supervisor
R. Dekker
Title
Stochastic solutions vs. scenario analysis for product recovery
network design
Abstract
For product recovery network design under uncertainty a strategic
location model is presented. We use data from a real case on recycling
sand from demolition waste in The Netherlands. The model is extended using
stochastic programming techniques to account for the uncertainties in demand
and supply. Computational results for twostage and threestage variants
are presented. The interpretation of the results is meant to give more
insight into decisionmaking under uncertainty for reverse logistics.
LITVAK, NELLY
EURANDOM
Eindhoven University of Technology
Building LG
P.O. Box 513
5600 MB Eindhoven
n.litvak@tue.nl
Supervisor
J.Wessels
Title
Order picking in carousel systems operating under the Nearest Item
heuristic
Abstract
A carousel (or paternoster) is an automated warehousing system consisting
of a large number of drawers rotating in a closed loop in either direction.
Such systems are mostly used for storage and retrieval of small and medium
sized goods, which are requested moderately often. The picker has a fixed
position in front of the carousel, which rotates the required items to
the picker.
An important performance characteristic is the travel time needed
to pick a list of items. In this presentation we study the travel time
under the Nearest Item (NI) heuristic, where the next item to be picked
is always the nearest one. This simple algorithm is frequently used in
real warehouses. We first give a tight upper bound for the travel time
under the NI heuristic. Then we present an exhaustive analysis of the travel
time assuming that the required items are uniformly distributed on the
carousel.
PEETERS, LEON
Rotterdam School of Management
Room F12
Erasmus University Rotterdam
P.O. Box 1738
3000 DR Rotterdam
lpeeters@fbk.eur.nl
Supervisors
J.A.E.E. van Nunen/L. Kroon
Title
Optimization for Cyclic Railway Timetabling
Abstract
Many European railway companies operate a cyclic timetable in which
trains leave every hour at the same time from a certain station for a certain
direction. In the Netherlands, NS Reizigers and Railned are currently using
a constraintpropagation algorithm called to search for feasible railway
timetables. We present an optimization model for cyclic railway timetabling
that provides an extension of this feasibility model, which is based on
Periodic Event Scheduling. The optimization model enables one to (i) search
for a timetable that is optimal, and (ii) to gain insight into the necessary
changes to an infeasible instance, by allowing a penalized violation of
the constraints. We use a mixed integer programming formulation for the
problem, where the integer variables correspond to cycles in the graph
induced by the constraints. A heuristic for constructing a good cycle basis
is presented. Furthermore we propose preprocessing procedures that considerably
reduce the size of problem instances. The usefulness of both the formulation
and the preprocessing procedures is illustrated by the application of the
model to the Dutch Intercity train network for several objective functions,
e.g. minimizing travelers' waiting time, maximizing timetable robustness,
and minimizing the number of trains needed to operate the timetable.
POP, PETRICA
Department of Discrete Mathematics, Operations Research and Statistics
Faculty of Applied Mathematics
University of Twente
P.O. Box 217
7500 AE Enschede
ppop@math.utwente.nl
Supervisor
U. Faigle
Title
Relaxation Methods for the Generalized Minimum Spanning Tree Problem
Abstract
We consider the Generalized Minimum Spanning Tree Problem (GMSTP).
Given an undirected graph whose nodes are partitioned into clusters, the
GMSTP is then to find a minimumcost tree which includes exactly one node
from each cluster. It is known that the GMSTP is an NPhard problem and
even finding a near optimal solution is also an NPhard problem. We introduce
several mixed integer programming formulations of the problem and examine
the polyhedra defined by the linear programming relaxations of these formulations.
Based on a new formulation of the GMST problem we give a heuristic
solution, a lower bound procedure, an upper bound procedure and discuss
the advantages of our approach in comparizon with an earlier method.
POPOV, NICOLAI
Department of Mathematics
University of Leiden
P.O. Box 9512
2300 RA Leiden
popov@math.leidenuniv.nl
Supervisors
A. Hordijk/F.M.Spieksma
Title
Large deviation principle for a transient deflected twodimensional
random walk on the nonnegative integers
Abstract
Click here for the postscript file.
SLEPTCHENKO, ANDREI
Faculty of Technology & Management
Operation Methods and System Theory
University of Twente
P.O. Box 217
7500 AE Enschede
a.sleptchenko@sms.utwente.nl
Supervisor
A. van Harten
Title
On multiclass multiserver queueing and spare parts management
Abstract
Multiclass multiserver queuing problems are a generalization of
the wellknown M/M/k situation to arrival processes with clients of N types
that require exponentially distributed service with different averaged
service time. Problems of this sort arise naturally in various applications,
such as spare parts management, for example. Here we are going to present
a procedure to construct exact solutions of the stationary state equations,
and to illustrate it with numerical results."
SMITS, SANNE
Faculty of Technology Management
Eindhoven University of Technology
Paviljoen E6
P.O. Box 513
5600 MB Eindhoven
s.r.smits@tm.tue.nl
Supervisor
A.G. de Kok
Title
Approximations for the waiting time in (s,nQ)inventory models for
different type of consolidation policies
Abstract
The coordination of the inventory managemment and the transportation
management is crucial for an efficient management of the supply chain.Transportation
management includes the application of different types of shipment consolidation
policies. The consolidation policy coordinates the shipment processes of
different ccustomer orders for the same (intermediate) destination at the
warehouse, and this can lead to a reduction in the transport costs.
Shipment consolidation implies that two or more customer orders
are combined such that a larger quantity can be dispachted on the same
vehicle. We distinguish between two types of consolidation policy. The
"quantity" policy, which dispatches the orders when a given quantity is
consolidated. The "time" policy, which dispatches the orders when the shipping
date is expired.
In this paper we assume that customer orders originate from customer
warehouses.
Each customer warehouse control multiple product inventories according
to (s,nQ)policies. I.e. whenever the inventory level drops below the reorder
level s, a quantity nQ is ordered such that the inventory position is raised
to a level between s and s+Q.
The focus of the paper is on the determination of the customer order
lead time behaviour. The waiting time due to c
In the inventory control the lead times play an important role.
The waiting time due to the consolidation of the shipment is included in
the lead time. A change in the shipping date for the "time" policy or a
change in the shipped quantity for the "quantity" policy leads to a change
in the lead times and hence this leads to a reevaluation of variables
in the inventory control to keep the same service degree.
In this paper, we will derive approximations for the lead time behaviour
in a multi echelon (s,nQ)inventory models when the products are shipped
according to different types of consolidation policies. The derived approximations
are tested through extensive computer simulation.
A further step in this research will be to find close to optimal
values for the batchsizes and the shipping date in the "time" policy and
close to optimal values for the batchsizes and the shipped quantity in
the "quantity" policy.
UITERT, MIRANDA VAN
CWI  Center for Mathematics and Computer Science
Department PNA 2
P.O. Box 94079
1090 GB Amsterdam
email : miranda@cwi.nl
Supervisor
S.C. Borst
Title
Generalized Processor Sharing Queues with Heterogeneous Traffic
Classes
Abstract
We consider a queue fed by a mixture of lighttailed and heavytailed
traffic. The two traffic flows are served in accordance with the Generalized
Processor Sharing (GPS) discipline. GPSbased scheduling algorithms, such
as Weighted Fair Queueing (WFQ), have emerged as an important mechanism
for achieving service differentiation in integrated networks.
We analyze the asymptotic behavior of the workload distribution
of the lighttailed traffic flow under the assumption that its GPS weight
is larger than its traffic intensity. The GPS mechanism ensures that the
workload of the lighttailed flow is bounded above by that in an isolated
system, which consists of the lighttailed flow served in isolation at
a constant rate equal to its GPS weight.
We show that the workload distribution is in fact asymptotically
equivalent to that in the isolated system, multiplied with a certain prefactor,
which accounts for the presence of the heavytailed flow. The prefactor
represents the probability that the heavytailed flow is backlogged long
enough for the lighttailed flow to reach overflow. The results provide
crucial qualitative insight in the typical overflow scenario.
