Thomas Stidsen

• PhD
• Professor (Associate) at Technical University of Denmark

Researchers have studied the nurse rostering problem for multiple decades. Initially, the formulations were rather primitive including only a few necessary restrictions, but down the road, the formulations have become more complex. Nonetheless, a fraction of the research reaches implementation in practice, and many wards still schedule nurses manua...

Combinatorial optimisation can be applied in a multitude of areas, and this article presents its application to the optimisation of LEGO constructions. Building LEGO constructions is an inherently combinatorial problem, with a large but finite number of possible brick combinations. The objective was to make an aesthetically pleasing construction wh...

Despite the multi-objective nature of the nurse rostering problem (NRP), most NRP formulations employ a single evaluation function that minimizes the weighted sum of constraint violations. When solving the NRP in practice, the focus should be on obtaining compromise solutions: those with appropriate trade-offs between different constraints. Due to...

Daily scheduling of surgical operations is a complicated and recurrent problem in the literature on health care optimization. In this study, we present an often overlooked approach to this problem that incorporates a rolling and overlapping planning horizon. The basis of our modeling approach is a Markov decision process, where patients are schedul...

In this paper we present two mixed-integer programming formulations for the curriculum based course timetabling problem (CTT). We show that the formulations contain underlying network structures by dividing the CTT into two separate models and then connect the two models using flow formulation techniques. The first mixed-integer programming formula...

At universities, the timetable plays a large role in the daily life of students and staff, showing when and where lectures are given. But whenever a schedule is executed in a dynamic environment, disruptions will occur. It is then desirable to find a new timetable similar to the old one, so only a few people will be affected. This leads to a minimu...

University course timetabling covers the task of assigning rooms and time periods to courses while ensuring a minimum violation of soft constraints that define the quality of the timetable. These soft constraints can have attributes that make it difficult for mixed-integer programming solvers to find good solutions fast enough to be used in a pract...

The emergency department is a key element of acute patient flow, but due to high demand and an alternating rate of arriving patients, the department is often challenged by insufficient capacity. Proper allocation of resources to match demand is, therefore, a vital task for many emergency departments. Constrained by targets on patient waiting time,...

In this paper, we considered the problem of Curriculum-Based Course Timetabling, i.e., assigning weekly lectures to a time schedule and rooms. We developed a Column Generation algorithm based on a pattern formulation of the time scheduling part of the problem by Bagger et al. (2016). The pattern formulation is an enumeration of all schedules by whi...

A large number of the real world planning problems which are today solved using Operations Research methods are actually multiobjective planning problems, but most of them are solved using singleobjective methods. The reason for converting, i.e. simplifying, multiobjective problems to singleobjective problems is that no standard multiobjective solv...

In this paper we applied Benders' decomposition to the Curriculum-Based Course Timetabling (CBCT) problem. The objective of the CBCT problem is to assign a set of lectures to time slots and rooms. Our approach was based on segmenting the problem into time scheduling and room allocation problems. The Benders' algorithm was then employed to generate...

EURO-k conferences are among the largest Operations Research conferences in the world, typically including more than 2000 presentations. As opposed to many other conferences, EURO-k conferences are hierarchically organized, and the conference schedule should reflect this structure to make navigation easier and more logical. In this article we prese...

University timetabling has traditionally been studied as an operational problem where the goal is to assign lectures to rooms and timeslots and create timetables of high quality for students and teachers. Two other important decision problems arise before this can be solved: what rooms are necessary, and in which teaching periods? These decisions m...

Educational timetabling problems require the assignment of times and resources to events, while sets of required and desirable constraints must be considered. The XHSTT format was adopted in this work because it models the main features of educational timetabling and it is the most used format in recent studies in the field. This work presents new...

Recently, the XHSTT format for high school timetabling was introduced. It provides a uniform way of modeling problem instances and corresponding solutions. The format supports a wide variety of constraints, and currently 38 real-life instances from 11 different countries are available. Thereby, the XHSTT format serves as a common ground for researc...

Protecting communication networks against failures is becoming increasingly important as they have become an integrated part of our society. Cable failures are fairly common, but it is unacceptable for a single cable failure to disconnect communication for more than a few seconds—hence protection schemes are employed. In contrast to manual interven...

The Elective Course Student Sectioning (ECSS) problem is a yearly recurrent planning problem at the Danish high schools. The problem is of assigning students to elective classes given their requests such that as many requests are fulfilled and the violations of the soft constraints are minimized. This paper presents an Adaptive Large Neighborhood S...

Most real-world optimization problems are multiobjective by nature, involving noncomparable objectives. Many of these problems can be formulated in terms of a set of linear objective functions that should be simultaneously optimized over a class of linear constraints. Often there is the complicating factor that some of the variables are required to...

Most real-world optimization problems are multiobjective by nature, involving noncomparable objectives. Many of these problems can be formulated in terms of a set of linear objective functions that should be simultaneously optimized over a class of linear constraints. Often there is the complicating factor that some of the variables are required to...

Purpose Applied life cycle assessment (LCA) studies often lead to a comparison of rather few alternatives; we call this the “ad hoc LCA approach.” This can seem surprising since applied LCAs normally cover countless options for variations and derived potentials for improvements in a product life cycle. In this paper, we will suggest an alternative...

In the different stages of the educational system, the demand for efficient planning is increasing. This article treats the $$mathcal NP $$ NP -hard Consultation Timetabling Problem, a recurrent planning problem for the high schools in Denmark, which has not been described in the literature before. Two versions of the problem are considered, the Pa...

Protecting communication networks against failures is becoming increasingly important as they have become an integrated part of our society. Cable failures are fairly common, but it is unacceptable for a single cable failure to disconnect communication for more than a few seconds - hence protection schemes are employed. In contrast to manual interv...

This paper addresses an integrated vehicle routing and driver scheduling problem arising at the largest fresh meat producer in Denmark. The problem consists of a one-week planning horizon, heterogeneousvehicles,and driverswith predefinedworkregulations.Theseregulationsinclude,among other things, predefined workdays, fixed starting time, maximum wee...

Efficient planning increasingly becomes an indispensable tool for management of both companies and public organizations. This is also the case for high school management in Denmark, because the growing individual freedom of the students to choose courses makes planning much more complex. Due to reforms, elective courses are today an important part...

This paper presents a comparison of various recovery methods in terms of capacity efficiency with the underlying aim of reducing control plane load. In particular, a method where recovery requests are bundled towards the destination (Shortcut Span Protection) is evaluated can compared against traditional recovery methods. The optimization model is...

This chapter presents an advanced shared protection approach called Failure Dependent Path Protection (FDPP). Under this approach, several protection paths can be assigned to connections in the context of a shared protection framework. After formalizing the survivable online routing problem, two possible implementations are compared, one based on h...

This chapter is devoted to the analysis and modeling of some problems related to the optimal usage of the label space in label switching networks. Label space problems concerning three different technologies and architectures – namely Multi-protocol Label Switching (MPLS), Ethernet VLAN-Label Switching (ELS) and All-Optical Label Switching (AOLS) –...

This paper presents a comparison of recovery methods in terms of capacity efficiency. In particular, a method where recovery requests are bundled towards the destination (Shortcut Span Protection) is evaluated against traditional recovery methods. Our simulation results show that Shortcut Span Protection uses more capacity than the unbundled relate...

Reliable communication has become crucial in today's information society. Modern communication networks are required to deliver reliable communication to their customers. Unfortunately, protection against network failures significantly hampers efficient utilization of network investments, because the associated routing problems become much harder....

Most network operators have considered reducing LSR label spaces (number of labels used) as a way of simplifying management of underlaying virtual private networks (VPNs) and therefore reducing operational expenditure (OPEX). The IETF outlined the label merging feature in MPLS-allowing the configuration of multipoint-to-point connections (MP2P)-as...

In this paper we present the generalized fixed-charge network design (GFCND) problem. The GFCND problem is an instance of the so-called generalized network design problems. In such problems, clusters instead of nodes have to be interconnected by a network. The network interconnecting the clusters is a fixed-charge network, and thus the GFCND proble...

In this paper, we present a hierarchical optimization method for finding feasible true 0–1 solutions to finite-element-based topology design problems. The topology design problems are initially modelled as non-convex mixed 0–1 programs. The hierarchical optimization method is applied to the problem of minimizing the weight of a structure subject to...

Protection of communication against network failures is becoming increasingly important, and here we present what we believe to be the most capacity-efficient protection method possible, the complete rerouting protection method, when we require that all communication should be restored in the case of a single link network failure. We present a line...

In this paper we present a multi-objective genetic algorithm to solve the problem of mapping a set of task graphs onto a heterogeneous multiprocessor platform. The objective is to meet all real-time deadlines subject to minimizing system cost and power consumption, while staying within bounds on local memory sizes and interface buffer sizes. Our ap...

We consider the problem of designing hierarchical two layer ring networks. The top layer consists of a federal-ring which establishes connection between a number of node disjoint metro-rings in a bottom layer. The objective is to minimize the costs of links in the network, taking both the fixed link establishment costs and the link capacity costs i...

During the last decade telecommunication operators have been deploying WDM (Wavelength Division Multiplexing) technology to satisfy the exponential growth in global communication. While facilitating the advanced information society of today, this has also led to a higher dependency on the networks, and furthermore the high capacity utilization of o...

A key development of telecommunication systems during the past two decades has been the evolution from the circuit-switched network toward the packet-switched network paradigm. Many operators are now migrating their PSTNs from circuit switched networks. This new approach is often called the next-generation network NGN enables network operators to r...

A key development of telecommunication systems during the past two decades has been the evolution from the circuit-switched network toward the packet-switched network paradigm. Many operators are now migrating their PSTNs from circuit switched networks. This new approach is often called the next-generation network NGN enables network operators to r...

The experimental results reported in many papers suggest that making an appropriate a priori choice of an evolutionary method for a nonlinear parameter optimization problem remains an open question. It seems that the most promising approach at this stage of research is experimental, involving the design of a scalable test suite of constrained optim...

We present here a full multifibre optical WDM network design problem formulated as an integer linear problem. The design problem consists in laying out ducts, fibres, routes, and wavelengths, given a set of nodes, the duct and fibre prices, and the traffic demands. We compare different methods for solving the design problem. These are integer linea...

This paper focuses on the issue of evaluation of constraints handling methods, as the advantages and disadvantages of various methods are not well understood. The general way of dealing with constraints -- whatever the optimization method -- is by penalizing infeasible points. However, there are no guidelines on designing penalty functions. Some su...

The experimental results reported in many papers suggest that making an appropriate a priori choice of an evolutionary method for a nonlinear parameter optimization problem remains an open question. It seems that the most promising approach at this stage of research is experimental, involving a design of a scalable test suite of constrained optimiz...

Parameter optimization has been a prime target for evolutionary algorithms for a number of years. Genetic algorithms, evolution strategies, and evolutionary programming have dealt with a variety of nonlinear programming problems. There is a growing evidence that evolutionary algorithms are well suited for optimization of real valued multi-modal dif...

. Genetic Algorithms have been an active research area for more than three decades, but the industrial applications of this search technique have been scarce. There may be several reasons for this. The EVALIA 1 project (EVolutionary ALgorithms for Industrial Applications) attempts to test the value of Genetic Algorithms on realistic industrial prob...

Jobshop scheduling is considered a standard problem to solve by means of Genetic Algorithms (GA) and a number of articles have been written about this subject (1). In spite of this, on ly few applications of GA have been published (2), (3) on real jobshop scheduling problems. Even though some applications may not be published for business related r...

Communication networks are increasingly required to be reliable. In this paper we only consider single cable failures (7, 12). The classic path protection method employed in circuit switched networks is 1+1 protection, where two cable disjoint circuits (and hence arc disjoint circuits) are established and actively used. In case an arc fails on one...