Ranking paths in stochastic time-dependent networks

The research paper “Ranking paths in stochastic time-dependent networks” have been accepted for publication in European Journal of Operational Research. In this paper we address optimal routing problems in networks where travel times are both stochastic and time-dependent. In these networks, the best route choice is not necessarily a path, but rather a time-adaptive strategy that assigns successors to nodes as a function of time. Nevertheless, in some particular cases an origin-destination path must be chosen a priori, since time-adaptive choices are not allowed. Unfortunately, finding the a priori shortest path is an NP-hard problem. We propose a solution method for the a priori shortest path problem, and we show that it can be easily extended to the ranking of the first K shortest paths. Our method exploits the solution of the time-adaptive routing problem as a relaxation of the a priori problem. Computational results are presented showing that, under […]

MCDM 2013 in Malaga

From 17-22 June 2013 I have been participating in MCDM 2013 in Malaga. The 22nd International Conference on Multiple Criteria Decision Making. The conference was held in a nice atmosphere and the city can be recommended. I coauthored a talk about the Bicriterion Stochastic Knapsack Problem which is a problem we currently are working on.

Course on supervision

During the past months I have been participating in a course about supervision. The course, which runs over several days, covers master, PhD and fellow supervision and use results from research, practice and experiences from the participants. It aims at strengthen the participants’ overall supervisory skills in the form of clear and specific criteria-based guidance better feedback when commenting on text from the student establishment and maintenance of a mutually binding supervision agreement initial, ongoing and final project evaluation and feedback using meta-communication of supervision content and processes The course included video-filming ourselves in doing supervision which was a good way of seeing how one actually perform. The course can be recommended to all university staff doing supervision.

New publication

The research paper “Inventory control in a lost-sales setting with information about supply lead times” have been accepted for publication in International Journal of Production Economics. We consider a periodically reviewed single-item inventory system in a lost sales setting where at most one order can be outstanding at a time and compare the performance of an inventory model assuming informed lead times to a model assuming uninformed independent and identically distributed lead times.

Sets, sums and constraints – A suvival guide for Business School students

Often when I teach students at our Business School they have a hard time understanding compact linear programming (LP) formulations. So here it is,  a short introduction to some of the concepts you need to know for understanding compact LP formulations. Sets A set is a group of elements, e.g. $\{1,2,4\}$ is a set with 3 elements, namely, $1,2$ and $4$ and $A=\{(2,3),(4,5),(6,8),(5,6)\}$ is a set called $A$ with 4 elements (pairs), namely,  $(2,3),(4,5),(6,8)$ and $(5,6)$. Note that in the last case each element is a pair $(i,j)$. Sets containing pairs are often used when we formulate LPs based on network problems where the pair $(i,j)$ denote the arc/edge from node $i$ to node $j$.

Why shortest paths not always are the same

I often hear people complain about the GPS they use for routing. Some do a better job than others. This is due to two things: the algorithm which may vary and the network used. Here I will focus on the network. Often the algorithms have to use different networks because: the network has not been updated or a different provider is used. This is illustrated in this post showing that the total length in the networks used by TomTom and OpenStreetMap are quite different. As a result the shortest path algorithm will find different optimal paths as illustrated in this post.

Why Waiting Is Torture

This is the title of a good article about the psychological principles about waiting. This is something that we as researchers in Operations Research often forget and calls for more interdisciplinary research in this area. Why Waiting Is Torture, New York Times

EURO 2012

This week I am participating in the EURO conference in Vilnius where I have a joint talk “Spare parts sharing with joint optimization of maintenance and inventory policies” with my college Christian Larsen.

VeRoLog 2012

The VeRoLog 2012 conference in Boglogna has just ended. I gave a talk about “Ranking shortest paths in Stochastic time-denpendent networks”. The conference had approximately 160 participants.

Using logvrp.com when introducing students to VRP

In my recent course on transportation and distribution systems for undergraduate business students I used a week talking about vehicle routing problems (VRP). Since their math skills aren’t that high the focus was not on algorithms instead I tried to talk about the general structure of the problem. Here the site logvrp.com is very good and provide a way of making a realistic problem case. Logvrp is a web based vehicle route planner software cable of minimize transportation costs and to minimize fleet mileages. It uses Google maps to display routes and trips. Moreover, it supports many different VRP problems. Current two heuristics are implemented and can be run simultaneously on a problem instance. This is a good thing since the students can see that the heuristics behave differently on the same instance and that there is no general winner. I created an exercise about a small-sized company located in […]