Richard F. Hartl

University of Vienna | UniWien · Institut für Betriebswirtschaftslehre

The dual-resource-constrained re-entrant flexible flow shop scheduling problem represents a specialised variant of the flow shop scheduling problem, inspired by real-world scenarios in screen printing industries. Besides the well-known flow shop structure, stages consist of identical parallel machines and operations may re-enter the same stage mult...

The COVID-19 pandemic has devastated lives and economies around the world. Initially a primary response was locking down parts of the economy to reduce social interactions and, hence, the virus' spread. After vaccines have been developed and produced in sufficient quantity, they can largely replace broad lock downs. This paper explores how lockdown...

We consider the vehicle routing problem with stochastic demands (VRPSD), a stochastic variant of the well-known VRP in which demands are only revealed upon arrival of the vehicle at each customer. Motivated by the significant recent progress on VRPSD research, we begin this paper by summarizing the key new results and methods for solving the proble...

One of the main obstacles in horizontal collaborative transportation is the proper distribution of gains among partners. In practice, a posteriori gain sharing mechanisms rarely guarantee that all partners feel treated fairly. In this paper, we present the multi-depot vehicle routing problem with profit fairness (MDVRP-PF), a bi-objective optimizat...

We consider the vehicle routing problem with stochastic demands (VRPSD), a stochastic variant of the well-known VRP in which demands are only revealed upon arrival of the vehicle at each customer. Motivated by the significant recent progress on VRPSD research, we begin this paper by summarizing the key new results and methods for solving the proble...

One of the main obstacles in horizontal collaborative transportation is the proper distribution of gains among partners. In practice, a posteriori gain sharing mechanisms rarely guarantee that all partners feel treated fairly. In this paper, we present the multi-depot vehicle routing problem with profit fairness (MDVRP-PF), a bi-objective optimizat...

This paper considers a capital accumulation game where the installation costs of investments are lowered by the firm’s own capital stock because of learning and by the competitor’s capital stock because of spillover effects. To properly understand the impact of the two capital stocks, we consider six information structures which differ in whether a...

Traditionally, individuals and firms have bought and owned resources. For instance, owning a car was a status symbol for people. In recent years, this focus has changed. Young people just want to have some convenient means of transportation available. Sharing cars, bikes, or rides is also acceptable and has the advantage of increased sustainability...

Carriers can remarkably reduce transportation costs and emissions when they collaborate, for example through a platform. Such gains, however, have only been investigated for relatively small problem instances with low numbers of carriers. We develop auction-based methods for large-scale dynamic collaborative pickup and delivery problems, combining...

In determining the durability of its product a firm faces a trade off. Performing a policy of planned obsolescence by making their products less durable implies that the consumer needs to replace them earlier, which thus enhances demand. However, a lower quality of the product will result in a lower reputation, which in turn will affect demand nega...

Most nations have responded to the COVID-19 pandemic by locking down parts of their economies starting in early 2020 to reduce the infectious spread. The optimal timing of the beginning and end of the lockdown, together with its intensity, is determined by the tradeoff between economic losses and improved health outcomes. These choices can be model...

Progress in digitalization opens opportunities to capture accurate transportation logistics data and provide advanced decision support, which leads into the question of how to efficiently exploit this progress in order to improve solution quality in transportation services. Here we address this issue in the context of a dynamic and stochastic patie...

Some of today's greatest challenges in urban environments concern individual mobility and rapid parcel delivery. Given the surge of e-commerce and the ever-increasing volume of goods to be delivered, we explore possible logistic solutions by proposing algorithms to add parcel-transport services to ride-hailing systems. Toward this end, we present a...

Competitive markets, increased fuel costs, and underutilized vehicle fleets are characteristics that currently define the logistics sector. Given an increasing pressure to act in a manner that is economically and ecologically efficient, mechanisms that help to benefit from idle capacities are on the rise. In the Sharing Economy, collaborative usage...

This paper deals with a Genetic Programming (GP) approach for solving flexible shop scheduling problems. The adopted approach aims to generate priority rules in the form of an expression tree for dispatching jobs. Therefore, in a list-scheduling algorithm, the available jobs can be ranked using the tree-based priority rules generated using GP. Thes...

Logistics providers have to utilize available capacities efficiently in order to cope with increasing competition and desired quality of service. One possibility to reduce idle capacity is to build coalitions with other players on the market. While the willingness to enter such coalitions does exist in the logistics industry, the success of collabo...

To achieve logistic efficiency and customer convenience in last-mile delivery processes, a system with alternative delivery points in the form of locker box stations can be used. In such a system, customers can be served either at their home address within a certain time window, or at a locker box station where parcels can be picked up at any time....

Cloud manufacturing is an emerging concept that enables the orchestration, matching, and sharing of services or resources among collaboration partners or intra-plant facilities. In this context, we introduce the capacitated multi-level lot sizing problem with transshipments and set up carry-over. We consider components that can only be produced by...

One of the principal ways nations are responding to the COVID-19 pandemic is by locking down portions of their economies to reduce infectious spread. This is expensive in terms of lost jobs, lost economic productivity, and lost freedoms. So it is of interest to ask: What is the optimal intensity with which to lockdown, and how should that intensity...

The rising competition in the logistics sector forces companies to be more economically efficient. One of the major sources of inefficiency is the incomplete usage of available resources, such as vehicles’ capacities. Mechanism that allow to better exploit such resources by enabling carrier collaborations are on the rise. Our study examines a centr...

Nations struggled to decide when and how to end COVID-19 inspired lockdowns, with sharply divergent views between those arguing for a resumption of economic activity and those arguing for continuing the lockdown in some form. We examine the choice between continuing or ending a full lockdown within a simple optimal control model that encompasses bo...

In many routing applications, it is necessary to place limits on the duration of the individual routes. When demands are stochastic and restocking during route execution is allowed, the durations of the resulting routes are also stochastic. In this paper, we consider the vehicle routing problem with stochastic demands and probabilistic duration con...

This paper focuses on a computational tool for scheduling and sequencing sugarcane vehicles for dump tippler machines at the mill yard of a sugar mill, which can be represented as an NP-hard problem for parallel capacitated machines with machine restrictions, job grouping, and sequencing independent setup time. This research aims to determine the o...

While collaborative vehicle routing has a significant potential to reduce transportation costs and emissions, current approaches are limited in terms of applicability, unrealistic assumptions, and low scalability. Centralized planning generally assumes full information and full control, which is often unacceptable for individual carriers. Combinato...

This paper considers the vehicle routing problem with stochastic demands under optimal restocking. We develop an exact algorithm that is effective for solving instances with many vehicles and few customers per route. In our experiments, we show that in these instances, solving the stochastic problem is most relevant (i.e., the potential gains over...

The aim of the present paper is to analyze how firms that sell durable goods should optimally combine continuous-time operational level planning with discrete decision making. In particular, a firm has to continuously adapt its capacity investments and sales strategy, but only at certain times it will introduce a new version of the durable good to...

The logistics industry is known to suffer from inefficiencies. Recently published numbers on the percentage of trucks running idle or almost idle on European road networks are alarming. Collaborative transportation offers opportunities to share capacities and thus reduce inefficiencies and environmental pollution considerably. It is therefore a hig...

This paper focuses on a computational tool for scheduling sugarcane vehicles for dump tippler machines operating at a sugar mill. This problem was defined as scheduling M parallel capacitated machines with a cyclic sequence where machine restriction, and sequencing independent setup time are included with the objective to minimise makespan. To solv...

The marketing-mix of price-quality and advertising-quality relationship is well studied. Less understood is the price-advertising-quality relationship. This article fills the gap, investigating the interplay between price, advertising, and quality in an optimal control model. Our results generalize the condition of Dorfman-Steiner in a dynamic cont...

In proposing a machine learning approach for a flow shop scheduling problem with alternative resources, sequence-dependent setup times, and blocking, this paper seeks to generate a tree-based priority rule in terms of a well-performing decision tree (DT) for dispatching jobs. Furthermore, generating a generic DT and RF that yields competitive resul...

Given an increasing pressure to act economically and ecologically efficient, mechanisms to benefit from idle capacities are on the rise. In this “sharing economy”, carrier collaborations are a key concept. We assess the benefit of sharing information in auction-based carrier collaborations where carriers seek to exchange transportation requests in...

We analyze the problem of a firm that sells durable goods. In particular, we investigate how this firm optimally combines continuous-time operational-level planning (continuously deciding on capacity investment) with discrete decision making (when to launch a new generation of the product, how to price a particular generation of the product). We fi...

This paper analyzes a dynamic model of the firm. We focus on the effect of investment in green energy. We explicitly take into account that green energy has a positive side effect, namely that it contributes to the goodwill of the firm and thus increases demand. Different models are proposed and the solutions range from monotonic saddle point conve...

The paper considers the problem of an investor that has the option to acquire a firm. Initially this firm is run as to maximize shareholder value, where the shareholders are risk averse. To do so it has to decide each time on investment and dividend levels. The firm’s capital stock can be financed by equity and debt, where less solvable firms pay a...

Collaborative operations planning is a key element of modern supply chains. We introduce the collaborative multi-level lot-sizing problem with cost synergies. This arises if producers can realise reductions of their costs by providing more than one product in a specific time horizon. Since producers are typically not willing to reveal critical info...

After decades of intensive research on the vehicle routing problem (VRP), many highly efficient single‐objective heuristics exist for a multitude of VRP variants. But when new side‐objectives emerge—such as service quality, workload balance, pollution reduction, consistency—the prevailing approach has been to develop new, problem‐specific, and incr...

In collaborative logistics, carriers form coalitions in order to perform parts of their operational business jointly. Combinatorial auctions can be used to exchange transportation requests without having to reveal critical information. Collaborators submit requests for exchange to a common pool. The requests in the pool are then grouped into bundle...

Logistics networks are constantly evolving such that new and more varied structures arise and need to be studied. Carriers are aiming for opportunities to save costs by efficient planning. Motivated by this, we define the two-region multi-depot pickup and delivery problem. A region in this setting refers to an area where customers and depots are lo...

We present a model for the single-vehicle routing problem with stochastic demands (SVRPSD) with optimal restocking. The model is derived from a characterization of the SVRPSD as a Markov decision process (MDP) controlled by a certain class of policies, and is valid for general discrete demand probability distributions. We transform this MDP into an...

We assess the potential total profit in collaborative pickup and delivery problems, where carriers are willing to exchange transportation requests. For this, we design an adaptive large neighborhood search method that is used to generate solutions of publicly available but yet unsolved test instances. Our computational study reveals that collaborat...

Unsuccessful delivery attempts, or failed hits, are still a recurring problem in the fulfillment of e-commerce orders to private customers. In this paper, we consider a parcel delivery company interested in optimizing the rate of successful deliveries. By doing so, the company is able to offer a differentiated service, increasing customer satisfact...

In horizontal collaborations, carriers form coalitions in order to perform parts of their logistics operations jointly. By exchanging transportation requests among each other, they can operate more efficiently and in a more sustainable way. This exchange of requests can be organized through combinatorial auctions, where collaborators submit request...

This paper considers the vehicle routing problem with stochastic demands (VRPSD) under optimal restocking. We develop an exact algorithm that is effective for solving instances with many vehicles and few customers per route. In our experiments, we show that in these instances solving the stochastic problem is most relevant (i.e., the potential gain...

This paper considers the vehicle routing problem with stochastic demands (VRPSD) under optimal restocking. We develop an exact algorithm that is effective for solving instances with many vehicles and few customers per route. In our experiments, we show that in these instances solving the stochastic problem is most relevant (i.e., the potential gain...

We introduce an electric vehicle routing problem combining conventional, plug-in hybrid, and electric vehicles. Electric vehicles are constrained in their service range by their battery capacity, and may require time-consuming recharging operations at some specific locations. Plug-in hybrid vehicles have two engines, an internal combustion engine a...

This paper considers the effect of investment in solar panels on optimal dynamic firm behavior. To do so, an optimal control model is analyzed that has as state variables goodwill and green capital stock. Following current practice in companies like Tesla and Google, we take into account that the use of green energy has positive goodwill effects. A...

In this study we investigate the decision problem of a central authority in pickup and delivery carrier collaborations. Customer requests are to be redistributed among participants, such that the total cost is minimized. We formulate the problem as multi-depot traveling salesman problem with pickups and deliveries. We apply three well-established e...

Collaboration has been one of the important trends in vehicle routing. A typical mechanism to enable carrier collaboration is to use combinatorial auctions, where requests are not traded individually but are combined into bundles. Previous literature on carrier collaboration has focused on issues such as bundle formation or winner determination, ty...

Two-echelon distribution systems are attractive from an economical standpoint and help keeping large vehicles out of city centers. Large trucks can be used to deliver goods to intermediate facilities in accessible locations, whereas smaller vehicles allow to reach the final customers. Due to their reduced size and emissions, companies have adopted...

In practical vehicle routing problems (VRPs), important non-monetary benefits can be achieved with more balanced operational plans which explicitly consider workload equity. This has motivated practitioners to include a wide variety of balancing criteria in decision support systems, and researchers to examine the properties of these criteria and th...

We consider the multi-vehicle one-to-one pickup and delivery problem with split loads, a NP-hard problem linked with a variety of applications for bulk product transportation, bike-sharing systems and inventory re-balancing. This problem is notoriously difficult due to the interaction of two challenging vehicle routing attributes, "pickups and deli...

This paper addresses the problem of a video game producer who starts out with a subscription-based business model but considers when, if ever, to switch to a free-to-play model, which price discriminates between typical users, who play for free, and heavy users who pay for acquiring extra features. The videogame producer has the possibility to adve...

In this study, we consider balancing problems of one- and two-sided assembly lines with real-world constraints like task or machine incompatibilities. First, we study the one-sided assembly line balancing problem (ALBP) with a limited number of machine types per workstation. Using a genetic algorithm (GA), we find optimal results for real-world ins...

Over the past two decades, equity aspects have been considered in a growing number of models and methods for vehicle routing problems (VRPs). Equity concerns most often relate to fairly allocating workloads and to balancing the utilization of resources, and many practical applications have been reported in the literature. However, there has been on...

In horizontal collaborations, carriers form coalitions in order to perform parts of their logistics operations jointly. By exchanging transportation requests among each other, they can operate more efficiently and in a more sustainable way. Collaborative vehicle routing has been extensively discussed in the literature. We identify three major strea...

One of the principle characteristics of additive manufacturing is that customers get access to their own design, i.e., they need not choose from a small number of standard products. The paper considers a framework where an incumbent with a standard technology produces a limited number of standard products, and faces a potential entrant with an addi...

Real-life problems are often characterized by conflicting optimization objectives. Consequently, there has been a growing interest not only in multi-objective models, but also in specialized multi-objective metaheuristics for solving those models. A wide variety of methods, e.g. NSGA-II, SPEA, IBEA, scatter search, Pareto local search, and many oth...

In curriculum-based course timetabling, lectures have to be assigned to periods and rooms, while avoiding overlaps between courses of the same curriculum. Taking into account the inherent complexity of the problem, a metaheuristic solution approach is proposed, more precisely an adaptive large neighborhood search, which is based on repetitively des...

The paper provides a framework that enables us to analyze the important topic of capital accumulation under technological progress.We describe an algorithm to solve Impulse Control problems, based on a (multipoint) boundary value problem approach. Investment takes place in lumps and we determine the optimal timing of technology adoptions as well as...

In our paper, we analyze new exact approaches for the multi-mode resource-constrained project scheduling (MRCPSP) problem with the aim of makespan minimization. For the single-mode RCPSP (SRCPSP) recent exact algorithms combine a Branch and Bound algorithm with principles from Constraint Programming (CP) and Boolean Satisfiability Solving (SAT). We...

The transportation industry expanded rapidly in a highly competitive environment. Logistics companies with insufficient volume of transport capacities are forced to make a selection of customers that they can integrate efficiently into their tours. This is of particular relevance in the pickup and delivery market, where shipments from several diffe...

In the literature on marketing models, the assumption of mixed word-of-mouth has been limited to the Bass diffusion model. Yet explicit leveraging of the originating factors of such assumption is lacking. Apart from that example, mixed word-of-mouth has been disregarded in contagion sales models. This paper bridges the gap by suggesting a sales mod...

For certain goods or services, the quality of the product can be assessed by customers only after consumption. We determine the optimal time paths for pricing, advertising and quality for a profit-maximizing firm facing demand that is influenced both by this experience quality as well as by advertising. In particular, there may exist two optimal tr...

In this paper, we address two optimisation problems arising in the context of city logistics and two-level transportation systems. The two-echelon vehicle routing problem and the two-echelon location routing problem seek to produce vehicle itineraries to deliver goods to customers, with transits through intermediate facilities. To efficiently solve...

Recently, equity aspects have been considered in a growing number of models and methods for vehicle routing problems (VRPs). Equity concerns most often relate to fairly allocating workloads and to balancing the utilization of resources, and many practical applications have been reported. However, there has been only limited discussion about how equ...

This paper proposes a new model for Cluster-tools with two load locks. Cluster-tools are widely used to automate single wafer processing in semiconductor industry. The load locks are the entry points into the vacuum of the Cluster-tool's mainframe. Usually there are two of them available. Each lot being processed, is dedicated to a single load-lock...

The organization of a specialized transportation system to perform transports for elderly and handicapped people is usually modeled as dial-a-ride problem. Users place transportation requests with specified pickup and delivery locations and times. The requests have to be completed under user inconvenience considerations by a specified fleet of vehi...

For variants of the single-mode resource-constrained project scheduling problem, state-of-the-art exact algorithms combine a Branch and Bound algorithm with principles from Constraint Programming and Boolean Satisfiability Solving. In our paper, we propose new exact approaches extending the above principles to the multi-mode RCPSP (MRCPSP) with gen...

This paper analyzes a two-echelon inventory problem with uncertain demand, where perishable products are distributed from a central warehouse to retail stores. To better deal with uncertainty, we propose a model in which, at a certain point in time during the operation time window, transshipments between retail stores are allowed in order to re-bal...

Due to new regulations and further technological progress in the field of electric vehicles, the research community faces the new challenge of incorporating the electric energy based restrictions into vehicle routing problems. One of these restrictions is the limited battery capacity which makes detours to recharging stations necessary, thus requir...

Research on dynamic and stochastic vehicle routing problems received increasing interest in the last decade. It considers a novel problem class, aiming at an appropriate handling of dynamic events combined with the incorporation of stochastic information about possible future events. This survey summarises the recent literature in this area. Beside...