Mediterranean Institute for Advanced Studies

Šempeter pri Gorici, Slovenia

Mediterranean Institute for Advanced Studies

Šempeter pri Gorici, Slovenia

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Bogataj M.,University of Ljubljana | Bogataj M.,Mediterranean Institute for Advanced Studies | Grubbstrom R.W.,Linkoping Institute of Technology | Grubbstrom R.W.,Mediterranean Institute for Advanced Studies
International Journal of Production Economics | Year: 2012

MRP Theory has been developed during the last 25 years for capturing processes concerning multi-level, multi-stage production-inventory systems in a compact way. Input-output analysis has been used to describe structures, and Laplace transforms to describe the timing relations. This theory has mainly dealt with assembly systems, in which each item has only one successor. The lead times for the assembly of an item have usually been constants and equal for all items entering a given successor. For such systems, the equations describing the flows of components may be written to include the generalised input matrix as the product of an input matrix containing needed amounts, and a diagonal lead time matrix with lead time operators along its main diagonal. On occasion, there has been a need to deviate from this representation enabling lead times to vary depending on which input item that is considered. This paper deals with how to represent lead times and similar output delays (in diverging, arborescent systems), when the assumption of equal times is relaxed, in order to retain the basic structure of the fundamental balance equations involved. The intention of this paper is to create a general taxonomy for the representation of timing in algebraic form for a variety of systems covering both assembly systems and arborescent systems (such as extraction, distribution and remanufacturing), as well as systems with mixed properties. For instance, this method may be used directly for the evaluation of investments in capacity or in the location of activities in a production network, or even in a global supply chain. © 2012 Elsevier B.V. All rights reserved.


Bogataj M.,University of Ljubljana | Bogataj M.,Mediterranean Institute for Advanced Studies | Grubbstrom R.W.,Linkoping Institute of Technology | Grubbstrom R.W.,Mediterranean Institute for Advanced Studies | Bogataj L.,University of Ljubljana
International Journal of Production Economics | Year: 2011

Inefficient locations for production, distribution and reverse logistics plants will result in excess costs no matter how well material requirements planning (MRP), inventory control, distribution and information sharing decisions are optimized. In this paper we study ways in which aspect of activity cell location decisions can be analyzed within an extended MRP model. This model has previously been extended by including distribution and reverse logistics components in a compact form, presented in Grubbström et al. (2007). Our aim is to demonstrate the basic differences between an approach to location problems with MRP under the same roof as the global supply chain, in which transportation time delays and direct transportation costs have substantial influence. We discuss possibilities of how to present location aspects in the supply chain model obtained from combining inputoutput analysis and Laplace transforms in four sub-systems, namely manufacturing, distribution, consumption and reverse logistics, and show how the transportation costs and lead time influenced by the location of all these activities affect the resulting net present value (NPV). Our aim is to build a model supporting decisions concerning the structure of a supply chain as an alternative to a mixed integer programming formulation. The model developed is based on the use of continuous functions describing spatial distributions of cost and customer demand. Continuous functions are embedded in the MRP extension previously introduced in Grubbström et al. (2007). Location decisions influence (i) production costs, because timing influences the cost of activities involved in creating a product, cf. (Grubbström and Bogataj, submitted for publication), and (ii) logistics costs, which refer to the procurement and physical transmission of materials through the supply chain. In this current paper we wish to combine both of these aspects into a comprehensive model, where we show the interaction between the space of flows and the space of places as Giovanni Arrighi distinguishes one from the other in his book The Long Twentieth Century. © 2010 Elsevier B.V. All rights reserved.


News Article | December 7, 2016
Site: phys.org

The isolation of ocean islands like the Galápagos prevents the arrival of large mammals, which disperse the seeds of many plants by ingesting them. In the absence of mammals, this function is filled by birds, tortoises, lizards and iguanas. To date, no investigation had been carried out into the role iguanas play with at least ten species of plants. The survival of many native and introduced plants depends in part on the role of animals in pollination and seed dispersal. The ingestion and subsequent expulsion of seeds in animal faeces means a proportion of them return to the soil at a more distant location. In addition to birds, the Galápagos giant tortoise is the animal that disperses most of seeds over great distances on the islands, followed by the endemic land iguanas, of which there are three species which feed on fruit and vegetation near ground level, as they do not climb. However apart from anecdotal records, their potential for seed dispersal had not to date been confirmed. A study published in the journal 'Integrative Zoology' demonstrates for the first time how by dispersing seeds, the Galápagos land iguana (Conolophus subcristatus) contributes to the survival of indigenous and introduced plants plant species on Fernandina Island, which covers 642 km2 of land. "We knew that female iguanas on this island cover large distances, around 10 kilometres, and climb up to 1,500 metres of altitude to lay their eggs at the island's volcanic crater," SINC was told by Anna Traveset from the Mediterranean Institute for Advanced Studies (CSIC-UIB), the lead author of the study. Between February 2010 and 2011, the researchers collected 160 faeces samples, in which they identified 5,705 seeds from 32 plant species. According to the team, at least 80% of the seeds (around 4,545) were damaged. With the remaining seeds, which remained intact after passing through the reptile's intestines, the team ran an experiment in which they planted 849 seeds from 29 plant species; only around 4% of these were germinated over 200 days later. "Considering the local abundance of land iguanas and the large amount of seeds ingested by these animals, even if only a small proportion germinates, they can be considered important for plant dissemination to new areas in this young island," Traveset writes in the article. In fact, some plants appear to benefit greatly from this action. According to the paper, 63% of the seeds belonged to native plants, a third of which were endemic to the Galápagos Islands. Six per cent were from introduced species and the remainder (31%) could not be identified. Explore further: Plants actively direct their seeds via wind or water towards suitable sites More information: Anna TRAVESET et al. Galápagos land iguana () as a seed disperser, Integrative Zoology (2016). DOI: 10.1111/1749-4877.12187


Rallus aquaticus skull (above) and Rallus "minutus" (below). Scale: 2 cm. / Alcover et al. When Charles Darwin visited the Azores islands in the 19th Century, the birds he observed were familiar to him. However, if he had travelled there 500 years before, he would have found an ornithofauna as particular as that of the Galápagos. The recent discovery in these Portuguese islands and in Madeira of five extinct species of rail, which lost the ability to fly due to having evolved on islands, confirms how fragile they are in the face of changes to their habitat like the ones that must have occurred after the first visits by humans over 500 years ago. In September 1826, the British naturalist Charles Darwin visited the Azores archipelago during the HMS Beagle's return voyage to the United Kingdom after more than four years travelling the world. In his diary he only mentions the existence of starlings, wagtails, finches and blackbirds; however, on the islands also lay the remains of other birds which populated the islands a few centuries before his visit. A new study, published in Zootaxa, now highlights the discovery of five extinct rail species, two in Madeira and three in the Azores. "The species of birds very probably disappeared following the arrival of humans and the animals that came with them, like mice, rats and cats," told Josep Antoni Alcover Paleontological exploration by Spanish, German and Portuguese researchers has made it possible to "discover new species of birds that very probably disappeared following the arrival of humans and the animals that came with them, like mice, rats and cats," SINC was told by Josep Antoni Alcover, a CSIC researcher working at the Mediterranean Institute for Advanced Studies (IMEDEA-CSIC/UIB) and co-author of the paper. The new species are: the Madeira rail (Rallus lowei), a flightless species with a very stout body; the Porto Santo rail (Rallus adolfocaesaris), graceful and probably not a very able flier; the São Miguel rail (Rallus carvaoensis), quite small, stout, flightless and with a somewhat curved beak; the Pico rail (Rallus montivagorum), larger than the São Miguel rail (but smaller than the continental species), graceful and with reduced flying capability; and the São Jorge rail (Rallus "minutus"), diminutive, relatively stout, with short legs, flightless and which does not have a definitive scientific name. According to dating obtained from the bones of these birds, or from those of other species found which were associated to them, these five extinct species lived until fairly recently, especially the Azores rails. "At least one of these species survived until the 15th Century, so we are looking at a very recent extinction process," stresses Alcover. According to the scientist, in Madeira the extinction may have been related to a possible visit by the Vikings (whether it was a colonisation is still not verified), who could have transported mice to the island. These would have brought about the disappearance of rails and other birds. "The bone remains of native bird species which are now appearing show that if Darwin had been able to study the fossils hidden on these islands, or if he had visited 500 years earlier, he would have found a much more singular ornithofauna, with many indigenous bird species, like that which was found on the Galápagos islands," Alcover highlights. Today, there are only 13 living rail species of the Rallus genus. "This is because other species, which only lived on islands, have disappeared recently," the expert clarifies. Two or three thousand insular rail species (rallids) are thought to have lived in the Pacific. In the Atlantic, only on the most remote islands, such as Tristan da Cunha and Gough, are there surviving indigenous rail species today; however, in the Antilles, Bermuda and on the islands of Ascension and Saint Helena, extinct species have been found. The extinct birds found on the islands of Macaronesia "were smaller in size than today's continental rails, such as the water rail (Rallus aquaticus), from which they very probably originate," says Alcover. Fossils uncovered also make it possible to verify that all these species had a reduced flying capability. "Some were completely incapable of regular flight," the researcher reveals. While on continents, rails live near water, on islands, they occupy more terrestrial habitats. The reason for this is that in order to live on islands, they evolve differently, to the point where they become indigenous insular species. This evolution implies changes in their size and body proportions and a reduction or complete loss of their ability to fly; this is why they tend to walk. "For that reason, the rails that reach islands and evolve on them lose their dispersive capacity: they cannot leave the islands and they are trapped in limited insular territories, which is why we observe an extremely reduced distribution area," the scientist explains. "The history of insular rails is an intense story of evolution, and frequently, extinction," Alcover highlights This circumstance also makes them very prone to extinction when there are changes to the islands' ecology (for example, when they are colonised by humans). "The history of insular rails is an intense story of evolution, and frequently, extinction," Alcover highlights. The tip of the iceberg of diversity The fossil remains surfacing on the Madeira and Azores archipelagos represent one part of all the diversity of animals that used to inhabit these islands, and are now beginning to be discovered. In addition to the five rail species the paper describes, there are other species, for example two indigenous species of scops owl. "This is only the tip of the iceberg of what is to come in terms of knowledge about the ornithological fauna native to these islands", according to the authors. "The existence in the past of indigenous species of scops owls and rails points to the great magnitude of the devastation birds suffered on Atlantic islands after the arrival of humans and the fauna that came with them," the scientists conclude. Explore further: An extinct species of scops owl has been discovered in Madeira Five new extinct species of rails (Aves: Gruiformes: Rallidae) from the Macaronesian Islands (North Atlantic Ocean)


News Article | March 15, 2016
Site: phys.org

But researchers say its demise could be avoided with a simple technique – the setting of fishing lines at night, when the bird does not dive for food. Experts put the global population of Balearic shearwaters (Puffinus mauretanicus) at about 7,200 breeding pairs and fewer than 30,000 individuals. The main threat to the bird is becoming entangled in fishing gear. The research is published in the Journal of Applied Ecology. Professor Tim Guilford of the Department of Zoology at the University of Oxford, a co-author of the study, said: 'The survival of adults from one year to the next, and especially of young adults, is much lower than we thought. The species is unsustainable – it is on the road to extinction.' The team, comprising scientists from Oxford, Southampton and Spain, and led by Meritxell Genovart of Palma de Mallorca's Mediterranean Institute for Advanced Studies, used modern techniques to model the sustainability of the current population, which is numbered at fewer than 30,000 birds. With declines of about 14% a year, complete extinction is predicted within about 60 years. Estimates of survival from the world's largest single colony, a remote cave on Mallorca where the Oxford team works, show that it is far below what is needed to maintain population growth or stability. However, policy changes such as setting fishing gear at night when the bird does not dive 'could make a massive difference,' according to Professor Guilford. He said: 'Fishermen do not want to catch seabirds on their lines or in their nets, but shearwaters can be persistent scavengers. Indeed, the researchers estimate that the recent European ban on discard waste in the Mediterranean will actually increase the risk to the Balearic shearwater population by reducing reproductive success, at least in the short term. 'However, this effect turns out to be much less important than the effect of bycatch on adult survival. Indeed, the models suggest that extinction is inevitable unless bycatch is eliminated or greatly reduced. 'By tracking breeding shearwaters with miniature on-board dive loggers, the team has shown that the birds do not dive for fish at night, so night-setting of fishing hooks might help avoid the bycatch problem.' Professor Guilford added: 'The science shows just how serious the problem is, but also that there is a technically simple solution – the setting of demersal long-lines at night. Now it is up to the politicians to decide.' More information: Meritxell Genovart et al. Demography of the critically endangered Balearic shearwater: the impact of fisheries and time to extinction, Journal of Applied Ecology (2016). DOI: 10.1111/1365-2664.12622


Grubbstrom R.W.,Linkoping Institute of Technology | Grubbstrom R.W.,Mediterranean Institute for Advanced Studies
International Journal of Production Economics | Year: 2014

The dynamic lotsizing problem concerns the determination of optimal batch quantities, when given required amounts appear at discrete points in time. The standard formulation assumes that no shortages are allowed and that replenishments are made instantaneously. For the case when no shortage is allowed, previously it has been demonstrated that the inner-corner condition for an optimal production plan in continuous time reduces the number of possible replenishment times to a finite set of given points at which either a replenishment is made, or not. The problem is thus turned into choosing from a set of zero/one decisions with 2n-1 alternatives, of which at least one solution must be optimal, where n is the number of requirement events. Recently, the instantaneous replenishment assumption has been replaced by allowing for a finite production rate, which turned the inner-corner condition into a condition of tangency between the cumulative demand staircase and cumulative production. In this paper we investigate relationships between optimal cumulative production and cumulative demand, when backlogging is permitted. The production rate is assumed constant and cumulative production will then be a set of consecutive ramps. Cumulative demand is a given staircase function. The net present value (NPV) principle is applied, assuming a fixed setup cost for each ramp, a unit production cost for each item produced and a unit revenue for each item sold at the time it is delivered. Among other results, it is shown that optimal cumulative production necessarily intersects the demand staircase. Instead of having 2n-1 production staircases as candidates for optimality, there are 2n-1 production structures as candidates. These are made up of sequences of batches, of which the set of batches may be optimised individually. Also is shown that the NPV of each batch has a unique timing maximum and behaves initially in a concave way and ends as convex. Results for the average cost approach are obtained from a zeroth/first order approximation of the objective function (NPV). © 2013 Published by Elsevier B.V.


Grubbstrom R.W.,Linkoping Institute of Technology | Grubbstrom R.W.,Mediterranean Institute for Advanced Studies
International Journal of Production Economics | Year: 2014

The dynamic lotsizing problem concerns the determination of optimally produced/delivered batch quantities, when demand, which is to be satisfied, is distributed over time in different amounts at different times. The standard formulation assumes that these batches are provided instantaneously, i.e. that the production rate is infinite. Using a cumulative geometrical representation for demand and production, it has previously been demonstrated that the inner-corner condition for an optimal production plan reduces the number of possible optimal replenishment times to a finite set of given points, at which replenishments can be made. The problem is thereby turned into choosing from a set of zero/one decisions, whether or not to replenish each time there is a demand. If n is the number of demand events, this provides 2n-1 alternatives, of which at least one solution must be optimal. This condition applies, whether an Average Cost approach or the Net Present Value principle is applied, and the condition is valid in continuous time, and therefore in discrete time. In the current paper, the assumption of an infinite production rate is relaxed, and consequences for the inner-corner condition are investigated. It is then shown that the inner-corner condition needs to be modified to a tangency condition between cumulative requirements and cumulative production. Also, we have confirmed the additional restriction for feasibility in the finite production case (provided by Hill, 1997), namely the production rate restriction. Furthermore, in the NPV case, one further necessary condition for optimality, the distance restriction concerning the proximity between adjacent production intervals, has been derived. In an example this condition has shown to reduce the number of candidate solutions for optimality still further. An algorithm leading to the optimal solution is presented. © 2012 Elsevier B.V.


Grubbstrom R.W.,Linkoping Institute of Technology | Grubbstrom R.W.,Mediterranean Institute for Advanced Studies
International Journal of Production Economics | Year: 2015

In this paper we attempt to provide a partial answer to the question of why energy is a scarce resource. Scarcity is a fundamental concept in the science of economics. If resources, goods or services were not in scarce supply, we need not economise when utilising them. Indeed, free commodities we need not pay for, their prices are zero, we attach no economic value to them, and their supply is in abundance - at least beyond the point at which our needs and wants are satisfied. However, energy is regarded as a scarce resource, although energy - as such - is not scarce. To describe energy as a useful and therefore a valuable quantity, to which a price may be attached, energy will thus have to be characterised in further dimensions than energy content alone. Apart from quantity, there is a need for a uniform qualitative measure of energy. The obvious field to revert to for such considerations is thermodynamics, which offers a method for defining a uniform measure for the qualitative content of energy, namely exergy. Although exergy is defined from purely physical properties, it is shown to have an important rôle to play when comparing the economic value of energy in different forms. In particular, this paper will focus on the economic value of heat, especially heat delivered through a district heating system.The concept of exergy is defined from maximising a work output reversibly taking an infinite time. However, for processes to run within finite horizons, entropy must be generated. This leads us to add finite time considerations from examining consequences from the assumed availability of so-called endo-reversible processes.In a small case example we show that heat appears to be overpriced compared to electricity from an exergetic point of view and that this is even more pronounced adopting finite time considerations. © 2015 Elsevier B.V.


Grubbstrm R.W.,Linkping Institute of Technology | Grubbstrm R.W.,Mediterranean Institute for Advanced Studies
International Journal of Production Research | Year: 2012

MRP theory provides a theoretical background for multi-level, multi-stage production-inventory systems (material requirements planning in a general sense) together with their economic evaluation, in particular applying the net present value principle. The theory combines the use of input-output analysis and Laplace transforms, the former for capturing product structures, and the latter for incorporating timing, including time lags, lead times, and output delays. In this paper, we consider any production policy, when given any external demand as a vector-valued function of time. It is shown that in order for available inventory to be kept at finite levels at any time, the lot-for-lot (L4L) solution must be valid for the time averages of production and deliveries, irrespective of the policy followed. This analysis is carried out using properties the Laurent expansions of the transforms involved. © 2012 Copyright Taylor and Francis Group, LLC.


Grubbstrom R.W.,Linkoping Institute of Technology | Grubbstrom R.W.,Mediterranean Institute for Advanced Studies | Tang O.,Linkoping Institute of Technology
International Journal of Production Economics | Year: 2012

MRP Theory combines the use of Input-Output Analysis and Laplace transforms, enabling the development of a theoretical background for multi-level, multi-stage production-inventory systems together with their economic evaluation, in particular applying the Net Present Value principle (NPV). In a recent paper (Grubbström et al., 2010), a general method for solving the dynamic lotsizing problem for a general assembly system was presented. It was shown there that the optimal production (completion) times had to be chosen from the set of times generated by the Lot-For-Lot (L4L) solution. Thereby, the problem could be stated in binary form by which the values of the binary decision variables represented either to make a production batch, or not, at each such time. Based on these potential times for production, the problem of maximising the Net Present Value or minimising the average cost could be solved, applying a single-item optimal dynamic lotsizing method, such as the Wagner-Whitin algorithm or the Triple Algorithm, combined with dynamic programming. This current paper follows up the former paper by investigating the complexity defined as the number of possible feasible solutions (production plans) to compare. We therefore investigate how properties of external demand timing and properties of requirements (Bill-of-Materials) have consequences on the size of this solution space. Explicit expressions are developed for how the total number of feasible production plans depends on numbers of external demand events on different levels for, in particular, the two extreme cases of a serial system and a full system (the latter, in which items have requirements of all existing types of subordinate items). A formula is also suggested for general systems falling in between these two extremes. For the most complex full system, it is shown that the number of feasible plans will be the product of elements taken from Sylvesters sequence (an instance of doubly exponential sequences) raised to powers depending on numbers of external demand events. © 2012 Elsevier B.V. All rights reserved.

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