The interface of buffer design and cyclic scheduling decisions in deterministic flow lines

In this paper, we address some issues on the interface of buffer design and cyclic scheduling decisions in a multi-product deterministic flow line. We demonstrate the importance of the above interface for the throughput performance of the flow line. In particular, we point out that the use of sequence-independent information, such as workload distribution and variability in processing times among stations, is not adequate to decide the optimal buffer configuration of the flow line. We formulate the buffer design problem for a fixed sequence of jobs as a general resource allocation problem, and suggest two effective heuristics for its solution. For the simultaneous buffer design and cyclic scheduling problem, we suggest an iterative scheme that builds on the effectiveness of the above heuristics. One of the side results of our extensive computational studies on this problem is that the general guidelines of buffer design in single-product flow lines with stochastic processing times are not directly transferable to the multiproduct deterministic flow line environment. © 1994 J.C. Baltzer AG, Science Publishers

Cyclic scheduling in flow lines: Modeling observations, effective heuristics and a cycle time minimization procedure

In this paper we address the cyclic scheduling problem in flow lines. We develop a modeling framework and an integer programming formulation of the problem. We subsequently present exact and approximate solution procedures. The exact solution procedure is a branch-and-bound algorithm which uses Lagrangian and station-based relaxations of the integer programming formulation of the problem as the lower bounding method. Our heuristic procedures show a performance superior to the available ones in the literature. Finally, we address the stability issue in cyclic scheduling, demonstrate its relationship to the work-in-progress inventory control of a flow line, and present a very simple procedure to generate stable schedules in flow lines. © 1996 John Wuey & Sons, Inc

Χαρακτηριστικά του Πληθωριστικού Δυναμικού και το Φάσμα των Διακυμάνσεων Καμπυλότητας

Η πιθανότητα δημιουργίας αρχέγονων μελανών οπών έχει προσελκύσει πολύ ενδιαφέρον τα τελευταία χρόνια, αφού είναι πιθανό να αποτελούν ένα μεγάλο μέρος της σκοτεινής ύλης. Η δημιουργία τους απαιτεί την παρουσία μεγάλων διαταραχών της πυκνότητας στο πρώιμο Σύμπαν, οι οποίες μπορούν να υποδηλωθούν από μία ενίσχυση στο αρχέγονο φάσμα ισχύος των διακυμάνσεων καμπυλότητας κατά αρκετές τάξεις μεγέθους. Στην παρούσα εργασία μελετάμε τέτοιες περιπτώσεις στις οποίες η ενίσχυση προέρχεται από χαρακτηριστικά σαν σκαλοπάτια στο πληθωριστικό δυναμικό, έτσι ώστε να παραβιάζεται η προσέγγιση της αργής εξέλιξης. Εστιάζουμε σε ένα μοντέλο με ένα πέδιο και μελετάμε την εξέλιξη των διαταραχών του πεδίου μέσω της εξίσωσης Mukhanov-Sasaki στο πλαίσιο της θεωρίας κοσμολογικών διαταραχών. Λαμβάνοντας υπόψη την κβαντική περιγραφή του πεδίου, μπορεί να υπολογιστεί το φάσμα ισχύος των διαταραχών αυτών. Εκτός από την αριθμητική ανάλυση, παρουσιάζουμε ακόμα μια αναλυτική προσέγγιση στον υπολογισμό αυτού του φάσματος ισχύος. Ενα ενδιαφέρον χαρακτηριστικό που εμφανίζεται και στους αριθμητικούς και στους αναλυτικούς υπολογισμούς, είναι ταλαντώσεις στο φάσμα των διακυμάνσεων καμπυλότητας. Αυτές οι ταλαντώσεις μπορούν να αποτυπωθούν στο φάσμα των βαρυτικών κυμάτων και μια ενδεχόμενη ανίχνευση τους μπορεί να παρέχει πληροφορίες για πιθανά χαρακτηριστικά στη πληθωριστική δυναμική.The creation of primordial black holes has attracted a lot of interest in recent years, as they may constitute a large part of the dark matter. Their production requires the presence of strong density perturbations in the early universe, which would correspond to the enhancement of the primordial power spectrum of curvature perturbations by several orders of magnitude. We study cases where this enhancement is generated by step-like features in the inflaton potential, so that the slow-roll approximation is violated. We focus on a single-field model and we study the evolution of the field perturbations through the Mukhanov-Sasaki equation. Starting from the quantum description of the field in the Bunch-Davies vacuum, we compute the power spectrum numerically. We also present an analytical approach that explains many of its properties. An interesting feature that is predicted by both numerical and analytical calculations is strong oscillations in the power spectrum of curvature perturbations. These oscillations can be imprinted on the spectrum of gravitational waves induced at second order in perturbation theory. A possible detection of these gravitational waves by near-future space interferometers, like LISA, can provide details of possible strong features in the inflationary dynamics

Inventory Management and Financial Hedging of Storable Commodities

Under revision for resubmitting to Management Science</p

Expected time delay in multi-item inventory systems with correlated demands

This paper considers multi-item inventory systems where a customer order may require several different items (i.e., demands are correlated across items) and customer satisfaction is measured by the time delays seen by the customers. Most inventory models on time delay in the literature assume each demand only requires one item (i.e., demands are not correlated across items or are independent). In this paper, we derive an exact expression for the expected total time delay. We show that when items are actually correlated, assuming items are independent leads to an overestimate of the total time delay. However, (1) it is extremely difficult in practice to obtain the demand information for all demand types (especially in a system with tens of thousands of part numbers), and (2) the problem becomes too complicated to be of practical interest when the correlation is considered. We then explore the possibility of including the demand information partially and develop bounds for the time delays. © 1999 John Wiley & Sons, Inc. Naval Research Logistics 46: 671–688, 1999Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/35020/1/5_ftp.pd