Modeling the operation of multireservoir systems using decomposition and stochastic dynamic programming

Stochastic dynamic programming models are attractive for multireservoir control problems because they allow non‐linear features to be incorporated and changes in hydrological conditions to be modeled as Markov processes. However, with the exception of the simplest cases, these models are computationally intractable because of the high dimension of the state and action spaces involved. This paper proposes a new method of determining an operating policy for a multireservoir control problem that uses stochastic dynamic programming, but is practical for systems with many reservoirs. Decomposition is first used to reduce the problem to a number of independent subproblems. Each subproblem is formulated as a low‐dimensional stochastic dynamic program and solved to determine the operating policy for one of the reservoirs in the system. © 2006 Wiley Periodicals, Inc. Naval Research Logistics, 2006     

Waiting times for M/G/1 queues with service-time or delay-dependent server vacations

This article shows how to determine the stationary distribution of the virtual wait in M/G/1 queues with either one-at-a-time or exhaustive server vacations, depending on either service times or accrued workload. For the first type of dependence, each vacation time is a function of the immediately preceding service time or of whether the server finds the system empty after returning from vacation. In this way, it is possible to model situations such as long service times followed by short vacations, and vice versa. For the second type of dependence, the vacation time assigned to an arrival to follow its service is a function of the level of virtual wait reached. By this device, we can model situations in which vacations may be shortened whenever virtual delays have gotten excessive. The method of analysis employs level-crossing theory, and examples are given for various cases of service and vacation-time distributions. A closing discussion relates the new model class to standard M/G/1 queues where the service time is a sum of variables having complex dependencies. © 1992 John Wiley & Sons, Inc.     

Economic lot scheduling of fully loaded processes with external setups

Without restricting the class of permissible schedules, we derive optimal schedules for economic lot scheduling problems that are fully loaded, have external setups, and have only two products. The fully loaded condition accurately represents certain types of bottlenecks. We show that the optimal schedule must have the Wagner-Whitin property. We also develop a measure of aggregate inventory, derive an optimal steady-state aggregate inventory policy, and provide conditions under which the aggregate inventory level of an optimal schedule must approach a steady state. By restricting the class of permissible schedules to rotation cycle schedules, we extend these results to more than two products.     

Fencing the bear? Explaining US foreign policy towards Russian interventions

Despite the burgeoning literature on Russia’s renewed power politics, little attention has been paid to the fact that US reactions towards Russia’s military interventions were all but coherent. The USA has chosen weak measures in Georgia in 2008 (shaming) compared to its assertive response in Ukraine in 2014 (sanctions, hard deterrence). This article assesses the explanatory power of neorealist, liberal and constructivist theories for the variation in US reactions towards Russian interventions in Georgia and Ukraine. Our argument is that the constructivist perspective explains the cases best as it highlights the power and communality of normative assessments. The Ukraine crisis was perceived by the USA as a violation of core international norms, especially the non-use of force and the principle of territorial integrity. Relevant international norm carriers shared this assessment of the conflict. In contrast, the perception of the Georgian war centred on the issue of democracy promotion. While democracy is an important aim of US foreign policy, it does not summon the same normative importance as general principles of international law. Furthermore, the perception of the Georgian war remained contested among Western allies, which decreased the communality of the normative assessment.     

Understanding armed groups and party politics

Abstract

Political parties with an armed history are not unusual, yet how these groups function in politics after the transition has largely been ignored. This special issue examines armed groups in party politics, using single and comparative case studies. The introduction forwards five recommendations for future research: (1) We need to see more comparisons across taken for granted boundaries; (2) the consequences for democracy should figure more prominently in our analysis of armed groups; (3) think more critically about standards and conceptual tools; (4) critically examine the interaction between levels of analysis; and (5) methodological pluralism would enrich the field.     

Case Study Commentary and Analysis: The Moral Sword of Damocles

Case summary, by James Cook (Case Study Editor):

In the final issue of the 2015 volume of the Journal of Military Ethics, we published a case study entitled “Coining an Ethical Dilemma: The Impunity of Afghanistan’s Indigenous Security Forces”, written by Paul Lushenko. The study detailed two extra-judicial killings (EJKs) by Afghan National Police (ANP) personnel in an area stabilized and overseen by a US-led Combined Task Force (CTF). To deter further EJKs following the first incident, the CTF’s commander reported the incidents up his chain of command and used the limited tools at his disposal to influence local indigenous officials directly. Apparently, the ANP unit took no notice. In his commentary on the case study, Paul Robinson considered moral compromise in war more generally. Coalition troops in Afghanistan, for instance, have encountered not just EJKs but also sexual abuse of minors, killing of non-combatants, kidnapping, torture, and widespread corruption. What should the soldier on the ground do if indigenous personnel violate Laws of Armed Conflict (LOAC) with impunity? Refusing to serve will not right or prevent moral wrongs, while staying on to fight the good but futile fight will mire the soldier in moral compromise. “?… [S]oldiers faced with this dilemma have no good options. The systemic failings surrounding them mean that it is probable that nothing they do will help”. In a concluding note, I suggested that while an individual soldier may indeed have no good options, as Paul Robinson suggests, that soldier’s military and nation at large are obliged to do what they can. At least, they must keep to the moral high ground so as not to give indigenous security forces an excuse to misbehave, and determine the nature of crimes such as EJKs: are they outlaw acts or in fact endorsed by the indigenous culture and perhaps even government? Below Colonel Dave Barnes, himself a veteran of Operation Enduring Freedom, analyzes Paul Lushenko’s case study at “?…?the local, tactical level: If a commander is in this situation – where her unit witnesses an EJK or other war crime – what should she do?”     

Controlling rates in a double queue

A double-ended queue with a Poisson arrival pattern is examined in a situation where the rates depend (in a restricted sense) on both the time and the state of the system. Under some circumstances, the rates can be controlled. This article studies the distribution of the difference in queue sizes for each member of a large class of control strategies and introduces the problem of determining the optimal times at which the control should be in effect in order to maximize certain objective functions.     

Minimum-cost mixtures of area and point defenses assuming simultaneous attack

It is desired to select numbers of area and point interceptors that minimize the cost of such defensive missiles under the condition that the maximum total expected damage produced by an unknown number of attacking missiles A be bounded above by a given function of A. Area coverages may overlap. The attacker is assumed to know the numbers of area and point interceptors and to launch a simultaneous attack (of arbitrary size A) against all targets, which is optimal against the given defenses. The defender is assumed to observe the attack and then allocate his area and point interceptors against attacking missiles so as to minimize the total expected damage. Upper and lower bounds on the minimal cost are obtained by solving integer programming problems.