Small Vertex Cover makes Petri Net Coverability and Boundedness Easier

The coverability and boundedness problems for Petri nets are known to be Expspace-complete. Given a Petri net, we associate a graph with it. With the vertex cover number k of this graph and the maximum arc weight W as parameters, we show that coverability and boundedness are in ParaPspace. This means that these problems can be solved in space O(ef(k,W)poly(n)), where ef(k,W) is some exponential function and poly(n) is some polynomial in the size of the input. We then extend the ParaPspace result to model checking a logic that can express some generalizations of coverability and boundedness.Comment: Full version of the paper appearing in IPEC 201

Nesting Depth of Operators in Graph Database Queries: Expressiveness Vs. Evaluation Complexity

Designing query languages for graph structured data is an active field of research, where expressiveness and efficient algorithms for query evaluation are conflicting goals. To better handle dynamically changing data, recent work has been done on designing query languages that can compare values stored in the graph database, without hard coding the values in the query. The main idea is to allow variables in the query and bind the variables to values when evaluating the query. For query languages that bind variables only once, query evaluation is usually NP-complete. There are query languages that allow binding inside the scope of Kleene star operators, which can themselves be in the scope of bindings and so on. Uncontrolled nesting of binding and iteration within one another results in query evaluation being PSPACE-complete. We define a way to syntactically control the nesting depth of iterated bindings, and study how this affects expressiveness and efficiency of query evaluation. The result is an infinite, syntactically defined hierarchy of expressions. We prove that the corresponding language hierarchy is strict. Given an expression in the hierarchy, we prove that it is undecidable to check if there is a language equivalent expression at lower levels. We prove that evaluating a query based on an expression at level i can be done in $\Sigma_i$ in the polynomial time hierarchy. Satisfiability of quantified Boolean formulas can be reduced to query evaluation; we study the relationship between alternations in Boolean quantifiers and the depth of nesting of iterated bindings.Comment: Improvements from ICALP 2016 review comment

International Trade Policy towards Monopoly and Oligopoly

This paper highlights the importance of product differentiation and endogenous R&D in determining the optimal R&D policy, in a model where investment in cost reducing R&D is committed before firms compete in a differentiated-goods third-country export market. R&D is always taxed in oligopolies for high degrees of product differentiation. For lower degrees of product differentiation the duopoly is subsidized or the government remains inactive. In contrast, the monopoly is always subsidized. The government with a duopoly may be active or inactive depending on the degree of product differentiation. Thus, we may observe a reversal in the sign of the optimal R&D policy if the degree of product differentiation changes or, alternatively, if there is a change in the number of firms. Similar qualitative results hold if trade policy uses output subsidies, instead of R&D promotion

STATE TRADING IN AGRICULTURE: AN ANALYTICAL FRAMEWORK

This paper highlights some of the recent concerns regarding agricultural state trading enterprises (STEs) and proposes an analytical framework to examine the trade impacts of such entities. Issues associated with discriminatory pricing, exclusive rights to sell and purchase commodities, and unfair competitive advantage vis-a-vis private traders are expected to be major concerns on the export side, while on the import side, the relevance of tariffication in the presence of STEs is being questioned. Our paper proposes that, in most instances, tariff equivalents are the most relevant methodology to quantify the trade impacts of agricultural STEs. But, obtaining empirical information that would enable the calculation of such measures is not an easy task. To that end, a classification scheme that highlights the different types of STEs in terms of their ability to distort trade is proposed. Quantification can then focus on those most likely to impact trade.International Relations/Trade,

Rapidly reconfigurable slow-light system based on off-resonant Raman absorption

We present a slow-light system based on dual Raman absorption resonances in warm rubidium vapor. Each Raman absorption resonance is produced by a control beam in an off-resonant Λ system. This system combines all optical control of the Raman absorption and the low-dispersion broadening properties of the double Lorentzian absorption slow light. The bandwidth, group delay, and central frequency of the slow-light system can all be tuned dynamically by changing the properties of the control beam. We demonstrate multiple pulse delays with low distortion and show that such a system has fast switching dynamics and thus fast reconfiguration rates

What You Must Remember When Transforming Datawords

Streaming Data String Transducers (SDSTs) were introduced to model a class of imperative and a class of functional programs, manipulating lists of data items. These can be used to write commonly used routines such as insert, delete and reverse. SDSTs can handle data values from a potentially infinite data domain. The model of Streaming String Transducers (SSTs) is the fragment of SDSTs where the infinite data domain is dropped and only finite alphabets are considered. SSTs have been much studied from a language theoretical point of view. We introduce data back into SSTs, just like data was introduced to finite state automata to get register automata. The result is Streaming String Register Transducers (SSRTs), which is a subclass of SDSTs. We use origin semantics for SSRTs and give a machine independent characterization, along the lines of Myhill-Nerode theorem. Machine independent characterizations for similar models are the basis of learning algorithms and enable us to understand fragments of the models. Origin semantics of transducers track which positions of the output originate from which positions of the input. Although a restriction, using origin semantics is well justified and is known to simplify many problems related to transducers. We use origin semantics as a technical building block, in addition to characterizations of deterministic register automata. However, we need to build more on top of these to overcome some challenges unique to SSRTs

THE CURRENT WTO AGRICULTURAL NEGOTIATIONS: OPTIONS FOR PROGRESS; SYNTHESIS

World Trade Organization, International Relations/Trade,