Bi-Criteria and Approximation Algorithms for Restricted Matchings

In this work we study approximation algorithms for the \textit{Bounded Color Matching} problem (a.k.a. Restricted Matching problem) which is defined as follows: given a graph in which each edge $e$ has a color $c_e$ and a profit $p_e \in \mathbb{Q}^+$, we want to compute a maximum (cardinality or profit) matching in which no more than $w_j \in \mathbb{Z}^+$ edges of color $c_j$ are present. This kind of problems, beside the theoretical interest on its own right, emerges in multi-fiber optical networking systems, where we interpret each unique wavelength that can travel through the fiber as a color class and we would like to establish communication between pairs of systems. We study approximation and bi-criteria algorithms for this problem which are based on linear programming techniques and, in particular, on polyhedral characterizations of the natural linear formulation of the problem. In our setting, we allow violations of the bounds $w_j$ and we model our problem as a bi-criteria problem: we have two objectives to optimize namely (a) to maximize the profit (maximum matching) while (b) minimizing the violation of the color bounds. We prove how we can "beat" the integrality gap of the natural linear programming formulation of the problem by allowing only a slight violation of the color bounds. In particular, our main result is \textit{constant} approximation bounds for both criteria of the corresponding bi-criteria optimization problem

Term Limits: Do they really Affect Fiscal Policy Choices?

According to reputational models of political economy, a term limit may change the behavior of a chief executive because he does not have to stand for election. We test this hypothesis in a sample of 52 countries over the period 1977-2000, using government spending, social and welfare spending and deficit as policy choice variables using panel data estimation techniques. We are unable to find significant differences in the behavior of term-limited and non term-limited chief executives. This is in contrast with some previous empirical results based on U.S. states and international data.term limits, comparative politics, fiscal policy

The ‘problem of problem choice’: A model of sequential knowledge production within scientific communities cientific communities.

In this paper we present an original model of sequential problem choice within scientific communities. Disciplinary knowledge is accumulated by solving problems emerging in a growing tree-like web of research areas. Knowledge production is sequential since the problems solved generate new problems that may be handled. The model allows us to study how the reward system in science influences the scientific community in stochastically selecting at each period its research agendas, and the long term resulting disciplines. We present some evidence on a decrease in the generation of new areas, a path dependency in specialization, and circumstances under which collapsing dynamics arise.Sequential Problem Choice; Stochastic Process; Tree; Graph Theory; Scientific Knowledge; Academics; Reward System

The Allocation of Software Development Resources In ‘Open Source’ Production Mode

This paper aims to develop a stochastic simulation structure capable of describing the decentralized, micro-level decisions that allocate programming resources both within and among open source/free software (OS/FS) projects, and that thereby generate an array of OS/FS system products each of which possesses particular qualitative attributes. The core or behavioral kernel of simulation tool presented here represents the effects of the reputational reward structure of OS/FS communities (as characterized by Raymond 1998) to be the key mechanism governing the probabilistic allocation of agents’ individual contributions among the constituent components of an evolving software system. In this regard, our approach follows the institutional analysis approach associated with studies of academic researchers in “open science” communities. For the purposes of this first step, the focus of the analysis is confined to showing the ways in which the specific norms of the reward system and organizational rules can shape emergent properties of successive releases of code for a given project, such as its range of functions and reliability. The global performance of the OS/FS mode, in matching the functional and other characteristics of the variety of software systems that are produced with the needs of users in various sectors of the economy and polity, obviously, is a matter of considerable importance that will bear upon the long-term viability and growth of this mode of organizing production and distribution. Our larger objective, therefore, is to arrive at a parsimonious characterization of the workings of OS/FS communities engaged across a number of projects, and their collective productive performance in dimensions that are amenable to “social welfare” evaluation. Seeking that goal will pose further new and interesting problems for study, a number of which are identified in the essay’s conclusion. Yet, it is argued that that these too will be found to be tractable within the framework provided by refining and elaborating on the core (“proof of concept”) model that is presented in this paper.

Ambiguity and macroeconomics:a rationale for price stickiness

This paper deals with the emergence of price stickiness, that is nominal price elasticity below one, in the wake of nominal shocks. The setting of analysis is a general equilibrium model with both ambiguity and rational expectations. Ambiguity and macroeconomics are linked exploiting a micro-founded framework. Ambiguity concerns the lack of knowledge of firms about the relationship between changes in the aggregated stock of money and in the money distribution across heterogeneous consumers in the economy. Ambiguity is represented through a multiple priors approach. It is shown that price stickiness can emerge even if a change in the money supply level does not alter the distribution of money across consumers (uniform monetary policy). The key assumption made in the paper is that attitude towards ambiguity of firms is asymmetric: ambiguity aversion towards uncertain positive outcomes (gains) and ambiguity seeking towards negative outcomes (losses). By focusing on the dynamics of beliefs following a change in the stock of money that does not alter the money distribution, it is shown that money neutrality remains true in the long runAmbiguity, multiple priors, incomplete information, price stickiness

SimCode: Agent-based Simulation Modelling of Open-Source Software Development

We present an original modeling tool, which can be used to study the mechanisms by which free/libre and open source software developers’ code-writing efforts are allocated within open source projects. It is first described analytically in a discrete choice framework, and then simulated using agent-based experiments. Contributions are added sequentially to either existing modules, or to create new modules out of existing ones: as a consequence, the global emerging architecture forms a hierarchical tree. Choices among modules reflect expectations of peer- regard, i.e. developers are more attracted a) to generic modules, b) to launching new ones, and c) to contributing their work to currently active development sites in the project. In this context, we are able – particularly by allowing for the attractiveness of “hot spots”-- to replicate the high degree of concentration (measured by Gini coefficients) in the distributions of modules sizes. The latter have been found by empirical studies to be a characteristic typical of the code of large projects, such as the Linux kernel. Introducing further a simple social utility function for evaluating the mophology of “software trees,” it turns out that the hypothesized developers’ incentive structure that generates high Gini coefficients is not particularly conducive to producing self-organized software code that yields high utility to end-users who want a large and diverse range of applications. Allowing for a simple governance mechanism by the introduction of maintenance rules reveals that “early release” rules can have a positive effect on the social utility rating of the resulting software trees.

“It Takes All Kinds”: A Simulation Modeling Perspective on Motivation and Coordination in Libre Software Development Projects

This paper presents a stochastic simulation model to study implications of the mechanisms by which individual software developers’ efforts are allocated within large and complex open source software projects. It illuminates the role of different forms of “motivations-at-the-margin” in the micro-level resource allocation process of distributed and decentralized multi-agent engineering undertakings of this kind. We parameterize the model by isolating the parameter ranges in which it generates structures of code that share certain empirical regularities found to characterize actual projects. We find that, in this range, a variety of different motivations are represented within the community of developers. There is a correspondence between the indicated mixture of motivations and the distribution of avowed motivations for engaging in FLOSS development, found in the survey responses of developers who were participants in large projects.free and open source software (FLOSS), libre software engineering, maintainability, reliability, functional diversity, modularity, developers’ motivations, user-innovation, peer-esteem, reputational reward systems, agent-based modeling, stochastic simulation, stigmergy, morphogenesis.

Coordination, Division of Labor, and Open Content Communities: Template Messages in Wiki-Based Collections

In this paper we investigate how in commons based peer production a large community of contributors coordinates its efforts towards the production of high quality open content. We carry out our empirical analysis at the level of articles and focus on the dynamics surrounding their production. That is, we focus on the continuous process of revision and update due to the spontaneous and largely uncoordinated sequence of contributions by a multiplicity of individuals. We argue that this loosely regulated process, according to which any user can make changes to any entry, while allowing highly creative contributions, has to come into terms with potential issues with respect to the quality and consistency of the output. In this respect, we focus on emergent, bottom up organizational practice arising within the Wikipedia community, namely the use of template messages, which seems to act as an effective and parsimonious coordination device in emphasizing quality concerns (in terms of accuracy, consistency, completeness, fragmentation, and so on) or in highlighting the existence of other particular issues which are to be addressed. We focus on the template "NPOV" which signals breaches on the fundamental policy of neutrality of Wikipedia articles and we show how and to what extent imposing such template on a page affects the production process and changes the nature and division of labor among participants. We find that intensity of editing increases immediately after the "NPOV" template appears. Moreover, articles that are treated most successfully, in the sense that "NPOV" disappears again relatively soon, are those articles which receive the attention of a limited group of editors. In this dimension at least the distribution of tasks in Wikipedia looks quite similar to what is know about the distribution in the FLOSS development process