Empirical studies of open source evolution

Copyright @ 2008 Springer-VerlagThis chapter presents a sample of empirical studies of Open Source Software (OSS) evolution. According to these studies, the classical results from the studies of proprietary software evoltion, such as Lehman’s laws of software evolution, might need to be revised, if not fully, at least in part, to account for the OSS observations. The book chapter also summarises what appears to be the empirical status of each of Lehman’s laws with respect to OSS and highlights the threads to validity that frequently emerge in these empirical studies. The chapter also discusses related topics for further research

Border Enforcement and Selection of Mexican Immigrants in the United States

Since 1986 the United States has made considerable efforts to curb illegal immigration. This has resulted in an increase in migration costs for undocumented immigrants. More stringent border enforcement either deters potential illegal immigrants from coming to the U.S., or moves the point of crossing for illegal immigrants from traditional crossing routes to more inhospitable routes. These changes are likely to place a heavier burden on illegal immigrant women as they are more likely to be kidnapped, smuggled, or raped when crossing illegally. If migration costs are not the same for all migrants, higher migrating costs may result in a change in the number and in the composition of immigrants to the United States. In the face of higher migration costs, only immigrants with relatively high expected benefits of migration will choose to migrate. Based on our theoretical model, we test for three empirical results that are associated with a stronger selection of immigrant women from Mexico relative to men as a result of higher migration costs: 1) A decrease in the relative flow of older and highly educated undocumented immigrant women relative to men; 2) A change in the skill composition of immigrant women to men; and 3) An increase in the average earnings of those groups most affected by increased migration costs. Using data from the 1990, 2000 Decennial Census, and from the 2006-2008 American Community Survey we empirically confirm these predictions.selection, gender, immigration, border enforcement

Dispersive spherical optical model of neutron scattering from Al27 up to 250 MeV

A spherical optical model potential (OMP) containing a dispersive term is used to fit the available experimental database of angular distribution and total cross section data for n + Al27 covering the energy range 0.1- 250 MeV using relativistic kinematics and a relativistic extension of the Schroedinger equation. A dispersive OMP with parameters that show a smooth energy dependence and energy independent geometry are determined from fits to the entire data set. A very good overall agreement between experimental data and predictions is achieved up to 150 MeV. Inclusion of nonlocality effects in the absorptive volume potential allows to achieve an excellent agreement up to 250 MeV.Comment: 13 figures (11 eps and 2 jpg), 3 table

Total electron scattering cross sections from thiophene for the (1-300 eV) impact energy range

Experimental electron scattering cross sections for thiophene in the impact energy range from 1 to 300 eV have been measured with a magnetically confined electron transmission-beam apparatus. Random uncertainty limits have been estimated to be less than 5%, and systematic errors derived from acceptance angle limitations have also been identified and evaluated. Experimental values are compared with our previous low energy (1-15 eV) R-matrix and intermediate/high energy (15-300 eV) IAM-SCAR+I calculations finding reasonable agreement, within the combined uncertainty limits. Some of the low energy shape and core-excited resonances predicted by previous calculations are experimentally confirmed in this study

Bogomolny equations for vortices in the noncommutative torus

We derive Bogomolny-type equations for the Abelian Higgs model defined on the noncommutative torus and discuss its vortex like solutions. To this end, we carefully analyze how periodic boundary conditions have to be handled in noncommutative space and discussed how vortex solutions are constructed. We also consider the extension to an $U(2)\times U(1)$ model, a simplified prototype of the noncommutative standard model.Comment: 23 pages, no figure

Branching Deregulation and Merger Optimality

The U.S. banking industry has been characterized by intense merger activity in the absence of economies of scale and scope. We claim that the loosening of geographic constraints on U.S. banks is responsible for this consolidation process, irrespective of value-maximizing motives. We demonstrate this by putting forward a theoretical model of banking competition and studying banks’ strategic responses to geographic deregulation. We show that even in the absence of economies of scale and scope, bank mergers represent an optimal response. Also, we show that the consolidation process is characterized by merger waves and that some equilibrium mergers are not profitable per se -they yield losses- but become profitable as the waves of mergers unfold.Banking Competition, Deregulation, Mergers

Leptonic universality breaking in Upsilon decays as a probe of new physics

In this work we examine the possible existence of new physics beyond the standard model which could modify the branching fractions of the leptonic (mainly tauonic) decays of bottomonium vector resonances below the $B\bar{B}$ threshold. The decay width is factorized as the product of two pieces: a) the probability of an intermediate pseudoscalar color-singlet $b\bar{b}$ state (coupling to the dominant Fock state of the Upsilon via a magnetic dipole transition) and a soft (undetected) photon; b) the annihilation width of the $b\bar{b}$ pair into two leptons, mediated by a non-standard CP-odd Higgs boson of mass about 10 GeV, introducing a quadratic dependence on the lepton mass in the partial width. The process would be unwittingly ascribed to the $\Upsilon$ leptonic channel thereby (slightly) breaking lepton universality. A possible mixing of the pseudoscalar Higgs and bottomonium resonances is also considered. Finally, several experimental signatures to check out the validity of the conjecture are discussed.Comment: LaTeX, 22 pages, 2 EPS figure