Guidelines for Teaching Object Orientation with Java

How to best teach object orientation to first year students is currently a topic of much debate. One of the tools suggested to aid in this task is BlueJ, an integrated development environment specifically designed for teaching. BlueJ supports a unique style of introduction of OO concepts. In this paper we discuss a set of problems with OO teaching, present some guidelines for better course design and show how BlueJ can be used to make significant improvements to introductory OO courses. We end by esenting a description of a possible project sequence using this teaching approach

Is Explicit Congestion Notification usable with UDP?

We present initial measurements to determine if ECN is usable with UDP traffic in the public Internet. This is interesting because ECN is part of current IETF proposals for congestion control of UDPbased interactive multimedia, and due to the increasing use of UDP as a substrate on which new transport protocols can be deployed. Using measurements from the author’s homes, their workplace, and cloud servers in each of the nine EC2 regions worldwide, we test reachability of 2500 servers from the public NTP server pool, using ECT(0) and not-ECT marked UDP packets. We show that an average of 98.97% of the NTP servers that are reachable using not-ECT marked packets are also reachable using ECT(0) marked UDP packets, and that ~98% of network hops pass ECT(0) marked packets without clearing the ECT bits. We compare reachability of the same hosts using ECN with TCP, finding that 82.0% of those reachable with TCP can successfully negotiate and use ECN. Our findings suggest that ECN is broadly usable with UDP traffic, and that support for use of ECN with TCP has increased

On non-normality and classification of amplification mechanisms in stability and resolvent analysis

We seek to quantify non-normality of the most amplified resolvent modes and predict their features based on the characteristics of the base or mean velocity profile. A 2-by-2 model linear Navier-Stokes (LNS) operator illustrates how non-normality from mean shear distributes perturbation energy in different velocity components of the forcing and response modes. The inverse of their inner product, which is unity for a purely normal mechanism, is proposed as a measure to quantify non-normality. In flows where there is downstream spatial dependence of the base/mean, mean flow advection separates the spatial support of forcing and response modes which impacts the inner product. Success of mean stability analysis depends on the normality of amplification. If the amplification is normal, the resolvent operator written in its dyadic representation reveals that the adjoint and forward stability modes are proportional to the forcing and response resolvent modes. If the amplification is non-normal, then resolvent analysis is required to understand the origin of observed flow structures. Eigenspectra and pseudospectra are used to characterize these phenomena. Two test cases are studied: low Reynolds number cylinder flow and turbulent channel flow. The first deals mainly with normal mechanisms and quantification of non-normality using the inverse inner product of the leading forcing and response modes agrees well with the product of the resolvent norm and distance between the imaginary axis and least stable eigenvalue. In turbulent channel flow, structures result from both normal and non-normal mechanisms. Mean shear is exploited most efficiently by stationary disturbances while bounds on the pseudospectra illustrate how non-normality is responsible for the most amplified disturbances at spatial wavenumbers and temporal frequencies corresponding to well-known turbulent structures

Neutral carbon and CO in 76 (U)LIRGs and starburst galaxy centers A method to determine molecular gas properties in luminous galaxies

We present fluxes in both neutral carbon [CI] lines at the centers of 76 galaxies with FIR luminosities between 10^{9} and 10^{12} L(o) obtained with Herschel-SPIRE and with ground-based facilities, along with the J=7-6, J=4-3, J=2-1 12CO and J=2-1 13CO line fluxes. We investigate whether these lines can be used to characterize the molecular ISM of the parent galaxies in simple ways and how the molecular gas properties define the model results. In most starburst galaxies, the [CI]/13CO flux ratio is much higher than in Galactic star-forming regions, and it is correlated to the total FIR luminosity. The [CI](1-0)/CO(4-3), the [CI](2-1) (2-1)/CO(7-6), and the [CI] (2-1)/(1-0) flux ratios are also correlated, and trace the excitation of the molecular gas. In the most luminous infrared galaxies (LIRGs), the ISM is fully dominated by dense and moderately warm gas clouds that appear to have low [C]/[CO] and [13CO]/[12CO] abundances. In less luminous galaxies, emission from gas clouds at lower densities becomes progressively more important, and a multiple-phase analysis is required to determine consistent physical characteristics. Neither the CO nor the [CI] velocity-integrated line fluxes are good predictors of H2 column densities in individual galaxies, and X(CI) conversion factors are not superior to X(CO) factors. The methods and diagnostic diagrams outlined in this paper also provide a new and relatively straightforward means of deriving the physical characteristics of molecular gas in high-redshift galaxies up to z=5, which are otherwise hard to determine

The excitation of near-infrared H2 emission in NGC 253

Because of its large angular size and proximity to the Milky Way, NGC 253, an archetypal starburst galaxy, provides an excellent laboratory to study the intricacies of this intense episode of star formation. We aim to characterize the excitation mechanisms driving the emission in NGC 253. Specifically we aim to distinguish between shock excitation and UV excitation as the dominant driving mechanism, using Br\gamma, H_2 and [FeII] as diagnostic emission line tracers. Using SINFONI observations, we create linemaps of Br\gamma, [FeII]_{1.64}, and all detected H_2 transitions. By using symmetry arguments of the gas and stellar gas velocity field, we find a kinematic center in agreement with previous determinations. The ratio of the 2-1 S(1) to 1-0 S(1) H_2 transitions can be used as a diagnostic to discriminate between shock and fluorescent excitation. Using the 1-0 S(1)/2-1 S(1) line ratio as well as several other H_2 line ratios and the morphological comparison between H_2 and Br\gamma and [FeII], we find that excitation from UV photons is the dominant excitation mechanisms throughout NGC 253. We employ a diagnostic energy level diagram to quantitatively differentiate between mechanisms. We compare the observed energy level diagrams to PDR and shock models and find that in most regions and over the galaxy as a whole, fluorescent excitation is the dominant mechanism exciting the H_2 gas. We also place an upper limit of the percentage of shock excited H_2 at 29%. We find that UV radiation is the dominant excitation mechanism for the H_2 emission. The H_2 emission does not correlate well with Br\gamma but closely traces the PAH emission, showing that not only is H_2 fluorescently excited, but it is predominately excited by slightly lower mass stars than O stars which excite Br\gamma, such as B stars

Morita Equivalence of Noncommutative Supertori

In this paper we study the extension of Morita equivalence of noncommutative tori to the supersymmetric case. The structure of the symmetry group yielding Morita equivalence appears to be intact but its parameter field becomes supersymmetrized having both body and soul parts. Our result is mainly in the two dimensional case in which noncommutative supertori have been constructed recently: The group $SO(2,2,V_{\Z}^0)$, where $V_{\Z}^0$ denotes Grassmann even number whose body part belongs to ${\Z}$, yields Morita equivalent noncommutative supertori in two dimensions.Comment: LaTeX 18 pages, the version appeared in JM

Finite energy shifts in SU(n) supersymmetric Yang-Mills theory on T^3xR at weak coupling

We consider a semi-classical treatment, in the regime of weak gauge coupling, of supersymmetric Yang-Mills theory in a space-time of the form T^3xR with SU(n)/Z_n gauge group and a non-trivial gauge bundle. More specifically, we consider the theories obtained as power series expansions around a certain class of normalizable vacua of the classical theory, corresponding to isolated points in the moduli space of flat connections, and the perturbative corrections to the free energy eigenstates and eigenvalues in the weakly interacting theory. The perturbation theory construction of the interacting Hilbert space is complicated by the divergence of the norm of the interacting states. Consequently, the free and interacting Hilbert furnish unitarily inequivalent representation of the algebra of creation and annihilation operators of the quantum theory. We discuss a consistent redefinition of the Hilbert space norm to obtain the interacting Hilbert space and the properties of the interacting representation. In particular, we consider the lowest non-vanishing corrections to the free energy spectrum and discuss the crucial importance of supersymmetry for these corrections to be finite.Comment: 31 pages, 1 figure, v4 Minor changes, references correcte