Julia: A Fresh Approach to Numerical Computing

Bridging cultures that have often been distant, Julia combines expertise from the diverse fields of computer science and computational science to create a new approach to numerical computing. Julia is designed to be easy and fast. Julia questions notions generally held as "laws of nature" by practitioners of numerical computing: 1. High-level dynamic programs have to be slow. 2. One must prototype in one language and then rewrite in another language for speed or deployment, and 3. There are parts of a system for the programmer, and other parts best left untouched as they are built by the experts. We introduce the Julia programming language and its design --- a dance between specialization and abstraction. Specialization allows for custom treatment. Multiple dispatch, a technique from computer science, picks the right algorithm for the right circumstance. Abstraction, what good computation is really about, recognizes what remains the same after differences are stripped away. Abstractions in mathematics are captured as code through another technique from computer science, generic programming. Julia shows that one can have machine performance without sacrificing human convenience.Comment: 37 page

Array operators using multiple dispatch: a design methodology for array implementations in dynamic languages

Arrays are such a rich and fundamental data type that they tend to be built into a language, either in the compiler or in a large low-level library. Defining this functionality at the user level instead provides greater flexibility for application domains not envisioned by the language designer. Only a few languages, such as C++ and Haskell, provide the necessary power to define $n$-dimensional arrays, but these systems rely on compile-time abstraction, sacrificing some flexibility. In contrast, dynamic languages make it straightforward for the user to define any behavior they might want, but at the possible expense of performance. As part of the Julia language project, we have developed an approach that yields a novel trade-off between flexibility and compile-time analysis. The core abstraction we use is multiple dispatch. We have come to believe that while multiple dispatch has not been especially popular in most kinds of programming, technical computing is its killer application. By expressing key functions such as array indexing using multi-method signatures, a surprising range of behaviors can be obtained, in a way that is both relatively easy to write and amenable to compiler analysis. The compact factoring of concerns provided by these methods makes it easier for user-defined types to behave consistently with types in the standard library.Comment: 6 pages, 2 figures, workshop paper for the ARRAY '14 workshop, June 11, 2014, Edinburgh, United Kingdo

What matters to African firms ? the relevance of perceptions data

Can perceptions data help us understand investment climate constraints facing the private sector? Or do firms simply complain about everything? In this paper, the authors provide a picture of how firms'views on constraints differ across countries in Sub-Saharan Africa. Using the World Bank's Enterprise Surveys database, they find that reported constraints reflect country characteristics and vary systematically by level of income-the most elemental constraints to doing business (power, access to finance, ability to plan ahead) appear to be most binding at low levels of income. As countries develop and these elemental constraints are relaxed, governance-related constraints become more problematic. As countries move further up the income scale and the state becomes more capable, labor regulation is perceived to be more of a problem-business is just one among several important constituencies. The authors also consider whether firm-level characteristics-such as size, ownership, exporter status, and firms'own experience-affect firms'views on the severity of constraints. They find that, net of country and sector fixed effects and firm characteristics, firms'views do reflect their experience as evidenced by responses to other questions in surveys. The results suggest that there are both country-level and firm-level variations in the investment climate. Turning to the concept of"binding constraints,"the Enterprise Surveys do not generally suggest one single binding constraint facing firms in difficult business climates. However, there do appear to be groups of constraints that matter more at different income levels, with a few elemental constraints being especially important at low levels and a few regulatory constraints at high levels, but a difficult range of governance-related constraints at intermediate levels. Adjusting to a constraint does not mean that firms then do not recognize it-for example, generator-owning firms are not distinguishable from other firms when ranking electricity as a constraint. Overall, firms do appear to discriminate between constraints in a reasonable way. Their views can provide a useful first step in the business-government consultative process and help in prioritizing more specific behavioral analysis and policy reforms.,Emerging Markets,Microfinance,Governance Indicators,Access to Finance

To Formalize or Not to Formalize? Comparisons of Microenterprise Data from Southern and East Africa

Why do firms choose to locate in the informal sector? Researchers often argue that the high cost of regulation prevents informal firms from becoming formal and productive. Our results point to a more nuanced story. Using data from surveys of microenterprises in South Africa, Namibia, Botswana, Kenya, Uganda, Tanzania, and Rwanda, we find that the labor productivity of informal firms is virtually indistinguishable from that of formal firms in East Africa, but very different in Southern Africa. We provide a theoretical model to explain this result, based on the key assumption that firms may evade taxes subject to a cost (or concealment cost) that is increasing and convex in the firm’s employment size. Consequently, the productivity distributions reflect the differences in concealment costs and the opportunity cost of formality. Greater enforcement of laws and better provision of services such as finance and electricity to formally registered firms in Southern Africa means that firms are more likely to register; those that do not are likely to be operating as “survivalist” firms. But in East Africa, weak enforcement of tax payment and no significant difference in access to services between formal and informal firms means that these variables do not explain the allocation of firms across the informal-formal divide. We conclude that in countries with weak business environments, informal firms are just as likely as formal firms to increase their productivity as they grow. Thus, interventions to increase productivity and lower the cost of formality may be helpful. But in countries with strong business environments such as those in Southern Africa, owners of informal firms are likely to be better off entering the labor market as wage labor. In the latter case, investment in education or vocational training is probably more important.microenterprise; informal sector; Southern Africa; East Africa; formalization; regulations

REST/NRSF Knockdown Alters Survival, Lineage Differentiation and Signaling in Human Embryonic Stem Cells.

REST (RE1 silencing transcription factor), also known as NRSF (neuron-restrictive silencer factor), is a well-known transcriptional repressor of neural genes in non-neural tissues and stem cells. Dysregulation of REST activity is thought to play a role in diverse diseases including epilepsy, cancer, Down's syndrome and Huntington's disease. The role of REST/NRSF in control of human embryonic stem cell (hESC) fate has never been examined. To evaluate the role of REST in hESCs we developed an inducible REST knockdown system and examined both growth and differentiation over short and long term culture. Interestingly, we have found that altering REST levels in multiple hESC lines does not result in loss of self-renewal but instead leads to increased survival. During differentiation, REST knockdown resulted in increased MAPK/ERK and WNT signaling and increased expression of mesendoderm differentiation markers. Therefore we have uncovered a new role for REST in regulation of growth and early differentiation decisions in human embryonic stem cells

Message-passing for Maximum Weight Independent Set

We investigate the use of message-passing algorithms for the problem of finding the max-weight independent set (MWIS) in a graph. First, we study the performance of the classical loopy max-product belief propagation. We show that each fixed point estimate of max-product can be mapped in a natural way to an extreme point of the LP polytope associated with the MWIS problem. However, this extreme point may not be the one that maximizes the value of node weights; the particular extreme point at final convergence depends on the initialization of max-product. We then show that if max-product is started from the natural initialization of uninformative messages, it always solves the correct LP -- if it converges. This result is obtained via a direct analysis of the iterative algorithm, and cannot be obtained by looking only at fixed points. The tightness of the LP relaxation is thus necessary for max-product optimality, but it is not sufficient. Motivated by this observation, we show that a simple modification of max-product becomes gradient descent on (a convexified version of) the dual of the LP, and converges to the dual optimum. We also develop a message-passing algorithm that recovers the primal MWIS solution from the output of the descent algorithm. We show that the MWIS estimate obtained using these two algorithms in conjunction is correct when the graph is bipartite and the MWIS is unique. Finally, we show that any problem of MAP estimation for probability distributions over finite domains can be reduced to an MWIS problem. We believe this reduction will yield new insights and algorithms for MAP estimation.Comment: Submitted to IEEE Transactions on Information Theor

Response to Dr. Murphy and Dr. Ferguson regarding comment on our paper: retrospective evaluation of antimicrobial prophylaxis in prevention of surgical site infection in the pediatric population

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/110572/1/pan12598.pd

Alteplase Used in a Child with an Acute Ischemic Stroke

Stroke, also known as cerebrovascular accident (CVA), is a neurological injury caused by inadequate brain perfusion due to either ischemia or hemorrhage. There is an abundance of literature on the management of ischemic strokes in adults and this has led to well-defined diagnostic and treatment guidelines. However, the management of pediatric ischemic strokes is currently based on clinical experience of experts, recommendations of consensus guidelines, case studies and extrapolation from adult clinical trials. In this case report, a pediatric patient suffering from an ischemic stroke is successfully treated with IV alteplase (tPA)