Finger Search in Grammar-Compressed Strings

Grammar-based compression, where one replaces a long string by a small context-free grammar that generates the string, is a simple and powerful paradigm that captures many popular compression schemes. Given a grammar, the random access problem is to compactly represent the grammar while supporting random access, that is, given a position in the original uncompressed string report the character at that position. In this paper we study the random access problem with the finger search property, that is, the time for a random access query should depend on the distance between a specified index $f$, called the \emph{finger}, and the query index $i$. We consider both a static variant, where we first place a finger and subsequently access indices near the finger efficiently, and a dynamic variant where also moving the finger such that the time depends on the distance moved is supported. Let $n$ be the size the grammar, and let $N$ be the size of the string. For the static variant we give a linear space representation that supports placing the finger in $O(\log N)$ time and subsequently accessing in $O(\log D)$ time, where $D$ is the distance between the finger and the accessed index. For the dynamic variant we give a linear space representation that supports placing the finger in $O(\log N)$ time and accessing and moving the finger in $O(\log D + \log \log N)$ time. Compared to the best linear space solution to random access, we improve a $O(\log N)$ query bound to $O(\log D)$ for the static variant and to $O(\log D + \log \log N)$ for the dynamic variant, while maintaining linear space. As an application of our results we obtain an improved solution to the longest common extension problem in grammar compressed strings. To obtain our results, we introduce several new techniques of independent interest, including a novel van Emde Boas style decomposition of grammars

OmniFold: A Method to Simultaneously Unfold All Observables

Collider data must be corrected for detector effects ("unfolded") to be compared with many theoretical calculations and measurements from other experiments. Unfolding is traditionally done for individual, binned observables without including all information relevant for characterizing the detector response. We introduce OmniFold, an unfolding method that iteratively reweights a simulated dataset, using machine learning to capitalize on all available information. Our approach is unbinned, works for arbitrarily high-dimensional data, and naturally incorporates information from the full phase space. We illustrate this technique on a realistic jet substructure example from the Large Hadron Collider and compare it to standard binned unfolding methods. This new paradigm enables the simultaneous measurement of all observables, including those not yet invented at the time of the analysis.Comment: 8 pages, 3 figures, 1 table, 1 poem; v2: updated to approximate PRL versio

Testing of minerals and industrial by-products as oxygen carriers for chemical-looping combustion in a circulating fluidized-bed 300W laboratory reactor

Chemical-looping combustion (CLC) is a promising technology for future energy production with inherent CO2 separation. One approach is to use minerals or industrial by-products as oxygen carriers to reduce the costs of the process. This study focuses on the investigation of two iron-based oxygen carriers, which were examined under continuous operation in a 300 W laboratory reactor. Ilmenite is an iron–titanium oxide mineral, whereas iron oxide scale (IOS) is obtained as a by-product from the rolling of sheet steel. Syngas was used as a fuel – pure and with steam addition to suppress the formation of solid carbon. During the experiments the variables reactor temperature, fuel flow and air flow were changed. Furthermore the effect of steam addition to the fuel was investigated. Particle properties were compared over the span of 85 h of continuous operation for ilmenite and 37 h for IOS. The analysis is based on gas measurements from the actual CLC operation, but also on scanning electron microscopy, X-ray powder diffractometry and measurements of BET surface area and density. With ilmenite oxygen carrier it was possible to achieve full conversion of syngas up to about 190 Wth fuel equivalent at 900 °C. With design fuel flow of about 300 Wth at 900 °C the combustion efficiency was above 98%. There was almost no visible difference in reactivity of fresh activated particles and those used for 85 h. Combustion efficiency up to 99% was achieved with IOS oxygen carrier at 900 °C and about 100 Wth fuel equivalent. At 300 Wth fuel equivalent and 900 °C a combustion efficiency of only 90% could be reached. Both oxygen carriers were operated for tens of hours, which allowed for a better understanding of lifetime behavior and other basic characteristics. Whereas ilmenite oxygen-carrier particles were mostly stable over the course of 85 h of experiments, a large fraction of IOS oxygen-carrier particles had disintegrated to fines after only 37 h of experiments. The gathered data indicates that both oxygen carriers could be an alternative to synthesized particles, though with more drawbacks for IOS than for ilmenite

Slow dynamics of a colloidal lamellar phase

8 pagesInternational audienceWe used x-ray photon correlation spectroscopy to study the dynamics in the lamellar phase of a platelet suspension as a function of the particle concentration. We measured the collective diffusion coefficient along the director of the phase, over length scales down to the interparticle distance, and quantified the hydrodynamic interaction between the particles. This interaction sets in with increasing concentration and can be described qualitatively by a simplified model. No change in the microscopic structure or dynamics is observed at the transition between the fluid and the gel-like lamellar phases

Homeschooling during lockdown deepens inequality

The inequalities arising from homeschooling during lockdown will exacerbate existing inequalities in education, write Jake Anders (UCL), Lindsey Macmillan (UCL), Patrick Sturgis (LSE), and Gill Wyness (UCL). Their study uncovers deep divisions along the lines of gender and class when it comes to supporting children’s education during the COVID–19 lockdown

Inequalities in late adolescents’ educational experiences and wellbeing during the Covid-19 pandemic

While the health risks of Covid-19 for young people are low, they have borne a heavy cost of the pandemic through intense disruption to their education and social lives. These effects have not been experienced equally across social and demographic groups. Using data from a nationally representative survey of 4,000 young people in England linked to their education records, we study inequalities in late adolescents’ experiences of the Covid-19 pandemic. We find particularly stark inequalities by socio-economic status, with those from poorer families facing disadvantage on multiple fronts, particularly in their experiences of home learning, returning to school, and exam cancellations compared to their more advantaged peers. Gender and ethnic inequalities were more mixed, though young females reported significantly lower wellbeing scores than males. This evidence suggests that the pandemic has exacerbated existing inequalities, meaning policymakers concerned with increasing equity and social mobility now face an even bigger task than before