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This paper addresses the problem of estimating the size of a deep web data source that is accessible by queries only. Since most deep web data sources are noncooperative, a data source size can only be estimated by sending queries and analyzing the returning results. We propose an efficient estimator based on the capture-recapture method. First we derive an equation between the overlapping rate and the percentage of the data examined when random samples are retrieved from a uniform distribution. This equation is conceptually simple and leads to the derivation of an estimator for samples obtained by random queries. Since random queries do not produce random documents, it is well known that the traditional methods by random queries underestimate the size, i.e., those estimators have negative bias. Based on the simple estimator for random samples, we adjust the equation so that it can handle the samples returned by random queries. We conduct both simulation studies and experiments on corpora including Gov2, Reuters, Newsgroups, and Wikipedia. The results show that our method has small bias and standard deviation.

As an emerging frontier, dual-atom catalysts (DACs) have sparked broad interest in energy catalysis, however the undesired thermal atomic migration during synthesis process pose significant challenge in enabling further applications. Herein, an interfacial cladding strategy is reported to construct monodispersed dual-atom metal sites (metal = Fe, Cu, or Ir), derived from metal dimer molecule functionalized metal-organic frameworks. First, metal dimer molecule is immobilized at the surface of cubic ZIF-8 by the interfacial cladding of polydopamine, thus preventing the potentially thermal migration of metal atoms during pyrolysis. Then, the paired metal atoms are anchored onto a hollow carbon nanocage and achieve nitrogen coordinated dual-atom metal sites after annealing at 900 °C. Representatively, the resultant dual Fe catalysts exhibit remarkable activity for electrocatalytic oxygen reduction reaction with half-wave potential of 0.951 and 0.816 V in alkaline and acidic media, respectively. The findings open up an avenue for the rational design of dual-atom catalysts.

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Background Zygophyllum xanthoxylum is a succulent xerophyte with remarkable tolerance to diverse abiotic stresses. Previous studies have revealed important physiological mechanisms and identified functional genes associated with stress tolerance. However, knowledge of the regulatory genes conferring stress tolerance in this species is poorly understood. Results Here, we present a comprehensive analysis of regulatory genes based on the transcriptome of Z. xanthoxylum roots exposed to osmotic stress and salt treatments. Significant changes were observed in transcripts related to known and obscure stress-related hormone signaling pathways, in particular abscisic acid and auxin. Significant changes were also found among key classes of early response regulatory genes encoding protein kinases, transcription factors, and ubiquitin-mediated proteolysis machinery. Network analysis shows a highly integrated matrix formed by these conserved and novel gene products associated with osmotic stress and salt in Z. xanthoxylum . Among them, two previously uncharacterized NAC (NAM/ATAF/CUC) transcription factor genes, ZxNAC083 (Unigene16368_All) and ZxNAC035 (CL6534.Contig1_All), conferred tolerance to salt and drought stress when constitutively overexpressed in Arabidopsis plants. Conclusions This study provides a unique framework for understanding osmotic stress and salt adaptation in Z. xanthoxylum including novel gene targets for engineering stress tolerance in susceptible crop species. Electronic supplementary material The online version of this article (10.1186/s12870-019-1686-1) contains supplementary material, which is available to authorized users.