Showing papers by "Academia Sinica published in 2000"
TL;DR: It is proved that the most expressive vectors derived in the null space of the within-class scatter matrix using principal component analysis (PCA) are equal to the optimal discriminant vectorsderived in the original space using LDA.
1,447 citations
TL;DR: In this paper, the spatial and temporal distribution of summer monsoon precipitation during the Holocene has been reconstructed on the basis of geological data, including lake levels, pollen profiles, and loess/paleosol records.
857 citations
TL;DR: It is demonstrated that SMN2 can partially compensate for lack of SMN1, and the variable phenotypes of Smn−/−SMN2 mice reflect those seen in SMA patients, providing a mouse model for this disease.
Abstract: The survival motor neuron gene is present in humans in a telomeric copy, SMN1, and several centromeric copies, SMN2. Homozygous mutation of SMN1 is associated with proximal spinal muscular atrophy (SMA), a severe motor neuron disease characterized by early childhood onset of progressive muscle weakness. To understand the functional role of SMN1 in SMA, we produced mouse lines deficient for mouse Smn and transgenic mouse lines that expressed human SMN2. Smn-/- mice died during the peri-implantation stage. In contrast, transgenic mice harbouring SMN2 in the Smn-/- background showed pathological changes in the spinal cord and skeletal muscles similar to those of SMA patients. The severity of the pathological changes in these mice correlated with the amount of SMN protein that contained the region encoded by exon 7. Our results demonstrate that SMN2 can partially compensate for lack of SMN1. The variable phenotypes of Smn-/-SMN2 mice reflect those seen in SMA patients, providing a mouse model for this disease.
677 citations
TL;DR: A statistical analysis of the occurrences of 162 genera and 333 species confirms a sudden extinction event at 251.4 million years ago, coincident with a dramatic depletion of delta13C(carbonate) and an increase in microspherules.
Abstract: The Meishan section across the Permian-Triassic boundary in South China is the most thoroughly investigated in the world. A statistical analysis of the occurrences of 162 genera and 333 species confirms a sudden extinction event at 251.4 million years ago, coincident with a dramatic depletion of δ13Ccarbonate and an increase in microspherules.
586 citations
TL;DR: In this paper, the existence of weak solutions to the Cauchy problem for a one-dimensional shallow-water equation that is formally integrable and can be obtained by approximating directly the Hamiltonian for Euler's equation in the shallowwater regime was obtained.
Abstract: We obtain the existence of global-in-time weak solutions to the Cauchy problem for a one-dimensional shallow-water equation that is formally integrable and can be obtained by approximating directly the Hamiltonian for Euler's equation in the shallow-water regime. The solution is obtained as a limit of viscous approximation. The key elements in our analysis are some new a priori one-sided supernorm and space-time higher-norm estimates on the first-order derivatives. © 2000 John Wiley & Sons, Inc.
522 citations
TL;DR: Evidence support that Pik3CA is an oncogene in cervical cancer and PIK3CA amplification may be linked to cervical tumorigenesis.
Abstract: Amplification of chromosome arm 3q is the most consistent aberration in cervical cancer, and is implicated in the progression of dysplastic uterine cervical cells into invasive cancer. The present study employed the ‘positional candidate gene’ strategy to determine the contribution of PIK3CA, which is located in 3q26.3, in cervical tumorigenesis. PIK3CA is known to be involved in the PI 3-kinase/AKT signaling pathway, which plays an important role in regulating cell growth and apoptosis. The results of comparative genomic hybridization show that the 3q26.3 amplification was the most consistent chromosomal aberration in primary tissues of cervical carcinoma, and a positive correlation between an increased copy number of PIK3CA (detected by competitive PCR) and 3q26.3 amplification was found in tumor tissues and in cervical cancer cell lines. In cervical cancer cell lines harboring amplified PIK3CA, the expression of gene product (p110α) of PIK3CA was increased, and was subsequently associated with high kinase activity. In addition, transformation phenotypes in these lines, including increased cell growth and decreased apoptosis, were found to be significantly affected by the treatment of specific PI 3-kinase inhibitor, suggesting that increased expression of PIK3CA in cervical cancer may result in promoting cell proliferation and reducing apoptosis. These evidences support that PIK3CA is an oncogene in cervical cancer and PIK3CA amplification may be linked to cervical tumorigenesis.
443 citations
TL;DR: Effective intervention and management for loss event and major depressive episode among emotionally unstable subjects with a family tendency of suicidal behaviour, frequently also comorbid with alcohol or other substance dependence, may prove to be most effective for suicide prevention in different populations.
Abstract: Background Few studies of suicide have simultaneously examined the individual and combined effects of psychosocial and psychiatric risk factors.
Aims To do so in a representative sample of suicides.
Method A case—control psychological autopsy was conducted among 113 consecutive suicides and 226 living controls matched for age, gender, ethnicity and area of residence in Taiwan.
Results Five major risk factors (loss event, suicidal behaviour in first-degree relatives, ICD-10 major depressive episode, emotionally unstable personality disorder and substance dependence) were found to have independent effects on suicide from multivariate conditional logistic regression analysis.
Conclusions Effective intervention and management for loss event and major depressive episode among emotionally unstable subjects with a family tendency of suicidal behaviour, frequently also comorbid with alcohol or other substance dependence, may prove to be most effective for suicide prevention in different populations.
436 citations
TL;DR: A novel comparative proteomic approach for assembling human gene contigs and assisting gene discovery in Caenorhabditis elegans was presented, and over 150 putative full-length human gene transcripts were assembled upon further database analyses.
Abstract: Modern biomedical research greatly benefits from large-scale genome-sequencing projects ranging from studies of viruses, bacteria, and yeast to multicellular organisms, like Caenorhabditis elegans. Comparative genomic studies offer a vast array of prospects for identification and functional annotation of human ortholog genes. We presented a novel comparative proteomic approach for assembling human gene contigs and assisting gene discovery. The C. elegans proteome was used as an alignment template to assist in novel human gene identification from human EST nucleotide databases. Among the available 18,452 C. elegans protein sequences, our results indicate that at least 83% (15,344 sequences) of C. elegans proteome has human homologous genes, with 7,954 records of C. elegans proteins matching known human gene transcripts. Only 11% or less of C. elegans proteome contains nematode-specific genes. We found that the remaining 7,390 sequences might lead to discoveries of novel human genes, and over 150 putative full-length human gene transcripts were assembled upon further database analyses. [The sequence data described in this paper have been submitted to the
433 citations
TL;DR: The Gnetales may be viewed as extremely divergent conifers, and the many morphological similarities between angiosperms and G netales arose independently.
Abstract: Phylogenetic relationships among the five groups of extant seed plants are presently quite unclear. For example, morphological studies consistently identify the Gnetales as the extant sister group to angiosperms (the so-called “anthophyte” hypothesis), whereas a number of molecular studies recover gymnosperm monophyly, and few agree with the morphology-based placement of Gnetales. To better resolve these and other unsettled issues, we have generated a new molecular data set of mitochondrial small subunit rRNA sequences, and have analyzed these data together with comparable data sets for the nuclear small subunit rRNA gene and the chloroplast rbcL gene. All nuclear analyses strongly ally Gnetales with a monophyletic conifers, whereas all mitochondrial analyses and those chloroplast analyses that take into account saturation of third-codon position transitions actually place Gnetales within conifers, as the sister group to the Pinaceae. Combined analyses of all three genes strongly support this latter relationship, which to our knowledge has never been suggested before. The combined analyses also strongly support monophyly of extant gymnosperms, with cycads identified as the basal-most group of gymnosperms, Ginkgo as the next basal, and all conifers except for Pinaceae as sister to the Gnetales + Pinaceae clade. According to these findings, the Gnetales may be viewed as extremely divergent conifers, and the many morphological similarities between angiosperms and Gnetales (e.g., double fertilization and flower-like reproductive structures) arose independently.
431 citations
TL;DR: The analysis suggests that the current standard for arsenic in water of 50 microg/L is associated with a substantial increased risk of cancer and is not sufficiently protective of public health.
Abstract: The U.S. Environmental Protection Agency is under a congressional mandate to revise its current standard for arsenic in drinking water. We present a risk assessment for cancers of the bladder, liver, and lung from exposure to arsenic in water, based on data from 42 villages in an arseniasis-endemic region of Taiwan. We calculate excess lifetime risk estimates for several variations of the generalized linear model and for the multistage-Weibull model. Risk estimates are sensitive to the model choice, to whether or not a comparison population is used to define the unexposed disease mortality rates, and to whether the comparison population is all of Taiwan or just the southwestern region. Some factors that may affect risk could not be evaluated quantitatively: the ecologic nature of the data, the nutritional status of the study population, and the dietary intake of arsenic. Despite all of these sources of uncertainty, however, our analysis suggests that the current standard of 50 microg/L is associated with a substantial increased risk of cancer and is not sufficiently protective of public health.
405 citations
TL;DR: In this article, a review of the PDP phosphor activator was presented, and improvements of the phosphor for practicality were considered. And the correlation of luminescence characteristics to the host crystal structure and activator activator activation was explained.
TL;DR: In this article, a 500km-long belt of metamorphic exposures in the Qiangtang block provides an opportunity to study the internal structure of northern Tibetan crust, and it is shown that the deeper crust of much of northern Tibet consists of accretionary melange, in contrast to the continental crystalline crust of southern Tibet.
Abstract: A 500-km-long belt of metamorphic exposures in the Qiangtang block provides an opportunity to study the internal structure of northern Tibetan crust. Metamorphic rocks exposed at two widely separated areas along this belt consist of blueschist-bearing melange and are bounded by Late Triassic–Early Jurassic, domal, low-angle normal faults. We propose that this melange was underplated to the Qiangtang block and was subsequently exhumed by detachment faulting; both the underplating and the exhumation occurred during early Mesozoic southward subduction of oceanic lithosphere along the Jinsha suture. This model predicts that the deeper crust of much of northern Tibet consists of accretionary melange, in contrast to the continental crystalline crust of southern Tibet, and may account for north-south variations of Cenozoic tectonism in Tibet.
TL;DR: The result that arsenite at low concentrations increases oxidant levels and causes oxidative DNA damage in vascular smooth muscle cells may be important in arsenic-induced atherosclerosis.
Abstract: Arsenic is atherogenic, carcinogenic, and genotoxic. Because atherosclerotic plaque has been considered a benign smooth muscle cell tumor, we have studied the effects of arsenite on DNA integrity of human vascular smooth muscle cells. By using single-cell alkaline electrophoresis, apparent DNA strand breaks were detected in a 4-hour treatment with arsenite at a concentration above 1 micromol/L. DNA strand breaks of arsenite-treated cells were increased by Escherichia coli formamidopyrimidine-DNA glycosylase and decreased by diphenylene iodinium, superoxide dismutase, catalase, pyruvate, DMSO, or D-mannitol. Extract from arsenite-treated cells showed increased capacity for producing superoxide when NADH was included in the reaction mixture; however, addition of arsenite to extract from untreated cells did not increase superoxide production. The superoxide-producing ability of arsenite-treated cells was also suppressed by diphenylene iodinium, 4,5-dihydroxy-1, 2-benzenedisulfonic acid disodium salt (Tiron), or superoxide dismutase. Superoxide production and DNA strand breaks in arsenite-treated cells were also suppressed by transfecting antisense oligonucleotides of p22phox, an essential component of NADH oxidase. Treatment with arsenite also increased the mRNA level of p22phox. These results suggest that arsenite activates NADH oxidase to produce superoxide, which then causes oxidative DNA damage. The result that arsenite at low concentrations increases oxidant levels and causes oxidative DNA damage in vascular smooth muscle cells may be important in arsenic-induced atherosclerosis.
TL;DR: Based on micro-meteorological field measurement of NH3 volatilization from agricultural fields in different regions and under different cropping systems, the total NH3 VOLUME 7, 1990 emissions from China in 1990 were calculated to be 1.282 Tg N as discussed by the authors.
Abstract: Using the 1997 IPCC Guidelines for National Greenhouse Gas Inventory Methodology, and statistical data from the China Agricultural Yearbook, we estimated that the direct N2O emission from agricultural fields in China in 1990 was 0.282 Tg N. Based on micro-meteorological field measurement of NH3 volatilization from agricultural fields in different regions and under different cropping systems, the total NH3 volatilization from agricultural fields in China in 1990 was calculated to be 1.80 Tg N, which accounted for 11% of the applied synthetic fertilizer N. Ammonia volatilization from agricultural soil was related to the cropping system and the form of N fertilizer. Ammonia volatilization from paddy fields was higher than that from uplands, and NH4HCO3 had a higher potential of NH3 volatilization than urea. N loss through leaching from uplands in north China accounted for 0.5–4.2% of the applied synthetic fertilizer N. In south China, the leaching of applied N and soil N from paddy fields ranged from 6.75 to 27.0 kg N ha-1 yr-1, while N runoff was between 2.45 and 19.0 kg N ha-1 yr-1.
TL;DR: The lepton kinetic energy ranges from 0.2 to 40 GeV for e − and from 3 GeV to 6 GeVs for e + were measured by the Alpha Magnetic Spectrometer (AMS) during space shuttle flight STS-91 at altitudes near 380 km.
TL;DR: Melibiose, lactose, d-Galactose, alpha-methyl-d-galactopyranoside, N-acetylneuraminic acid, raffinose, and inulin were capable of inhibiting its hemagglutinating activity, with melibiose being the most potent.
TL;DR: A detailed analysis of the quantum dynamics establishes that a reactive resonance localized in the transition-state region is responsible for these remarkable observations in the F+HD reaction.
Abstract: Conclusive evidence is presented for the existence of a reactive resonance in the $\mathrm{F}+\mathrm{HD}$ reaction. In a molecular beam experiment, the resonance appears in the integral cross section as a distinct steplike feature, while in the differential cross section it is manifested as sharply varying forward-backward peaks in the product distribution. A detailed analysis of the quantum dynamics establishes that a reactive resonance localized in the transition-state region is responsible for these remarkable observations. At collision energies below $1\mathrm{kcal}/\mathrm{mol}$, the reaction proceeds almost exclusively through resonant tunneling with very little contribution from the more conventional direct mechanism.
TL;DR: The Steptoean positive carbon isotope excursion (SPICE) as discussed by the authors is a global oceanographic event that confirms intercontinental correlations between different biogeographic realms based on agnostids and other blue-water trilobites.
TL;DR: In this article, the Raman spectra of several compositions of (Mg, Fe, Ca)SiO3 pyroxenes were collected at ambient conditions and more than 10 Raman vibrational modes were observed for these pyroxene in the wavenumber range between 200 and 1200 cm−1.
Abstract: Raman spectra of several compositions of (Mg, Fe, Ca)SiO3 pyroxenes were collected at ambient conditions. More than 10 Raman vibrational modes were observed for these pyroxenes in the wavenumber range between 200 and 1200 cm−1. In general, these pyroxenes are characterized by (1) the Si-O stretching modes above 800 cm−1; (2) the Si-O bending modes between 500 and 760 cm−1; (3) SiO4 rotation and metal-oxygen translation modes below 500 cm−1. For a constant Ca content, frequencies of the Raman modes in the enstatite-ferrosilite (opx) and diopside-hedenbergite (cpx) series generally decrease with an increase in Fe content. This phenomenon is attributed to an increase in both the bonding lengths and the reduced mass as Fe2+ is substituted for Mg. However, two modes at ~900 cm−1 in the enstatite-ferrosilite series increase in frequencies as Fe content increases. A possible explanation is to the shortening in the Si-O-Si bridging bonding bonds when the M2 sites are preferentially occupied by the iron cation. The effect of Fe substituting for Mg on the frequency shift in cpx is less profound than opx because the larger M2 was occupied by calcium and the substitution of iron and magnesium in the M1 site results in a less significant change in the bond length. The major-element composition of the (Mg, Fe, Ca)-pyroxenes, especially the orthopyroxene series, can be semi-quantitatively determined on the basis of the peak positions of their characteristic Raman modes.
TL;DR: From extensive experiments, results indicate that the proposed cocktail watermarking scheme is remarkably effective in resisting various attacks, including combined ones.
Abstract: A novel image protection scheme called "cocktail watermarking" is proposed in this paper. We analyze and point out the inadequacy of the modulation techniques commonly used in ordinary spread spectrum watermarking methods and the visual model-based ones. To resolve the inadequacy, two watermarks which play complementary roles are simultaneously embedded into a host image. We also conduct a statistical analysis to derive the lower bound of the worst likelihood that the better watermark (out of the two) can be extracted. With this "high" lower bound, it is ensured that a "better" extracted watermark is always obtained. From extensive experiments, results indicate that our cocktail watermarking scheme is remarkably effective in resisting various attacks, including combined ones.
TL;DR: Immunolocalization data support the role of the 9-kD protein in facilitating adhesion of pollen tubes to the stylar transmitting tract epidermis in species such as lily.
Abstract: Flowering plants possess specialized extracellular matrices in the female organs of the flower that support pollen tube growth and sperm cell transfer along the transmitting tract of the gynoecium. Transport of the pollen tube cell and the sperm cells involves a cell adhesion and migration event in species such as lily that possess a transmitting tract epidermis in the stigma, style, and ovary. A bioassay for adhesion was used to isolate from the lily stigma/stylar exudate the components that are responsible for in vivo pollen tube adhesion. At least two stylar components are necessary for adhesion: a large molecule and a small (9 kD) protein. In combination, the two molecules induced adhesion of pollen tubes to an artificial stylar matrix in vitro. The 9-kD protein was purified, and its corresponding cDNA was cloned. This molecule shares some similarity with plant lipid transfer proteins. Immunolocalization data support its role in facilitating adhesion of pollen tubes to the stylar transmitting tract epidermis.
TL;DR: A high pressure phase of titanium dioxide with an alpha-PbO(2)-type structure has been identified in garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany as discussed by the authors.
Abstract: A high-pressure phase of titanium dioxide (TiO(2)) with an alpha-PbO(2)-type structure has been identified in garnet of diamondiferous quartzofeldspathic rocks from the Saxonian Erzgebirge, Germany. Analytical electron microscopy indicates that this alpha-PbO(2)-type TiO(2) occurred as an epitaxial nanometer-thick slab between twinned rutile bicrystals. Given a V-shaped curve for the equilibrium phase boundary of alpha-PbO(2)-type TiO(2) to rutile, the stabilization pressure of alpha-PbO(2)-type TiO(2) should be 4 to 5 gigapascals at 900 degrees to 1000 degrees C. This suggests a burial of continental crustal rocks to depths of at least 130 kilometers. The alpha-PbO(2)-type TiO(2) may be a useful pressure and temperature indicator in the diamond stability field.
TL;DR: In this paper, the authors investigated the spatial association of these Dabieshan orthogneisses with undoubted UHP schists and eclogites through conditions of Pmax at ca. 690°C-715°C and 36 kbar to Tmax at 4 kbar.
TL;DR: It is shown that, for a fixed fraction of nonlocal couplings, synchronous chaos is always a stable attractor in the thermodynamic limit and it is pointed out that randomness helps synchronization.
Abstract: In certain physical situations, extensive interactions arise naturally in systems. We consider one such situation, namely, small-world couplings. We show that, for a fixed fraction of nonlocal couplings, synchronous chaos is always a stable attractor in the thermodynamic limit. We point out that randomness helps synchronization. We also show that there is a size dependent bifurcation in the collective behavior in such systems.
TL;DR: A number of new local and parallel discretization and adaptive nite element algorithms are proposed and analyzed in this paper for elliptic boundary value problems and the main idea is to use a coarse grid to approximate the low frequencies and then to correct the resulted residue (which contains mostly high frequencies) by some local/parallel procedures.
Abstract: A number of new local and parallel discretization and adaptive nite element algorithms are proposed and analyzed in this paper for elliptic boundary value problems. These algorithms are motivated by the observation that, for a solution to some elliptic problems, low frequency components can be approximated well by a relatively coarse grid and high frequency components can be computed on a ne grid by some local and parallel procedure. The theoretical tools for analyzing these methods are some local a priori and a posteriori estimates that are also obtained in this paper for nite element solutions on general shape-regular grids. Some numerical experiments are also presented to support the theory. In this paper, we will propose some new parallel techniques for nite element computation. These techniques are based on our understanding of the local and global properties of a nite element solution to some elliptic problems. Simply speaking, the global behavior of a solution is mostly governed by low frequency components while the local behavior is mostly governed by high frequency compo- nents. The main idea of our new algorithms is to use a coarse grid to approximate the low frequencies and then to use a ne grid to correct the resulted residue (which contains mostly high frequencies) by some local/parallel procedures. Let us now give a somewhat more detailed but informal (and hopefully infor- mative) description of the main ideas and results in this paper. We consider the following very simple model problem posed on a convex polygonal domain R 2 : ( u + bru = f; in ;
TL;DR: It is shown that the RS domain derived from a human papillomavirus E2 transcriptional activator can target a heterologous protein to the nucleus, as it does in many other SR proteins, but insufficient for localization in speckles, and a role of TRN-SR2 in the cellular trafficking of phosphorylated SR proteins is likely reflected.
TL;DR: The necessary and sufficient condition for the permanence of two species and the extinction of one species or two species is obtained and the optimal harvesting policy and the threshold of the harvesting for sustainable development are obtained.
Abstract: In this paper, we establish a mathematical model of two species with stage structure and the relation of predator-prey, to obtain the necessary and sufficient condition for the permanence of two species and the extinction of one species or two species. We also obtain the optimal harvesting policy and the threshold of the harvesting for sustainable development.
TL;DR: In this paper, the authors discuss the origin of galactic angular momentum and the statistics of the present-day spin distribution, and present the maximum likelihood shear inversion procedure, which involves a constrained linear minimization.
Abstract: We discuss the origin of galactic angular momentum and the statistics of the present-day spin distribution. It is expected that the galaxy spin axes are correlated with the intermediate principal axis of the gravitational shear tensor. This allows one to reconstruct the shear field and thereby the full gravitational potential from the observed galaxy spin fields. We use the direction of the angular momentum vector without any information of its magnitude, which requires a measurement of the position angle and inclination on the sky of each disk galaxy. We present the maximum likelihood shear inversion procedure, which involves a constrained linear minimization. The theory is tested against numerical simulations. We find the correlation strength of nonlinear structures with the initial shear field and show that accurate large-scale density reconstructions are possible at the expected noise level.
TL;DR: In this article, the authors present results from dc magnetic susceptibility, specific-heat data, inelastic neutron-scattering measurements, and crystal-field calculations that strongly suggest that the ions in the antiferromagnetic pyrochlore lattice of corner-sharing tetrahedra remain in a collective paramagnetic state down to 70 mK.
Abstract: In a recent paper [J. S. Gardner et al., Phys. Rev. Lett. 82, 1012 (1999)] it was found that the ${\mathrm{Tb}}^{3+}$ magnetic moments in the ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ antiferromagnetic pyrochlore lattice of corner-sharing tetrahedra remain in a collective paramagnetic state down to 70 mK. In this paper we present results from dc magnetic susceptibility, specific-heat data, inelastic neutron-scattering measurements, and crystal-field calculations that strongly suggest that (i) the ${\mathrm{Tb}}^{3+}$ ions in ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ possess a moment of approximatively $5{\ensuremath{\mu}}_{\mathrm{B}},$ and (ii) the ground state g-tensor is extremely anisotropic below a temperature of ${O(10}^{0}) \mathrm{K},$ with Ising-like ${\mathrm{Tb}}^{3+}$ magnetic moments confined to point along a local cubic $〈111〉$ diagonal (e.g., towards the middle of the tetrahedron). Such a very large easy-axis Ising-like anisotropy along a $〈111〉$ direction dramatically reduces the frustration otherwise present in a Heisenberg pyrochlore antiferromagnet. The results presented herein underpin the conceptual difficulty in understanding the microscopic mechanism(s) responsible for ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ failing to develop long-range order at a temperature of the order of the paramagnetic Curie-Weiss temperature ${\ensuremath{\theta}}_{\mathrm{CW}}\ensuremath{\approx}\ensuremath{-}{10}^{1} \mathrm{K}.$ We suggest that dipolar interactions and extra perturbative exchange coupling(s) beyond nearest neighbors may be responsible for the lack of ordering of ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}.$
TL;DR: A posteriori error estimators of residual type are derived for piecewise linear finite element approximations to elliptic obstacle problems with instrumental ingredient an instrumental ingredient which requires minimal regularity, exhibits optimal approximation properties and preserves positivity.
Abstract: A posteriori error estimators of residual type are derived for piecewise linear finite element approximations to elliptic obstacle problems. An instrumental ingredient is a new interpolation operator which requires minimal regularity, exhibits optimal approximation properties and preserves positivity. Both upper and lower bounds are proved and their optimality is explored with several examples. Sharp a priori bounds for the a posteriori estimators are given, and extensions of the results to double obstacle problems are briefly discussed.