Претрага
276 items
-
Tektono-termalna evolucija kontakta Dinarida i Karpato-Balkanida u području Jastrepca
Dalibor Erak (2019)Predmet istraživanja doktorske disertacije je tektonska evolucija područja planine Jastrebac u centralnoj Srbiji, gde višefazna tektonska aktivnost od mezozoika do danas komplikuje rekonstrukciju strukturnih odnosa i geološke evolucije ovog prostora. Kompleksna višefazna evolucija planine Jastrebac u Srbiji je demonstrirana kombinovanjem kinematskih podataka prikupljenih na terenu i mikrostrukturnih opservacija sa dodatnim termohronološkim podacima “fission track” i podacima odredbe starosti metodom U-Pb na cirkonima. Gornjokredna ekshumacija bila je praćena gornjokredno-eocenskim navlačenjem i magmatizmom vezanim za kontinentalnu koliziju, pri čemu je akreciona ...... metamorphosed rocks of Veliki Jastrebac are used for corelation with their nonmetamorphosed equivalents. On that way, paleogeographic affinity of units that build area of Jastrebac Mountains is defined by the presence of three different tectonical units which were subducted and where rocks were m ...
... Surroundings, Serbia)", Stratigraphy and Geological Correlation 19(6), pp.631-640. Brun, J.-P., Faccenna, C., 2008. Exhumation of high-pressure rocks driven by slab rollback. Earth and Planetary Science Letters 272, 1-7. Brun, J.-P., Sokoutis, D., 2007. Kinematics of the Southern Rhodope Core ...
... pulse in a typical orogenic setting: The geochemical and geochronological record in the East Serbian latest Cretaceous to Palaeocene alkaline rocks. Lithos 180–181, 181-199. Dercourt, J., Gaetani, M., Vrielynck, B., Barrier, E., BijuDural, B., Brunet, M. F., Cadet, J. P., Crasquin, S., Sandulescu ...Dalibor Erak. Tektono-termalna evolucija kontakta Dinarida i Karpato-Balkanida u području Jastrepca, Beograd : Rudarsko-geološki fakultet, 2019
-
Groundwater management by riverbank filtration and an infiltration channel, the case of Obrenovac, Serbia
Dušan Polomčić, Bojan Hajdin, Zoran Stevanović, Dragoljub Bajić, Katarina Hajdin. "Groundwater management by riverbank filtration and an infiltration channel, the case of Obrenovac, Serbia" in Hydrogeology Journal, Berlin, Heidelberg : Springer, International Association of Hydrogeologists (2013). https://doi.org/10.1007/s10040-013-1025-9
-
Contribution to the geotechnical classification of municipal waste landfills in Serbia
Management, Monitoring, Policy and Law, Nature and Landscape Conservation, Geochemistry and Petrology, Waste Management and Disposal,Geotechnical Engineering and Engineering Geology,Water Science and Technology,Environmental Chemistry,Environmental EngineeringDragoslav Rakić, Irena Basarić, Laslo Čaki, Slobodan Ćorić. "Contribution to the geotechnical classification of municipal waste landfills in Serbia" in Environmental Geotechnics, Thomas Telford Ltd. (2018). https://doi.org/10.1680/jenge.18.00028
-
Hidrogeohemija prirodnih radioaktivnih elemenata u podzemnim vodama Srbije
Marina D. Ćuk (2018-02-23)Osnovni cilj doktorske disertacije predstavljao je definisanje prirodnih koncentracija (engl. background values) U, Th, i K u podzemnim vodama, i definisanje hidrogeohemijskih uslova koji utiču na distribuciju navedenih elemenata u podzemnim vodama...prirodni radioaktivni elementi, podzemna voda, geohemijske sredine, hidrogeohemijski uslovi, neparametarska statistička analiza, logistički model, hidrohemijsko modeliranje... gamma-ray spectrometry methods for estimating K, U, Th concentrations in rocks of the Sardinia batholith. Doctoral Dissertation. University of Sassari, School in Natural Science, Sassari. 178. Yanagi T. 2011. Island Arc Volcanic Rocks and the Upper Continental Crust. In: Yanagi T (eds). Arc Volcano ...
... of Environmental Sciences 4(1): 7 – 13. 117. Nikić Z, Kovačević J, Papić P. 2008. Uranium minerals of Bukulja mountain controls on storage reservoir water. In: Merkel B, Hasche-Berger A (eds.). Uranium, Mining and Hydrogeology. Springer, Springer – Verlag Berlin, Heidelberg, pp. 645 – 652. ...
... Aspects of Uranium Mining and Processing in Virginia. National Academies Press, Washington D.C. 23. Custodio E. 2004. Hydrogeology of volcanic rocks. In: Kovalevsky VS, Kruseman GP, Rushton KR (eds.). Groundwater Studies. An international guide for hydrogeological investigations. IHP-VI, Series ...Marina D. Ćuk. "Hidrogeohemija prirodnih radioaktivnih elemenata u podzemnim vodama Srbije" in Универзитет у Београду, Универзитет у Београду, Рударско-геолошки факултет (2018-02-23)
-
Study of the Synergetic Effect of Co-Pyrolysis of Lignite and High-Density Polyethylene Aiming to Improve Utilization of Low-Rank Coal
Ivan Kojić, Achim Bechtel, Nikoleta Aleksić, Dragana Životić, Snežana Trifunović, Gordana Gajica, Ksenija Stojanović (2021)... organic matter (OM) depends on its quantity, quality/type, and maturity. The most important factor controlling the potential of OM of sedimentary rocks to produce liquid hydrocarbons is the amount of hydrogen in kerogen [25]. At the beginning of catagenesis, kerogen type I, and kerogen type II, which ...
... ty, depositional environment, maturity, and hydrocarbon generation potential of sedimentary organic matter (OM) in the oil shale, coal, and source rocks [27–31]. In the last 15 years, much attention has been given to thermal treatment of coal/plastic blends. Namely, hydrocarbon plastic materials, PE ...Ivan Kojić, Achim Bechtel, Nikoleta Aleksić, Dragana Životić, Snežana Trifunović, Gordana Gajica, Ksenija Stojanović. "Study of the Synergetic Effect of Co-Pyrolysis of Lignite and High-Density Polyethylene Aiming to Improve Utilization of Low-Rank Coal" in Polymers, MDPI AG (2021). https://doi.org/10.3390/polym13050759
-
Asbestos-Based Pottery from Corsica: The First Fiber-Reinforced Ceramic Matrix Composite
Asbestos-containing pottery shards collected in the northeast of Corsica (Cap Corse) and dating from the 19th century, or earlier, have been analyzed by SEM-EDS, XRPD, FTIR and Raman microspectroscopy. Blue (crocidolite) and white (chrysotile) asbestos fiber bundles are observed in cross-sections. Most of the asbestos is partly or totally dehydroxylated, and some transformation to forsterite is observed to occur, indicative of a firing above 800 C. Examination of freshly fractured pieces shows a nonbrittle fracture with fiber pull-out, consistent with ...... Environ. Geol. 2016, 49, 77–88. [CrossRef] Rooney, J.; Tarling, M.; Smith, S.; Gordon, K. Sub-micron raman spectroscopy mapping,of serpentinite fault rocks. J. Raman Spectrosc. 2018, 49, 279-286. [CrossRef] Dodony, I.; Buseck, P.R. Serpentines close-up and intimate: An hrtem view. Int. Geol. Rev. 2004 ...Philippe Colomban, Aleksandar Kremenović. "Asbestos-Based Pottery from Corsica: The First Fiber-Reinforced Ceramic Matrix Composite" in Materials, MDPI AG (2020). https://doi.org/10.3390/ma13163597
-
Spatial assessment of the areas sensitive to degradation in the rural area of the municipality Čukarica
Nature and Landscape Conservation, Soil Science, Agronomy and Crop Science, Water Science and TechnologyNatalija Momirović, Ratko Kadović, Veljko Perović, Miloš Marjanović, Aleksandar Baumgertel. "Spatial assessment of the areas sensitive to degradation in the rural area of the municipality Čukarica" in International Soil and Water Conservation Research, Elsevier BV (2019). https://doi.org/10.1016/j.iswcr.2018.12.004
-
Determination of the groundwater-leakage mechanism (binary mixing) in a karstic dam site using thermometry and isotope approach (HPP Visegrad, Bosnia, and Herzegovina)
Earth-Surface Processes,Geology, Pollution, Soil Science, Water Science and Technology, Environmental Chemistry, Global and Planetary ChangeLjiljana Vasić, Saša Milanović, Anita Puskás-Preszner, Laszlo Palcsu. "Determination of the groundwater-leakage mechanism (binary mixing) in a karstic dam site using thermometry and isotope approach (HPP Visegrad, Bosnia, and Herzegovina)" in Environmental Earth Sciences, Springer Science and Business Media LLC (2020). https://doi.org/10.1007/s12665-020-08910-x
-
Lower Berriasian ammonites from Dedina (Golubac Mountains, eastern Serbia) and their biostratigraphic implication
... Royal Society of Edinburgh 53, 91—160. Spath, LE, 1923. VIIL. Appendix. On ammonites from New Zealand. In: Trechmann, C.T. (Ed.), The Jurassic Rocks of New Zealand, Quarterly Journal of the Geological Society of London, vol. 79, pp. 286—312. Spath, L.E., 1924. On the ammonites of the Speeton Clay ...
... Neocomian. Geological Magazine 61, 73—89. Spath, L.F., 1925. Ammonites and Aptychi. VII. In: Gregory, J.W. (Ed.), The Collection of Fossils and Rocks from Somaliland made by Messrs. Wyllie and Smellie, Monographs of the Geological Department of the Hunterian Museum, Glasgow University 1, pp. 111—164 ...Zdenek Vasícek , Dragoman Rabrenović, Petr Skupien, Barbara Radulović. "Lower Berriasian ammonites from Dedina (Golubac Mountains, eastern Serbia) and their biostratigraphic implication" in Cretaceous Research (2023). https://doi.org/10.1016/j.cretres.2023.105623
-
Models of Determining the Parameters of Rock Mass Oscillation Equation with Experimental and Mass Blastings
... Savić, L.; Ristović, I.; Trajković, S.; Savić, L. 2010: Experimental Research and Defining of the Optimum Bit Blade Sharpening Angle for Drilling Rocks and Ores; Technics Technologies Education Management (TTEM): Sarajevo, Bosnia and Herzegovina, 2010; Volume 5, pp. 205–217. 7. Medvedev, S.V. Seismics ...Suzana Lutovac, Branko Gluščević, Rade Tokalić, Jelena Majstorović, Čedomir Beljić. "Models of Determining the Parameters of Rock Mass Oscillation Equation with Experimental and Mass Blastings" in Minerals, Basel : MDPI AG, Basel, Switzerland (2018). https://doi.org/10.3390/min8020070
-
‘Blue-’ and ‘Brown-speckled’ pottery from Qalhât, the Sultanate of Oman (13–16th centuries): Comparison with traditional Omani 19–20 century productions
Representative ‘Blue speckled’ (13–16th centuries) and ‘Brown speckled’ (13–21st century) wares recovered from the excavations in Qalhât or collected around Bahlâ city (Sultanate of Oman) were characterized by XRPD, SEM-EDS, Raman/optical microspectroscopy, and thermal expansion. Minerals present in the body (quartz, augite and albite as major phases, calcite, dolomite, rutile, anatase, phosphate, hematite and magnetite as minor phases) were identified by XRPD and/or Raman spectroscopy. SEM-EDXS microanalysis was performed on the glazes in order to compare their composition, especially ...Liliana Gianni, Hélène Renel, Aleksandar Kremenović, Philippe Colomban. "‘Blue-’ and ‘Brown-speckled’ pottery from Qalhât, the Sultanate of Oman (13–16th centuries): Comparison with traditional Omani 19–20 century productions" in Boletín de la Sociedad Española de Cerámica y Vidrio, Elsevier BV (2020). https://doi.org/10.1016/j.bsecv.2020.06.005
-
Nelinearno dinamičko modelovanje potresa izazvanih promenom naponskog stanja pri izradi horizontalnih podzemnih prostorija
Srđan D. Kostić (2013-11-04)U ovoj disertaciji, cilj istraživanja bio je određivanje dinamičkih svojstava potresaizazvanih promenom naponskog stanja pri izradi horizontalnih podzemnih prostorija.Analiza potresa izvedena je sa dva aspekta – u prvom delu istraživanja izučavane suvremenske serije oscilovanja tla za vreme potresa, a u drugom delu vršena je analizastabilnosti fenomenološkog Baridž-Knopof modela bloka sa oprugom. Vremenske serijeoscilovanja tla proučavane su primenom postupka analize surogat podataka, kao ikorišćenjem metoda analize nelinearnih vremenskih serija (primena teoreme orazvijanju skalarne vremenske serije). Analiza vremenskih serija izvedena je ...potres, naponsko stanje, podzemne prostorije, bifurkacije, vremenskaserija, deterministički haos, stohastičnost, perturbacije, vremensko kašnjenje, jačina trenja... L'Aquila (Central Italy) seismic sequence as a chaotic process, Tectonophysics 496, 44– 52. 38. Dieterich, J.H. 1972. Time-dependent friction in rocks, Journal of Geophysical Research 77, 3690-3697. 39. Dieterich, J.H. 1979. Modeling of rock friction: 1; Experimental results and constitutive ...
... ve properties of faults with simulated gouge, in: Carter, N.L., Friedman, M., Logan, J.M., Sterns, D.W. (eds.): Mechanical behavior of crustal rocks, American Geophysical Union Monograph 24, Washington DC, 103-120. 41. Dieterich, J.H., Kilgore, B.D. 1994. Direct observation of frictional contacts: ...
... Seizmologija. Skripta za studente Rudarsko-geološkog fakulteta, Beograd. 62. Goldsby, D.L., T.E. Tullis. 2002. Low frictional strength of quartz rocks at subseismic slip rates, Geophysical Research Letters 29, 25-1-4. 63. Gomberg, J., Beeler, N., Blanpied, M.L., Bodin, P. 1998. Earthquake triggering ...Srđan D. Kostić. "Nelinearno dinamičko modelovanje potresa izazvanih promenom naponskog stanja pri izradi horizontalnih podzemnih prostorija" in Универзитет у Београду, Универзитет у Београду, Рударско-геолошки факултет (2013-11-04)
-
Nelinearno dinamičko modelovanje potresa izazvanih promenom naponskog stanja pri izradi horizontalnih podzemnih prostorija
Srđan D. Kostić (2013-11-04)U ovoj disertaciji, cilj istraživanja bio je određivanje dinamičkih svojstava potresaizazvanih promenom naponskog stanja pri izradi horizontalnih podzemnih prostorija.Analiza potresa izvedena je sa dva aspekta – u prvom delu istraživanja izučavane suvremenske serije oscilovanja tla za vreme potresa, a u drugom delu vršena je analizastabilnosti fenomenološkog Baridž-Knopof modela bloka sa oprugom. Vremenske serijeoscilovanja tla proučavane su primenom postupka analize surogat podataka, kao ikorišćenjem metoda analize nelinearnih vremenskih serija (primena teoreme orazvijanju skalarne vremenske serije). Analiza vremenskih serija izvedena je ...potres, naponsko stanje, podzemne prostorije, bifurkacije, vremenskaserija, deterministički haos, stohastičnost, perturbacije, vremensko kašnjenje, jačina trenja... L'Aquila (Central Italy) seismic sequence as a chaotic process, Tectonophysics 496, 44– 52. 38. Dieterich, J.H. 1972. Time-dependent friction in rocks, Journal of Geophysical Research 77, 3690-3697. 39. Dieterich, J.H. 1979. Modeling of rock friction: 1; Experimental results and constitutive ...
... ve properties of faults with simulated gouge, in: Carter, N.L., Friedman, M., Logan, J.M., Sterns, D.W. (eds.): Mechanical behavior of crustal rocks, American Geophysical Union Monograph 24, Washington DC, 103-120. 41. Dieterich, J.H., Kilgore, B.D. 1994. Direct observation of frictional contacts: ...
... Seizmologija. Skripta za studente Rudarsko-geološkog fakulteta, Beograd. 62. Goldsby, D.L., T.E. Tullis. 2002. Low frictional strength of quartz rocks at subseismic slip rates, Geophysical Research Letters 29, 25-1-4. 63. Gomberg, J., Beeler, N., Blanpied, M.L., Bodin, P. 1998. Earthquake triggering ...Srđan D. Kostić. "Nelinearno dinamičko modelovanje potresa izazvanih promenom naponskog stanja pri izradi horizontalnih podzemnih prostorija" in Универзитет у Београду, Универзитет у Београду, Рударско-геолошки факултет (2013-11-04)
-
Geochemical characterization of sediments from the archaeological site Vinča – Belo Brdo, Serbia
Gorica Veselinović, Dragana Životić, Kristina Penezić, Milica Kašanin-Grubin, Nevenka Mijatović, Jovana Malbašić, Aleksandra Šajnović (2020)Gorica Veselinović, Dragana Životić, Kristina Penezić, Milica Kašanin-Grubin, Nevenka Mijatović, Jovana Malbašić, Aleksandra Šajnović. "Geochemical characterization of sediments from the archaeological site Vinča – Belo Brdo, Serbia" in CATENA, Elsevier BV (2020). https://doi.org/10.1016/j.catena.2020.104914
-
A fuzzy-based decision support model for effectiveness evaluation - a case study of examination of bulldozers
Effectiveness evaluation is one of the basic components of engineering asset management and maintenance engineering in general. Effectiveness is an overall concept representing a measurement of quality of service level for the analyzed engineering system. This concept contains a series of partial indicators relating to time in operation and time for maintenance activities, as well as functional properties of the system. This article describes the analysis and structuring of partial indicators, as well as the development of a model ...Miloš Tanasijević, Predrag Jovančić, Dejan Ivezić, Uglješa Bugarić, Radiša Đurić. "A fuzzy-based decision support model for effectiveness evaluation - a case study of examination of bulldozers" in International Journal of Industrial Engineering: Theory, Applications and Practice (2019)
-
Просторни положај лежишта угља Ђурђевик: геофизичко-геолошки модел
Синиша Арсеновић (2019)Лежиште угља Ђурђевик представља источни део Бановићког угљеног басена и припада северној страни босанске офиолитске зоне унутрашњих Динарида у чији састав улазе стене палеозојске, мезозојске и кенозојске старости. Језерска доњомиоценска угљоносна серија изграђена је од лапоровито-глиновитих седимената са једним слојем мрког угља. Дебљина угљеног слоја креће се у интервалу од 5 до 25 m са генералним правцем пружања СЗ - ЈИ и падом према ЈЗ. У зони обухваћеноj геофизичким истраживањима изражена je пострудна тектоника у виду раседа мањег и ...... eastern part of the Banović coal basin and belongs to the northern side of the Bosnian ophiolite zone of the Inner Dinarides, that consists of the rocks of the Palaeozoic, Mesozoic and Cenozoic age. The Lower Miocene freshwater series consist of marl-clay sediment with one sub-bituminous coal seam ...Синиша Арсеновић. Просторни положај лежишта угља Ђурђевик: геофизичко-геолошки модел, Београд : Рударско-геолошки факултет, 2019
-
Pleistocene rhinoceros from Bogovina Cave: the first report of Stephanorhinus hundsheimensis Toula, 1902 (Mammalia, Rhinocerotidae) from Serbia
Finds of Pleistocene rhinoceros are rare in Serbia, and only one species (the woolly rhinoceros Coelodonta antiquitatis Blumenbach, 1799) has been reported so far. The current paper presents the dental material of an extinct so-called Hundsheim rhinoceros, Stephanorhinus hundsheimensis Toula, 1902 from Bogovina Cave (East ern Serbia). Both the morphological and metric characteristics of the teeth are consis tent with the attribution to S. hundsheimensis. Unfortunately, the rhinoceros material originated from an uncertain geological context, so there is no ...... Zoologica Cracoviensia, 39(1):117-120. Dokmanovic, P., Jemcov, I., Milanovic, S., and Hajdin, B. 2003. Hydrogeological risk factors of dam and reservoir construction-case example Bogovina. Materials and Geoenvironment, 50(1):105-108. Eisenmann, V., Alberdi, M.T., de Giuli, C., and Staesche, U. 1988 ...
... Hibbard, C.W., Gutentag, E.D., Smith, G.R., Lundberg, J.G., Holman, J.A., Feduccia, J.A., Rich, P.V., 1972. Early Pleistocene pre-glacial and glacial rocks and faunas of north-central Nebraska. Bulletin of the American Museum of Natural History, 148:77-148. Smith, J.B. and Dodson, P. 2003. A proposal ...Predrag Radović, Miloš Radonjić, Emmanuel Billia. "Pleistocene rhinoceros from Bogovina Cave: the first report of Stephanorhinus hundsheimensis Toula, 1902 (Mammalia, Rhinocerotidae) from Serbia" in Palaeontologia Electronica, Coquina Press (2020). https://doi.org/10.26879/985
-
Development of the Availability Concept by Using Fuzzy Theory with AHP Correction, a Case Study: Bulldozers in the Open-Pit Lignite Mine
Stevan Đenadić, Dragan Ignjatović, Miloš Tanasijević, Uglješa Bugarić, Ivan Janković, Tomislav Šubaranović (2019)Availability is one of the most used terms in maintainability engineering. This concept is used to denote: The quality of service of an engineering system, i.e., machines, weak points’ analysis, asset management, as well as making decisions in the process of life cycle management. Availability is an overall indicator and contains partial indicators that are oriented towards reliability, maintenance, and logistical support. Availability presents a variable value and changes in time and space. Usually, availability is shown as the ...... ISBN 0-13-101171-5. 18. Djuric, R.; Milisavljevic, V. Investigation on the relationship between reliability of track mechanism and dust content in rocks of lignite open pits. Maint. Reliab. 2016, 18, 142–150. [CrossRef] 19. Tanasijevic, M. A fuzzy-based decision support model for evaluation of mining ...Stevan Đenadić, Dragan Ignjatović, Miloš Tanasijević, Uglješa Bugarić, Ivan Janković, Tomislav Šubaranović. "Development of the Availability Concept by Using Fuzzy Theory with AHP Correction, a Case Study: Bulldozers in the Open-Pit Lignite Mine" in Energies, MDPI AG (2019). https://doi.org/10.3390/en12214044
-
Comprehensive Profiling of Microbiologically Induced CaCO3 Precipitation by Ureolytic Bacillus Isolates from Alkaline Soils
Olja Šovljanski, Lato Pezo, Jovana Stanojev, Branimir Bajac, Sabina Kovač, Elvira Tóth, Ivan Ristić, Ana Tomić, Aleksandra Ranitović, Dragoljub Cvetković, Siniša Markov (2021)MICP process, kinetic study, calcite, vaterite, MALDI-TOF identification, Bacillus licheniformis, Sporosarcina pasteurii... activity, short generation time, sporulation ability, etc. [12]. Therefore, isolation of these bacteria was performed from areas rich in carbonate rocks such as mines [4], soils [13], landfills [14], cement matrix [15], sand [16], active sludge from wastewater treatment [17], etc. However, most research ...Olja Šovljanski, Lato Pezo, Jovana Stanojev, Branimir Bajac, Sabina Kovač, Elvira Tóth, Ivan Ristić, Ana Tomić, Aleksandra Ranitović, Dragoljub Cvetković, Siniša Markov. "Comprehensive Profiling of Microbiologically Induced CaCO3 Precipitation by Ureolytic Bacillus Isolates from Alkaline Soils" in Microorganisms, MDPI AG (2021). https://doi.org/10.3390/microorganisms9081691
-
Osnovi hidrogeologije
Petar Dokmanović (2021)... Groundwater, Englewood Cliffs, New Jersey: Prentice-Hall Inc., USA 29. Gascoyne, M. & Kamieni D. C. (1994): The hydrogeochemistry of Fractured Plutonic Rocks in the Canadian Shield, Applied Hidrogeology, Volume 2, 2/1994, Journal of the International Association of Hydrogeologists, 43-50, Hanover. ...
... N. (2008): Groundwater resources (Sustainability, Mansgement and Restoration), McGraw- Hill, New York 42. Larson, J. (1984): Ground water in hard rocks, 1-228, UNESCO. 43. Lazić, M. (1990): Specijalna hidrogeologija I deo (meliorativna hidrogeologija), Rudarsko-geološki fakultet, Beograd 44. Lazić ...
... Percolation, Springer 75. Simić, D. (1988): Mikrobiologija, Naučna knjiga, Beograd. 76. Singhal BB, Gupta RP (2010): Applied Hydrology of Fractured Rocks, sec. edition, Springer 77. Stepanov, V. M. (1989): Vvedenie v strukturnuju gidrogeologiju, Nedra, Moskva. 78. Stepanović, B. (1962): Principi opšte ...Petar Dokmanović. Osnovi hidrogeologije, Beograd : Univerzitet u Beogradu, Rudarsko-geološki fakultet, 2021