Mostrando 901 - 920 Resultados de 2,962 Para Buscar '"Concèntric"', tiempo de consulta: 0.12s Limitar resultados
  1. 901
    Publicado 2012
    Tabla de Contenidos: “…5.3.1 Signal acquisition5.3.2 Signal conditioning; 5.3.3 Analogue to digital conversion; 5.3.4 Computation; 5.3.5 Input/output; 5.3.6 Communication; 5.4 Communications infrastructure and protocols for smart metering; 5.4.1 Home-area network; 5.4.2 Neighbourhood area network; 5.4.3 Data concentrator; 5.4.4 Meter data management system; 5.4.5 Protocols for communications; 5.5 Demand-side integration; 5.5.1 Services provided by DSI; 5.5.2 Implementations of DSI; 5.5.3 Hardware support to DSI implementations; 5.5.4 Flexibility delivered by prosumers from the demand side…”
    Libro electrónico
  2. 902
    por OECD
    Publicado 2020
    Tabla de Contenidos: “…-- More than five million (one in five) working adults in England have low basic skills -- Low basic skills in England are closely tied to adults' socio-economic backgrounds -- Only a small share of low-skilled workers in England are actually low-educated -- Low-skilled workers are quite concentrated in smaller enterprises -- Low-skilled workers are also concentrated in certain sectors and occupations -- Low-skilled workers and employers are not highly engaged in basic skills development -- About half of low-skilled workers in England do not participate in education or training -- Participation in basic skills programmes is declining, in line with broader training trends -- England's employers are not highly engaged in developing the basic skills of workers -- Why is participation and engagement of low-skilled workers and their employers in basic skills so low? …”
    Libro electrónico
  3. 903
    por Brown, Theodore
    Publicado 2021
    Tabla de Contenidos: “…3.4 Avogadro's Number and the Mole -- Molar Mass -- The Mole and Avogadro's Number -- Molar Mass -- Converting Between Masses, Moles, and Atoms/Molecules/Ions -- 3.5 Formula Weights and Elemental Compositions of Substances -- Molecular Formulas from Empirical Formulas -- Combustion Analysis -- 3.6 Reaction Stoichiometry -- 3.7 Limiting Reactants -- Theoretical and Percent Yields -- Chapter Summary and Key Terms -- Learning Outcomes -- Key Equations -- Exercises -- Additional Exercises -- Integrative Exercises -- Design an Experiment -- Strategies for Success Problem Solving -- Chemistry and Life Glucose Monitoring -- Strategies for Success Design an Experiment -- Chapter 4: Reactions in Aqueous Solution -- 4.1 General Properties of Aqueous Solutions -- Electrolytes and Nonelectrolytes -- How Compounds Dissolve in Water -- Strong and Weak Electrolytes -- 4.2 Precipitation Reactions -- Solubility Guidelines for Ionic Compounds -- Exchange (Metathesis) Reactions -- Ionic Equations and Spectator Ions -- 4.3 Acids, Bases, and Neutralization Reactions -- Acids -- Bases -- Strong and Weak Acids and Bases -- Identifying Strong and Weak Electrolytes -- Neutralization Reactions and Salts -- Neutralization Reactions with Gas Formation -- 4.4 Oxidation-Reduction Reactions -- Oxidation and Reduction -- Oxidation Numbers -- Oxidation of Metals by Acids and Salts -- The Activity Series -- 4.5 Concentrations of Solutions -- Molarity -- Expressing the Concentration of an Electrolyte -- Interconverting Molarity, Moles, and Volume -- Dilution -- 4.6 Solution Stoichiometry and Chemical Analysis -- Titrations -- Chapter Summary and Key Terms -- Learning Outcomes -- Key Equations -- Exercises -- Additional Exercises -- Integrative Exercises -- Design an Experiment -- Chemistry Put to Work Antacids -- Strategies for Success Analyzing Chemical Reactions…”
    Libro electrónico
  4. 904
    por Brown, Theodore
    Publicado 2021
    Tabla de Contenidos: “…3.4 Avogadro's Number and the Mole -- Molar Mass -- The Mole and Avogadro's Number -- Molar Mass -- Converting Between Masses, Moles, and Atoms/Molecules/Ions -- 3.5 Formula Weights and Elemental Compositions of Substances -- Molecular Formulas from Empirical Formulas -- Combustion Analysis -- 3.6 Reaction Stoichiometry -- 3.7 Limiting Reactants -- Theoretical and Percent Yields -- Chapter Summary and Key Terms -- Learning Outcomes -- Key Equations -- Exercises -- Additional Exercises -- Integrative Exercises -- Design an Experiment -- Strategies for Success Problem Solving -- Chemistry and Life Glucose Monitoring -- Strategies for Success Design an Experiment -- Chapter 4: Reactions in Aqueous Solution -- 4.1 General Properties of Aqueous Solutions -- Electrolytes and Nonelectrolytes -- How Compounds Dissolve in Water -- Strong and Weak Electrolytes -- 4.2 Precipitation Reactions -- Solubility Guidelines for Ionic Compounds -- Exchange (Metathesis) Reactions -- Ionic Equations and Spectator Ions -- 4.3 Acids, Bases, and Neutralization Reactions -- Acids -- Bases -- Strong and Weak Acids and Bases -- Identifying Strong and Weak Electrolytes -- Neutralization Reactions and Salts -- Neutralization Reactions with Gas Formation -- 4.4 Oxidation-Reduction Reactions -- Oxidation and Reduction -- Oxidation Numbers -- Oxidation of Metals by Acids and Salts -- The Activity Series -- 4.5 Concentrations of Solutions -- Molarity -- Expressing the Concentration of an Electrolyte -- Interconverting Molarity, Moles, and Volume -- Dilution -- 4.6 Solution Stoichiometry and Chemical Analysis -- Titrations -- Chapter Summary and Key Terms -- Learning Outcomes -- Key Equations -- Exercises -- Additional Exercises -- Integrative Exercises -- Design an Experiment -- Chemistry Put to Work Antacids -- Strategies for Success Analyzing Chemical Reactions…”
    Libro electrónico
  5. 905
    Publicado 2017
    Tabla de Contenidos: “…Cover -- Title page -- Copyright -- Contents -- Preface -- Nomenclature -- Chapter 1 - Nanofluid: Definition and Applications -- 1.1 - Introduction -- 1.1.1 - Definition of nanofluid -- 1.1.2 - Model description -- 1.1.3 - Conservation equations -- 1.1.3.1 - Single-phase model -- 1.1.3.2 - Two-phase model -- 1.1.3.2.1 - Continuity equation -- 1.1.3.2.2 - Nanoparticle continuity equation -- 1.1.3.2.3 - Momentum equation -- 1.1.3.2.4 - Energy equation -- 1.1.4 - Physical properties of the nanofluids for single-phase model -- 1.1.4.1 - Density -- 1.1.4.2 - Specific heat capacity -- 1.1.4.3 - Thermal expansion coefficient -- 1.1.4.4 - The electrical conductivity -- 1.1.4.5 - Dynamic viscosity -- 1.1.4.6 - Thermal conductivity -- 1.2 - Simulation of nanofluid flow and heat transfer -- 1.2.1 - Semianalytical methods -- 1.2.2 - Runge-Kutta method -- 1.2.3 - Finite difference method -- 1.2.4 - Finite volume method -- 1.2.5 - Finite element method -- 1.2.6 - Control volume-based finite element method -- 1.2.7 - Lattice Boltzmann method -- References -- Chapter 2 - Nanofluid Natural Convection Heat Transfer -- 2.1 - CuO-water nanofluid hydrothermal analysis in a complex-shaped cavity -- 2.1.1 - Problem definition and mathematical model -- 2.1.1.1 - Problem definition -- 2.1.1.2 - Basic idea of CVFEM -- 2.1.2 - Effects of active parameters -- 2.2 - Natural convection heat transfer in a nanofluid filled inclined L-shaped enclosure -- 2.2.1 - Problem definition -- 2.2.2 - Effects of active parameters -- 2.3 - Natural convection heat transfer in a nanofluid filled enclosure with elliptic inner cylinder -- 2.3.1 - Problem definition -- 2.3.2 - Effects of active parameters -- 2.4 - Natural convection in a nanofluid filled concentric annulus between an outer square cylinder and an inner circular cy... -- 2.4.1 - Problem definition and mathematical model…”
    Libro electrónico
  6. 906
    por Kovari, Peter
    Publicado 2004
    Tabla de Contenidos: “…An architectural model for coexistent applications -- 3.1 Coexisting heterogeneous technologies -- 3.1.1 Layered application model -- 3.1.2 Concentric layered application model -- 3.1.3 Bridging layers and address spaces -- 3.1.4 Interoperation layer abstraction -- 3.1.5 Summary -- Part 2 Scenarios -- Chapter 4. …”
    Libro electrónico
  7. 907
    Publicado 2000
    Tabla de Contenidos: “…1.4.3.2.2 Nitrogen(II) Oxide Reduction Process1.4.3.2.3 Nitrate Reduction Process (DSM/HPO-Stamicarbon); References for Chapter 1.4.3: Hydroxylamine; 1.4.4 Nitric Acid; 1.4.4.1 Economic Importance; 1.4.4.2 Manufacture; 1.4.4.2.1 Fundamentals of Nitric Acid Manufacture; 1.4.4.2.2 Plant Types; 1.4.4.2.3 Process Description; 1.4.4.2.4 Manufacture of Highly Concentrated Nitric Acid; 1.4.4.2.5 Tail Gases from Nitric Acid Manufacture; 1.4.4.3 Nitric Acid Applications; References for Chapter 1.4.4: Nitric Acid; 1.5 Phosphorus and its Compounds; 1.5.1 Phosphorus and Inorganic Phosphorus Compounds…”
    Libro electrónico
  8. 908
    Publicado 2007
    Tabla de Contenidos: “…Separation of Solid Matter; 3.2.2.1 Filtration; 3.2.2.2 Centrifugation; 3.2.2.3 Extraction; 3.2.2.4 Flocculation and Flotation; 3.2.3 Concentration; 3.2.3.1 Thermal Methods; 3.2.3.2 Precipitation; 3.2.3.3 Ultrafiltration; 3.2.4 Purification; 3.2.4.1 Crystallization; 3.2.4.2 Electrophoresis; 3.2.4.3 Chromatography; 3.2.5 Product Formulation; 3.2.6 Waste Disposal; 3.3 Immobilization; 3.3.1 Definitions…”
    Libro electrónico
  9. 909
    por OECD
    Publicado 2022
    Tabla de Contenidos: “…Vers une croissance verte -- Relever les défis environnementaux à l'heure de la sortie de l'UE et du COVID-19 -- Le Royaume-Uni a des objectifs climatiques ambitieux et une stratégie à l'échelle de l'économie visant à atteindre le zéro net d'ici à 2050 -- La pollution de l'air a été réduite mais demeure préoccupante pour la santé -- Les actifs naturels se détériorent et les objectifs de biodiversité n'ont pas été atteints -- Améliorer la gouvernance et la gestion de l'environnement -- Les approches divergentes entre les juridictions décentralisées nécessitent une coordination plus étroite -- L'évaluation de la réglementation et des politiques est rigoureuse, mais l'évaluation environnementale de la législation et des politiques non environnementales pourrait être améliorée -- Les bonnes pratiques en matière de réglementation doivent être préservées -- Promouvoir les investissements et les instruments économiques pour la croissance verte -- L'effort de relance verte s'est concentré sur les transports durables…”
    Libro electrónico
  10. 910
    Publicado 2023
    Tabla de Contenidos: “…Intro -- Foreword -- Acknowledgements -- Abbreviations and acronyms -- Executive summary -- Key findings -- Key recommendations -- Beyond the RDAs - Key recommendations to higher-level bodies -- 1 Recommendations for Romania and considerations for regional planning and innovation -- Key insights -- The ability of Romanian RDAs to implement regional development and innovation policy could be further reinforced -- A wide range of resources can be mobilised to implement regional plans -- Innovation remains undeveloped and widely misunderstood in Romania -- The Association of Regional Development Agencies of Romania (ROREG) could play a greater role in co-ordinating and speaking for Romania's RDAs -- Recommendations for Romanian RDAs -- Recommendations for ROREG -- Recommendations for Romania's national government -- Governance considerations for RDAs and policy makers -- Reference -- 2 Overview of regional development and innovation in Romania -- Romania faces significant and growing regional disparities -- Economic disparities between regions are large and growing larger -- Disparities are visible in other regional development areas -- Innovation is concentrated in Bucharest-Ilfov -- Subnational governments are key players in delivering regional development policy -- Setting the scene: The important role of Romanian RDAs in regional development -- Romanian RDAs are key actors in advancing regional development -- RDAs can leverage strategic planning and innovation to advance regional development -- New opportunities and challenges: Becoming Managing Authorities in the 2021-27 programming period -- Conclusion -- References -- Notes -- 3 Strategic planning in Romanian Regional Development Agencies -- Strategy, planning and performance measurement -- The RDAs' longstanding experience can help them deliver as regional Managing Authorities…”
    Libro electrónico
  11. 911
    Tabla de Contenidos: “…ARMY POLICY CONCERNING USE OF OBSCURANTS -- SUBCOMMITTEE TASK -- DEFINITIONS OF EXPOSURE GUIDANCE LEVELS -- APPROACH TO DEVELOPING EXPOSURE GUIDANCE LEVELS -- Emergency Exposure Guidance Levels (EEGLs) -- Short-Term Public Emergency Guidance Levels (SPEGLs) -- Permissible Exposure Guidance Levels (PEGLs) and Permissible Public Exposure Guidance Levels (PPEGLs) -- SUMMARY OF APPROACH -- REFERENCES -- 2 Diesel-Fuel Smoke -- BACKGROUND INFORMATION -- Military Applications -- Physical and Chemical Properties -- TOXICOKINETICS -- TOXICITY SUMMARY -- Effects in Humans -- Effects in Animals -- Dermal and Ocular Exposures -- Lethality -- Eye and Skin Irritation -- Carcinogenic Effects -- Inhalation Exposures -- One-Time Exposures -- Repeated 15- or 60-Minute Exposures -- Repeated 4-Hour Exposures -- Repeated Exposures: Relative Significance of Frequency, Duration, and Concentration -- Screening Tests for Mutagenicity -- Summary of Toxicity Data -- Noncancer Toxicity -- Carcinogenicity -- LIMITS EXISTING RECOMMENDED EXPOSURE -- SUBCOMMITTEE EVALUATION AND RECOMMENDATIONS -- Military Exposures -- Emergency Exposure Guidance Level (EEGL)1 -- Permissible Exposure Guidance Level (PEGL)2 -- Public Exposures -- Short-Term Public Emergency Guidance Level (SPEGL)3 -- Permissible Public Exposure Guidance Level (PPEGL)4 -- Summary of Subcommittee Recommendations -- RESEARCH NEEDS -- REFERENCES -- 3 Fog-Oil Smoke -- BACKGROUND INFORMATION -- Military Applications -- Physical and Chemical Properties…”
    Libro electrónico
  12. 912
    por OECD
    Publicado 2023
    Tabla de Contenidos: “…-- The job of licensed practical nurse in Alberta saw a spike in demand during the pandemic -- Alberta's Welders and Machinists had one of the steepest declines in the average number of OJPS during the peak of the pandemic -- References -- Notes -- 4 Assessing the impact of digitalisation in Alberta through the lens of big data -- The digital transition: Challenges and opportunities for Alberta going forward -- Identifying digital occupations and the volume of job postings available in Alberta -- The geography of Alberta's demand for digital occupations -- Trends in online job postings for group one of the digital professionals -- The demand for data mining analysts and business intelligence analysts has been rapidly growing pre-pandemic -- Data scientists and data engineers are amongst the occupations with the fastest rate of growth across digital occupations -- Demand for Computer Scientists and Cyber Security Professionals has been trending upwards in Alberta -- Skill requirements for occupations in the group of Computer and data analysts / administrators -- Trends in online job postings for group two of the digital professionals -- Software developers, programmers and engineers concentrate a high volume of online job postings in Alberta -- Trends in online job postings for group three of the digital professionals -- The demand for ICT supporting roles has experience more rapid growth in Alberta than in Canada -- Trends in online job postings for group four of the digital professionals -- Demand for managerial roles as well as for HR and marketing professionals has been steadily increasing, and rapidly growing since mid-2020 -- How quickly are digital skill demands permeating Alberta's workplaces?…”
    Libro electrónico
  13. 913
    Publicado 2018
    Tabla de Contenidos: “…-Pilar Marco Studies towards hcTnI Immunodetection Using Electrochemical Approaches Based on Magnetic Microbeads, Reprinted from: Sensors 2018, 18, 2457, doi: 10.3390/s18082457 -- Anna Malec, Georgios Kokkinis, Christoph Haiden and Ioanna Giouroudi / Biosensing System for Concentration Quantification of Magnetically Labeled E. coli in Water Samples, Reprinted from: Sensors 2018, 18, 2250, doi: 10.3390/s18072250 -- Zhen Yang, Huanhuan Wang, Pengfei Guo, Yuanyuan Ding, Chong Lei and Yongsong Luo / A Multi-Region Magnetoimpedance-Based Bio-Analytical System for Ultrasensitive Simultaneous Determination of Cardiac Biomarkers Myoglobin and C-Reactive Protein, Reprinted from: Sensors 2018, 18, 1765, doi: 10.3390/s18061765 -- Jen-Jie Chieh, Wen-Chun Wei, Shu-Hsien Liao, Hsin-Hsein Chen, Yen-Fu Lee, Feng-Chun Lin, Ming-Hsien Chiang, Ming-Jang Chiu, Herng-Er Horng and Shieh-Yueh Yang / Eight-Channel AC Magnetosusceptometer of Magnetic Nanoparticles for High-Throughput and Ultra-High-Sensitivity Immunoassay, Reprinted from: Sensors 2018, 18, 1043, doi: 10.3390/s18041043 -- Alexander P. …”
    Libro electrónico
  14. 914
    Tabla de Contenidos:
    Libro electrónico
  15. 915
    Publicado 2017
    Tabla de Contenidos: “…11 The Genie and the Bottle: Reflections on the Fate of the Geneva Protocol in the United States, 1918-1928 -- Abstract -- 1 Introduction -- 2 Science and the Great War -- 3 The Coming of Geneva -- 4 A Protocol Post-mortem -- 5 Summing up the Senate -- 6 Conclusion -- References -- 12 The Soldier's Body in Gas Warfare: Trauma, Illness, Rentennot, 1915-1933 -- Abstract -- 1 Introduction -- 2 Perceptions in the Field -- 3 Gas and Psyche -- 4 Weimar to the Nazi Period-the Need of the Traumatized -- 5 Summary -- References -- 13 Chemical Weapons Research on Soldiers and Concentration Camp Inmates in Nazi Germany -- Abstract -- 1 Organizational Structures of Chemical Warfare Research in Germany -- 2 Chemical Weapons Research on Humans in Military and Academic Institutions -- 3 Experiments in Concentration Camps -- 3.1 Sachsenhausen -- 3.2 Natzweiler -- 3.2.1 The Sulphur Mustard Experiments of August Hirt -- 3.2.2 The Phosgene Experiments of Otto Bickenbach -- 3.2.3 New Series of Phosgene Experiments in June and August 1944 -- 3.3 Neuengamme -- 4 Conclusion -- References -- 14 No Retaliation in Kind: Japanese Chemical Warfare Policy in World War II -- Abstract -- References -- 15 The 1925 Geneva Protocol: China's CBW Charges Against Japan at the Tokyo War Crimes Tribunal -- Abstract -- 1 Introduction -- 2 World War II and the Post-war IMTFE -- 3 Retrieving CBW Evidence from China, 1946 -- 4 Conclusion -- References -- Dual Use, Storage and Disposal of Chemical Weapons Today -- 16 The Reconstruction of Production and Storage Sites for Chemical Warfare Agents and Weapons from Both World Wars in the Context of Assessing Former Munitions Sites -- Abstract -- 1 Introduction -- 2 The Ammendorf Factory of Orgacid GmbH -- 3 The History of Falkenhagen Factory of Monturon GmbH, Development, the Structure of the Buildings, and Production…”
    Libro electrónico
  16. 916
    Tabla de Contenidos: “…Policies to counter drink-driving, such as enforcement of sobriety checkpoints or blood alcohol concentration limits -- 1.3.4. Policies to regulate alcohol marketing, such as regulation of advertising and sponsorships -- 1.3.5. …”
    Libro electrónico
  17. 917
    Tabla de Contenidos: “…Principales entreprises publiques -- Déréglementation et politique de la concurrence : la priorité doit être de réduire les abus... -- Tableau 14. Concentration du marché dans les industries de réseaux et dans les autres secteurs -- Gonflement des subventions agricoles -- Secteur financier : le mouvement de concentration se poursuit -- Tableau 15. …”
    Libro electrónico
  18. 918
    Publicado 2022
    Tabla de Contenidos: “…Cover -- Title Page -- Copyright -- Contents -- Preface -- List of Abbreviations -- Chapter 1 From Anion Recognition to Organocatalytic Chemical Reactions -- 1.1 Introduction and Background -- 1.1.1 Evolution of Thiourea‐Based Catalysts -- 1.1.2 Evolution of Triazole‐Based Catalysts -- 1.1.3 Progress on Halogen‐Binding‐Based Catalysts -- 1.1.4 Miscellaneous Anion‐Binding Catalysts -- 1.2 Concepts in Anion‐Binding Catalysis -- 1.2.1 Introduction -- 1.2.2 Anion‐Binding Catalysis in Addition Reactions -- 1.2.3 Anion‐Binding Catalysis in Substitution Reactions -- 1.2.4 Anion Binding in Cooperative Catalysis -- 1.2.5 Anion‐Binding in Lewis Acid Enhancement Catalysis -- 1.2.6 Anion‐Binding in Phase Transfer Catalysis -- 1.3 Summary and Outlook -- Acknowledgment -- References -- Chapter 2 Anion Recognition and Binding Constant Determination -- 2.1 Introduction to Supramolecular Chemistry and Binding Constant Determination -- 2.1.1 Chapter Overview -- 2.1.2 Supramolecular Chemistry and Its Connection to Anion‐Assisted Catalysis -- 2.1.3 Brief History of Advances in Supramolecular Anion Binding -- 2.1.4 Predicting the Model of Association and Simulating the Expected Species Distribution Profiles and Binding Curves -- 2.2 Equilibrium Constants, Binding Curves, Titration Conditions, and Errors -- 2.2.1 Physical Origins of Equilibrium Binding Constants -- 2.2.2 Explanation of the Basis for Titration Techniques and Binding Curves -- 2.2.3 Hirose's Rule and Picking the Right Concentration, Solvent, and Technique -- 2.2.4 Error Determination -- 2.3 Experimental Techniques: NMR Spectroscopy -- 2.3.1 When to Use -- 2.3.2 Slow Exchange vs. …”
    Libro electrónico
  19. 919
    Publicado 2011
    Tabla de Contenidos: “…Lewis Acid-Base Theory -- Hard and soft acids and bases -- Chapter 4: Chemical Reactions -- Types of chemical reactions -- Hydrolysis -- Oxidation-reduction reactions -- Oxidation number -- Oxidizing agents -- Reducing agents -- Oxidizing and reducing agents -- Strength of oxidants and reductants -- Equivalent weights of oxidizing and reducing agents -- Balancing of redox reactions -- Oxidation number method -- Ion-electron method -- Some important half reactions -- Chapter 5: Transition Metals, Lanthanides and Actinides -- General properties -- Atomic and ionic radii -- Bonding in Transition metals and its effect on properties -- Electrode Potential -- Oxidation states -- Paramagnetic nature of transition metal compounds -- Complex compound formation -- Colour of Transition metal compounds -- The d - d transition -- Colour and charge transfer -- Hydrolysis of Transition metal compounds -- Catalytic property -- Lanthanides and actinides -- Chapter 6: Coordination Chemistry -- Coordination number -- Types of ligands -- Chelates -- Conditions for complex formation -- Werner's coordination theory -- Nomenclature -- Valence bond model for complexes -- Crystal field model of bonding -- Stability and CF model -- Magnetic properties and CF model -- Colour and CF model -- Isomerism in coordination compounds -- Geometrical isomerism -- Optical isomerism -- Preparations of complex compounds -- Stability of complex compound in a solution -- Applicability of complex compounds -- Organometallic compounds -- Preparation of organometallies -- Bonding in organometallic compounds -- Bonding in alkene complexes -- Chapter 7: Abundance and metallurgy -- Chemical elements in the Earth's crust -- Cosmic abundance of elements -- Abundance in oceans -- Occurrence of Metals -- Metallurgy -- Terms used in metallurgical process -- Concentration of ore -- Leaching…”
    Libro electrónico
  20. 920
    por OECD
    Publicado 2023
    Tabla de Contenidos: “…Taux de fécondité -- 2.4. Concentration dans les zones densément peuplées -- 2.5. …”
    Libro electrónico