Geometric Algebra with Applications in Engineering

Download Geometric Algebra with Applications in Engineering PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 3540890688
Total Pages : 389 pages
Book Rating : 4.5/5 (48 download)

DOWNLOAD NOW!


Book Synopsis Geometric Algebra with Applications in Engineering by : Christian Perwass

Download or read book Geometric Algebra with Applications in Engineering written by Christian Perwass and published by Springer Science & Business Media. This book was released on 2009-02-11 with total page 389 pages. Available in PDF, EPUB and Kindle. Book excerpt: The application of geometric algebra to the engineering sciences is a young, active subject of research. The promise of this field is that the mathematical structure of geometric algebra together with its descriptive power will result in intuitive and more robust algorithms. This book examines all aspects essential for a successful application of geometric algebra: the theoretical foundations, the representation of geometric constraints, and the numerical estimation from uncertain data. Formally, the book consists of two parts: theoretical foundations and applications. The first part includes chapters on random variables in geometric algebra, linear estimation methods that incorporate the uncertainty of algebraic elements, and the representation of geometry in Euclidean, projective, conformal and conic space. The second part is dedicated to applications of geometric algebra, which include uncertain geometry and transformations, a generalized camera model, and pose estimation. Graduate students, scientists, researchers and practitioners will benefit from this book. The examples given in the text are mostly recent research results, so practitioners can see how to apply geometric algebra to real tasks, while researchers note starting points for future investigations. Students will profit from the detailed introduction to geometric algebra, while the text is supported by the author's visualization software, CLUCalc, freely available online, and a website that includes downloadable exercises, slides and tutorials.

Applications of Geometric Algebra in Computer Science and Engineering

Download Applications of Geometric Algebra in Computer Science and Engineering PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 146120089X
Total Pages : 479 pages
Book Rating : 4.4/5 (612 download)

DOWNLOAD NOW!


Book Synopsis Applications of Geometric Algebra in Computer Science and Engineering by : Leo Dorst

Download or read book Applications of Geometric Algebra in Computer Science and Engineering written by Leo Dorst and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 479 pages. Available in PDF, EPUB and Kindle. Book excerpt: Geometric algebra has established itself as a powerful and valuable mathematical tool for solving problems in computer science, engineering, physics, and mathematics. The articles in this volume, written by experts in various fields, reflect an interdisciplinary approach to the subject, and highlight a range of techniques and applications. Relevant ideas are introduced in a self-contained manner and only a knowledge of linear algebra and calculus is assumed. Features and Topics: * The mathematical foundations of geometric algebra are explored * Applications in computational geometry include models of reflection and ray-tracing and a new and concise characterization of the crystallographic groups * Applications in engineering include robotics, image geometry, control-pose estimation, inverse kinematics and dynamics, control and visual navigation * Applications in physics include rigid-body dynamics, elasticity, and electromagnetism * Chapters dedicated to quantum information theory dealing with multi- particle entanglement, MRI, and relativistic generalizations Practitioners, professionals, and researchers working in computer science, engineering, physics, and mathematics will find a wide range of useful applications in this state-of-the-art survey and reference book. Additionally, advanced graduate students interested in geometric algebra will find the most current applications and methods discussed.

Clifford (geometric) Algebras with Applications to Physics, Mathematics, and Engineering

Download Clifford (geometric) Algebras with Applications to Physics, Mathematics, and Engineering PDF Online Free

Author :
Publisher : Boston : Birkhäuser
ISBN 13 :
Total Pages : 544 pages
Book Rating : 4.3/5 (91 download)

DOWNLOAD NOW!


Book Synopsis Clifford (geometric) Algebras with Applications to Physics, Mathematics, and Engineering by : William Eric Baylis

Download or read book Clifford (geometric) Algebras with Applications to Physics, Mathematics, and Engineering written by William Eric Baylis and published by Boston : Birkhäuser. This book was released on 1996 with total page 544 pages. Available in PDF, EPUB and Kindle. Book excerpt: This volume offers a comprehensive approach to the theoretical, applied and symbolic computational aspects of the subject. Excellent for self-study, leading experts in the field have written on the of topics mentioned above, using an easy approach with efficient geometric language for non-specialists.

Geometric Algebra Computing

Download Geometric Algebra Computing PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 1849961085
Total Pages : 527 pages
Book Rating : 4.8/5 (499 download)

DOWNLOAD NOW!


Book Synopsis Geometric Algebra Computing by : Eduardo Bayro-Corrochano

Download or read book Geometric Algebra Computing written by Eduardo Bayro-Corrochano and published by Springer Science & Business Media. This book was released on 2010-05-19 with total page 527 pages. Available in PDF, EPUB and Kindle. Book excerpt: This useful text offers new insights and solutions for the development of theorems, algorithms and advanced methods for real-time applications across a range of disciplines. Its accessible style is enhanced by examples, figures and experimental analysis.

Geometric Algebra Applications Vol. II

Download Geometric Algebra Applications Vol. II PDF Online Free

Author :
Publisher : Springer Nature
ISBN 13 : 3030349780
Total Pages : 609 pages
Book Rating : 4.0/5 (33 download)

DOWNLOAD NOW!


Book Synopsis Geometric Algebra Applications Vol. II by : Eduardo Bayro-Corrochano

Download or read book Geometric Algebra Applications Vol. II written by Eduardo Bayro-Corrochano and published by Springer Nature. This book was released on 2020-06-19 with total page 609 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book presents a unified mathematical treatment of diverse problems in the general domain of robotics and associated fields using Clifford or geometric alge- bra. By addressing a wide spectrum of problems in a common language, it offers both fresh insights and new solutions that are useful to scientists and engineers working in areas related with robotics. It introduces non-specialists to Clifford and geometric algebra, and provides ex- amples to help readers learn how to compute using geometric entities and geomet- ric formulations. It also includes an in-depth study of applications of Lie group theory, Lie algebra, spinors and versors and the algebra of incidence using the universal geometric algebra generated by reciprocal null cones. Featuring a detailed study of kinematics, differential kinematics and dynamics using geometric algebra, the book also develops Euler Lagrange and Hamiltoni- ans equations for dynamics using conformal geometric algebra, and the recursive Newton-Euler using screw theory in the motor algebra framework. Further, it comprehensively explores robot modeling and nonlinear controllers, and discusses several applications in computer vision, graphics, neurocomputing, quantum com- puting, robotics and control engineering using the geometric algebra framework. The book also includes over 200 exercises and tips for the development of future computer software packages for extensive calculations in geometric algebra, and a entire section focusing on how to write the subroutines in C++, Matlab and Maple to carry out efficient geometric computations in the geometric algebra framework. Lastly, it shows how program code can be optimized for real-time computations. An essential resource for applied physicists, computer scientists, AI researchers, roboticists and mechanical and electrical engineers, the book clarifies and demon- strates the importance of geometric computing for building autonomous systems to advance cognitive systems research.

Clifford (Geometric) Algebras

Download Clifford (Geometric) Algebras PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 1461241049
Total Pages : 522 pages
Book Rating : 4.4/5 (612 download)

DOWNLOAD NOW!


Book Synopsis Clifford (Geometric) Algebras by : William E. Baylis

Download or read book Clifford (Geometric) Algebras written by William E. Baylis and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 522 pages. Available in PDF, EPUB and Kindle. Book excerpt: This volume is an outgrowth of the 1995 Summer School on Theoretical Physics of the Canadian Association of Physicists (CAP), held in Banff, Alberta, in the Canadian Rockies, from July 30 to August 12,1995. The chapters, based on lectures given at the School, are designed to be tutorial in nature, and many include exercises to assist the learning process. Most lecturers gave three or four fifty-minute lectures aimed at relative novices in the field. More emphasis is therefore placed on pedagogy and establishing comprehension than on erudition and superior scholarship. Of course, new and exciting results are presented in applications of Clifford algebras, but in a coherent and user-friendly way to the nonspecialist. The subject area of the volume is Clifford algebra and its applications. Through the geometric language of the Clifford-algebra approach, many concepts in physics are clarified, united, and extended in new and sometimes surprising directions. In particular, the approach eliminates the formal gaps that traditionally separate clas sical, quantum, and relativistic physics. It thereby makes the study of physics more efficient and the research more penetrating, and it suggests resolutions to a major physics problem of the twentieth century, namely how to unite quantum theory and gravity. The term "geometric algebra" was used by Clifford himself, and David Hestenes has suggested its use in order to emphasize its wide applicability, and b& cause the developments by Clifford were themselves based heavily on previous work by Grassmann, Hamilton, Rodrigues, Gauss, and others.

Geometric Algebra Applications Vol. I

Download Geometric Algebra Applications Vol. I PDF Online Free

Author :
Publisher : Springer
ISBN 13 : 3319748300
Total Pages : 753 pages
Book Rating : 4.3/5 (197 download)

DOWNLOAD NOW!


Book Synopsis Geometric Algebra Applications Vol. I by : Eduardo Bayro-Corrochano

Download or read book Geometric Algebra Applications Vol. I written by Eduardo Bayro-Corrochano and published by Springer. This book was released on 2018-06-20 with total page 753 pages. Available in PDF, EPUB and Kindle. Book excerpt: The goal of the Volume I Geometric Algebra for Computer Vision, Graphics and Neural Computing is to present a unified mathematical treatment of diverse problems in the general domain of artificial intelligence and associated fields using Clifford, or geometric, algebra. Geometric algebra provides a rich and general mathematical framework for Geometric Cybernetics in order to develop solutions, concepts and computer algorithms without losing geometric insight of the problem in question. Current mathematical subjects can be treated in an unified manner without abandoning the mathematical system of geometric algebra for instance: multilinear algebra, projective and affine geometry, calculus on manifolds, Riemann geometry, the representation of Lie algebras and Lie groups using bivector algebras and conformal geometry. By treating a wide spectrum of problems in a common language, this Volume I offers both new insights and new solutions that should be useful to scientists, and engineers working in different areas related with the development and building of intelligent machines. Each chapter is written in accessible terms accompanied by numerous examples, figures and a complementary appendix on Clifford algebras, all to clarify the theory and the crucial aspects of the application of geometric algebra to problems in graphics engineering, image processing, pattern recognition, computer vision, machine learning, neural computing and cognitive systems.

Geometric Algebra for Electrical Engineers

Download Geometric Algebra for Electrical Engineers PDF Online Free

Author :
Publisher : Createspace Independent Publishing Platform
ISBN 13 : 9781987598971
Total Pages : 282 pages
Book Rating : 4.5/5 (989 download)

DOWNLOAD NOW!


Book Synopsis Geometric Algebra for Electrical Engineers by : Peeter Joot

Download or read book Geometric Algebra for Electrical Engineers written by Peeter Joot and published by Createspace Independent Publishing Platform. This book was released on 2019-01-29 with total page 282 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book introduces the fundamentals of geometric algebra and calculus, and applies those tools to the study of electromagnetism. Geometric algebra provides a structure that can represent oriented point, line, plane, and volume segments. Vectors, which can be thought of as a representation of oriented line segments, are generalized to multivectors. A full fledged, but non-commutative (i.e. order matters) mul- tiplication operation will be defined for products of vectors. Namely, the square of a vector is the square of its length. This simple rule, along with a requirement that we can sum vectors and their products, essentially defines geometric algebra. Such sums of scalars, vectors and vector products are called multivectors. The reader will see that familiar concepts such as the dot and cross product are related to a more general vector product, and that algebraic structures such as complex numbers can be represented as multivectors. We will be able to utilize generalized complex exponentials to do rotations in arbitrarily oriented planes in space, and will find that simple geometric algebra representations of many geometric transformations are possible. Generalizations of the divergence and Stokes' theorems are required once we choose to work with multivector functions. There is an unfortunate learning curve required to express this gen- eralization, but once overcome, we will be left with a single powerful multivector integration theorem that has no analogue in conventional vector calculus. This fundamental theorem of geo- metric calculus incorporates Green's (area) theorem, the divergence theorem, Stokes' theorems, and complex residue calculus. Multivector calculus also provides the opportunity to define a few unique and powerful Green's functions that almost trivialize solutions of Maxwell's equations. Instead of working separately with electric and magnetic fields, we will work with a hybrid multivector field that includes both electric and magnetic field contributions, and with a mul- tivector current that includes both charge and current densities. The natural representation of Maxwell's equations is a single multivector equation that is easier to solve and manipulate then the conventional mess of divergence and curl equations are familiar to the reader. This book is aimed at graduate or advanced undergraduates in electrical engineering or physics. While all the fundamental results of electromagnetism are derived from Maxwell's equations, there will be no attempt to motivate Maxwell's equations themselves, so existing familiarity with the subject is desirable.

Foundations of Geometric Algebra Computing

Download Foundations of Geometric Algebra Computing PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 3642317944
Total Pages : 217 pages
Book Rating : 4.6/5 (423 download)

DOWNLOAD NOW!


Book Synopsis Foundations of Geometric Algebra Computing by : Dietmar Hildenbrand

Download or read book Foundations of Geometric Algebra Computing written by Dietmar Hildenbrand and published by Springer Science & Business Media. This book was released on 2012-12-31 with total page 217 pages. Available in PDF, EPUB and Kindle. Book excerpt: The author defines “Geometric Algebra Computing” as the geometrically intuitive development of algorithms using geometric algebra with a focus on their efficient implementation, and the goal of this book is to lay the foundations for the widespread use of geometric algebra as a powerful, intuitive mathematical language for engineering applications in academia and industry. The related technology is driven by the invention of conformal geometric algebra as a 5D extension of the 4D projective geometric algebra and by the recent progress in parallel processing, and with the specific conformal geometric algebra there is a growing community in recent years applying geometric algebra to applications in computer vision, computer graphics, and robotics. This book is organized into three parts: in Part I the author focuses on the mathematical foundations; in Part II he explains the interactive handling of geometric algebra; and in Part III he deals with computing technology for high-performance implementations based on geometric algebra as a domain-specific language in standard programming languages such as C++ and OpenCL. The book is written in a tutorial style and readers should gain experience with the associated freely available software packages and applications. The book is suitable for students, engineers, and researchers in computer science, computational engineering, and mathematics.

Clifford Algebras

Download Clifford Algebras PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 1461220440
Total Pages : 635 pages
Book Rating : 4.4/5 (612 download)

DOWNLOAD NOW!


Book Synopsis Clifford Algebras by : Rafal Ablamowicz

Download or read book Clifford Algebras written by Rafal Ablamowicz and published by Springer Science & Business Media. This book was released on 2012-12-06 with total page 635 pages. Available in PDF, EPUB and Kindle. Book excerpt: The invited papers in this volume provide a detailed examination of Clifford algebras and their significance to analysis, geometry, mathematical structures, physics, and applications in engineering. While the papers collected in this volume require that the reader possess a solid knowledge of appropriate background material, they lead to the most current research topics. With its wide range of topics, well-established contributors, and excellent references and index, this book will appeal to graduate students and researchers.

Clifford Algebra to Geometric Calculus

Download Clifford Algebra to Geometric Calculus PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 9789027725615
Total Pages : 340 pages
Book Rating : 4.7/5 (256 download)

DOWNLOAD NOW!


Book Synopsis Clifford Algebra to Geometric Calculus by : David Hestenes

Download or read book Clifford Algebra to Geometric Calculus written by David Hestenes and published by Springer Science & Business Media. This book was released on 1984 with total page 340 pages. Available in PDF, EPUB and Kindle. Book excerpt: Matrix algebra has been called "the arithmetic of higher mathematics" [Be]. We think the basis for a better arithmetic has long been available, but its versatility has hardly been appreciated, and it has not yet been integrated into the mainstream of mathematics. We refer to the system commonly called 'Clifford Algebra', though we prefer the name 'Geometric Algebra' suggested by Clifford himself. Many distinct algebraic systems have been adapted or developed to express geometric relations and describe geometric structures. Especially notable are those algebras which have been used for this purpose in physics, in particular, the system of complex numbers, the quaternions, matrix algebra, vector, tensor and spinor algebras and the algebra of differential forms. Each of these geometric algebras has some significant advantage over the others in certain applications, so no one of them provides an adequate algebraic structure for all purposes of geometry and physics. At the same time, the algebras overlap considerably, so they provide several different mathematical representations for individual geometrical or physical ideas.

Geometric Algebra for Computer Science

Download Geometric Algebra for Computer Science PDF Online Free

Author :
Publisher : Elsevier
ISBN 13 : 0080553109
Total Pages : 664 pages
Book Rating : 4.0/5 (85 download)

DOWNLOAD NOW!


Book Synopsis Geometric Algebra for Computer Science by : Leo Dorst

Download or read book Geometric Algebra for Computer Science written by Leo Dorst and published by Elsevier. This book was released on 2010-07-26 with total page 664 pages. Available in PDF, EPUB and Kindle. Book excerpt: Until recently, almost all of the interactions between objects in virtual 3D worlds have been based on calculations performed using linear algebra. Linear algebra relies heavily on coordinates, however, which can make many geometric programming tasks very specific and complex-often a lot of effort is required to bring about even modest performance enhancements. Although linear algebra is an efficient way to specify low-level computations, it is not a suitable high-level language for geometric programming. Geometric Algebra for Computer Science presents a compelling alternative to the limitations of linear algebra. Geometric algebra, or GA, is a compact, time-effective, and performance-enhancing way to represent the geometry of 3D objects in computer programs. In this book you will find an introduction to GA that will give you a strong grasp of its relationship to linear algebra and its significance for your work. You will learn how to use GA to represent objects and perform geometric operations on them. And you will begin mastering proven techniques for making GA an integral part of your applications in a way that simplifies your code without slowing it down. * The first book on Geometric Algebra for programmers in computer graphics and entertainment computing * Written by leaders in the field providing essential information on this new technique for 3D graphics * This full colour book includes a website with GAViewer, a program to experiment with GA

The Power of Geometric Algebra Computing

Download The Power of Geometric Algebra Computing PDF Online Free

Author :
Publisher : CRC Press
ISBN 13 : 1000461165
Total Pages : 202 pages
Book Rating : 4.0/5 (4 download)

DOWNLOAD NOW!


Book Synopsis The Power of Geometric Algebra Computing by : Dietmar Hildenbrand

Download or read book The Power of Geometric Algebra Computing written by Dietmar Hildenbrand and published by CRC Press. This book was released on 2021-09-30 with total page 202 pages. Available in PDF, EPUB and Kindle. Book excerpt: Geometric Algebra is a very powerful mathematical system for an easy and intuitive treatment of geometry, but the community working with it is still very small. The main goal of this book is to close this gap from a computing perspective in presenting the power of Geometric Algebra Computing for engineering applications and quantum computing. The Power of Geometric Algebra Computing is based on GAALOPWeb, a new user-friendly, web-based tool for the generation of optimized code for different programming languages as well as for the visualization of Geometric Algebra algorithms for a wide range of engineering applications. Key Features: Introduces a new web-based optimizer for Geometric Algebra algorithms Supports many programming languages as well as hardware Covers the advantages of high-dimensional algebras Includes geometrically intuitive support of quantum computing This book includes applications from the fields of computer graphics, robotics and quantum computing and will help students, engineers and researchers interested in really computing with Geometric Algebra.

Understanding Geometric Algebra for Electromagnetic Theory

Download Understanding Geometric Algebra for Electromagnetic Theory PDF Online Free

Author :
Publisher : John Wiley & Sons
ISBN 13 : 0470941634
Total Pages : 320 pages
Book Rating : 4.4/5 (79 download)

DOWNLOAD NOW!


Book Synopsis Understanding Geometric Algebra for Electromagnetic Theory by : John W. Arthur

Download or read book Understanding Geometric Algebra for Electromagnetic Theory written by John W. Arthur and published by John Wiley & Sons. This book was released on 2011-09-13 with total page 320 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book aims to disseminate geometric algebra as a straightforward mathematical tool set for working with and understanding classical electromagnetic theory. It's target readership is anyone who has some knowledge of electromagnetic theory, predominantly ordinary scientists and engineers who use it in the course of their work, or postgraduate students and senior undergraduates who are seeking to broaden their knowledge and increase their understanding of the subject. It is assumed that the reader is not a mathematical specialist and is neither familiar with geometric algebra or its application to electromagnetic theory. The modern approach, geometric algebra, is the mathematical tool set we should all have started out with and once the reader has a grasp of the subject, he or she cannot fail to realize that traditional vector analysis is really awkward and even misleading by comparison. Professors can request a solutions manual by email: [email protected]

Guide to Geometric Algebra in Practice

Download Guide to Geometric Algebra in Practice PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 0857298119
Total Pages : 458 pages
Book Rating : 4.8/5 (572 download)

DOWNLOAD NOW!


Book Synopsis Guide to Geometric Algebra in Practice by : Leo Dorst

Download or read book Guide to Geometric Algebra in Practice written by Leo Dorst and published by Springer Science & Business Media. This book was released on 2011-08-28 with total page 458 pages. Available in PDF, EPUB and Kindle. Book excerpt: This highly practical Guide to Geometric Algebra in Practice reviews algebraic techniques for geometrical problems in computer science and engineering, and the relationships between them. The topics covered range from powerful new theoretical developments, to successful applications, and the development of new software and hardware tools. Topics and features: provides hands-on review exercises throughout the book, together with helpful chapter summaries; presents a concise introductory tutorial to conformal geometric algebra (CGA) in the appendices; examines the application of CGA for the description of rigid body motion, interpolation and tracking, and image processing; reviews the employment of GA in theorem proving and combinatorics; discusses the geometric algebra of lines, lower-dimensional algebras, and other alternatives to 5-dimensional CGA; proposes applications of coordinate-free methods of GA for differential geometry.

Geometric Algebra for Physicists

Download Geometric Algebra for Physicists PDF Online Free

Author :
Publisher : Cambridge University Press
ISBN 13 : 1139643142
Total Pages : 647 pages
Book Rating : 4.1/5 (396 download)

DOWNLOAD NOW!


Book Synopsis Geometric Algebra for Physicists by : Chris Doran

Download or read book Geometric Algebra for Physicists written by Chris Doran and published by Cambridge University Press. This book was released on 2007-11-22 with total page 647 pages. Available in PDF, EPUB and Kindle. Book excerpt: Geometric algebra is a powerful mathematical language with applications across a range of subjects in physics and engineering. This book is a complete guide to the current state of the subject with early chapters providing a self-contained introduction to geometric algebra. Topics covered include new techniques for handling rotations in arbitrary dimensions, and the links between rotations, bivectors and the structure of the Lie groups. Following chapters extend the concept of a complex analytic function theory to arbitrary dimensions, with applications in quantum theory and electromagnetism. Later chapters cover advanced topics such as non-Euclidean geometry, quantum entanglement, and gauge theories. Applications such as black holes and cosmic strings are also explored. It can be used as a graduate text for courses on the physical applications of geometric algebra and is also suitable for researchers working in the fields of relativity and quantum theory.

Geometric Computing with Clifford Algebras

Download Geometric Computing with Clifford Algebras PDF Online Free

Author :
Publisher : Springer Science & Business Media
ISBN 13 : 3662046210
Total Pages : 559 pages
Book Rating : 4.6/5 (62 download)

DOWNLOAD NOW!


Book Synopsis Geometric Computing with Clifford Algebras by : Gerald Sommer

Download or read book Geometric Computing with Clifford Algebras written by Gerald Sommer and published by Springer Science & Business Media. This book was released on 2013-06-29 with total page 559 pages. Available in PDF, EPUB and Kindle. Book excerpt: This monograph-like anthology introduces the concepts and framework of Clifford algebra. It provides a rich source of examples of how to work with this formalism. Clifford or geometric algebra shows strong unifying aspects and turned out in the 1960s to be a most adequate formalism for describing different geometry-related algebraic systems as specializations of one "mother algebra" in various subfields of physics and engineering. Recent work shows that Clifford algebra provides a universal and powerful algebraic framework for an elegant and coherent representation of various problems occurring in computer science, signal processing, neural computing, image processing, pattern recognition, computer vision, and robotics.