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In mathematics, **multilinear algebra** extends the methods of linear algebra. Just as linear algebra is built on the concept of a vector and develops the theory of vector spaces, multilinear algebra builds on the concepts of *p*-vectors and multivectors with Grassmann algebra.

In a vector space of dimension *n*, one usually considers only the vectors. According to Hermann Grassmann and others, this presumption misses the complexity of considering the structures of pairs, triples, and general multivectors. Since there are several combinatorial possibilities, the space of multivectors turns out to have 2^{n} dimensions. The abstract formulation of the determinant is the most immediate application. Multilinear algebra also has applications in mechanical study of material response to stress and strain with various moduli of elasticity. This practical reference led to the use of the word tensor to describe the elements of the multilinear space. The extra structure in a multilinear space has led it to play an important role in various studies in higher mathematics. Though Grassmann started the subject in 1844 with his *Ausdehnungslehre*, and republished in 1862, his work was slow to find acceptance as ordinary linear algebra provided sufficient challenges to comprehension.

The topic of multilinear algebra is applied in some studies of multivariate calculus and manifolds where the Jacobian matrix comes into play. The infinitesimal differentials of single variable calculus become differential forms in multivariate calculus, and their manipulation is done with exterior algebra.

After Grassmann, developments in multilinear algebra were made in 1872 by Victor Schlegel when he published the first part of his *System der Raumlehre*, and by Elwin Bruno Christoffel. A major advance in multilinear algebra came in the work of Gregorio Ricci-Curbastro and Tullio Levi-Civita (see references). It was the *absolute differential calculus* form of multilinear algebra that Marcel Grossmann and Michele Besso introduced to Albert Einstein. The publication in 1915 by Einstein of a general relativity explanation for the precession of the perihelion of Mercury, established multilinear algebra and tensors as physically important mathematics.

Around the middle of the 20th century the study of tensors was reformulated more abstractly. The Bourbaki group's treatise *Multilinear Algebra* was especially influential—in fact the term *multilinear algebra* may have originated there.^{[ citation needed ]}

One reason at the time was a new area of application, homological algebra. The development of algebraic topology during the 1940s gave additional incentive for the development of a purely algebraic treatment of the tensor product. The computation of the homology groups of the product of two topological spaces involves the tensor product; but only in the simplest cases, such as a torus, is it directly calculated in that fashion (see Künneth theorem). The topological phenomena were subtle enough to need better foundational concepts; technically speaking, the Tor functors had to be defined.

The material to organise was quite extensive, including also ideas going back to Hermann Grassmann, the ideas from the theory of differential forms that had led to de Rham cohomology, as well as more elementary ideas such as the wedge product that generalises the cross product.

The resulting rather severe write-up of the topic, by Bourbaki, entirely rejected one approach in vector calculus (the quaternion route, that is, in the general case, the relation with Lie groups), and instead applied a novel approach using category theory, with the Lie group approach viewed as a separate matter. Since this leads to a much cleaner treatment, there was probably no going back in purely mathematical terms. (Strictly, the universal property approach was invoked; this is somewhat more general than category theory, and the relationship between the two as alternate ways was also being clarified, at the same time.)

Indeed, what was done is almost precisely to explain that *tensor spaces* are the constructions required to reduce multilinear problems to linear problems. This purely algebraic attack conveys no geometric intuition.

By re-expressing problems in terms of multilinear algebra, there is a clear and well-defined "best solution": the constraints the solution exerts are exactly those needed in practice. In general there is no need to invoke any *ad hoc* construction, geometric idea, or recourse to co-ordinate systems. In the category-theoretic jargon, everything is entirely * natural *.

In principle, the abstract approach can recover everything done via the traditional approach. In practice this may not seem so simple. On the other hand, the notion of *naturality* is consistent with the * general covariance * principle of general relativity. The latter deals with tensor fields (tensors varying from point to point on a manifold), but covariance asserts that the language of tensors is essential to the proper formulation of general relativity.

Some decades later the rather abstract view coming from category theory was tied up with the approach that had been developed in the 1930s by Hermann Weyl ^{[ how? ]} (by working through general relativity via abstract tensor analysis, and additionally in his book *The Classical Groups*). In a way this took the theory full circle, connecting once more the content of old and new viewpoints.

The subject matter of multilinear algebra has evolved less than the presentation down the years. Here are further pages centrally relevant to it:

- Bilinear operator
- Component-free treatment of tensors
- Cramer's rule
- Dual space
- Einstein notation
- Exterior algebra
- Exterior derivative
- Inner product
- Kronecker delta
- Levi-Civita symbol
- Metric tensor
- Mixed tensor
- Multilinear map
- Multilinear form
- Symmetric algebra, Symmetric power
- Symmetric tensor
- Tensor
- Tensor algebra, Free algebra
- Tensor contraction

There is also a glossary of tensor theory.

Some of the ways in which multilinear algebra concepts are applied:

**Differential geometry** is a mathematical discipline that uses the techniques of differential calculus, integral calculus, linear algebra and multilinear algebra to study problems in geometry. The theory of plane and space curves and surfaces in the three-dimensional Euclidean space formed the basis for development of differential geometry during the 18th century and the 19th century.

In mathematics, the **geometric algebra** (**GA**) of a vector space with a quadratic form is an algebra over a field, the Clifford algebra of a vector space with a quadratic form with its multiplication operation called the **geometric product**. The algebra elements are called multivectors, which contains both the scalars and the vector space .

In mathematics, a **tensor** is an algebraic object that describes a multilinear relationship between sets of algebraic objects related to a vector space. Objects that tensors may map between include vectors and scalars, and even other tensors. There are many types of tensors, including scalars and vectors, dual vectors, multilinear maps between vector spaces, and even some operations such as the dot product. Tensors are defined independent of any basis, although they are often referred to by their components in a basis related to a particular coordinate system.

**Hermann Günther Grassmann** was a German polymath, known in his day as a linguist and now also as a mathematician. He was also a physicist, general scholar, and publisher. His mathematical work was little noted until he was in his sixties.

In mathematics, the **exterior product** or **wedge product** of vectors is an algebraic construction used in geometry to study areas, volumes, and their higher-dimensional analogues. The exterior product of two vectors and , denoted by , is called a bivector and lives in a space called the *exterior square*, a vector space that is distinct from the original space of vectors. The magnitude of can be interpreted as the area of the parallelogram with sides and , which in three dimensions can also be computed using the cross product of the two vectors. More generally, all parallel plane surfaces with the same orientation and area have the same bivector as a measure of their oriented area. Like the cross product, the exterior product is anticommutative, meaning that for all vectors and , but, unlike the cross product, the exterior product is associative.

In mathematics, the modern component-free approach to the theory of a **tensor** views a tensor as an abstract object, expressing some definite type of multilinear concept. Their properties can be derived from their definitions, as linear maps or more generally; and the rules for manipulations of tensors arise as an extension of linear algebra to multilinear algebra.

In mathematics and physics, a **tensor field** assigns a tensor to each point of a mathematical space. Tensor fields are used in differential geometry, algebraic geometry, general relativity, in the analysis of stress and strain in materials, and in numerous applications in the physical sciences. As a tensor is a generalization of a scalar and a vector, a tensor field is a generalization of a scalar field or vector field that assigns, respectively, a scalar or vector to each point of space.

This is a **glossary of tensor theory**. For expositions of tensor theory from different points of view, see:

**Tullio Levi-Civita**, was an Italian mathematician, most famous for his work on absolute differential calculus and its applications to the theory of relativity, but who also made significant contributions in other areas. He was a pupil of Gregorio Ricci-Curbastro, the inventor of tensor calculus. His work included foundational papers in both pure and applied mathematics, celestial mechanics, analytic mechanics and hydrodynamics.

**Gregorio Ricci-Curbastro** was an Italian mathematician. He is most famous as the inventor of tensor calculus, but also published important works in other fields.

In differential geometry, an **affine connection** is a geometric object on a smooth manifold which *connects* nearby tangent spaces, so it permits tangent vector fields to be differentiated as if they were functions on the manifold with values in a fixed vector space. Connections are among the simplest methods of defining differentiation of the sections of vector bundles.

The **mathematics of general relativity** refers to various mathematical structures and techniques that are used in studying and formulating Albert Einstein's theory of general relativity. The main tools used in this geometrical theory of gravitation are tensor fields defined on a Lorentzian manifold representing spacetime. This article is a general description of the mathematics of general relativity.

In mathematics, a **differentiable manifold** is a type of manifold that is locally similar enough to a linear space to allow one to do calculus. Any manifold can be described by a collection of charts, also known as an atlas. One may then apply ideas from calculus while working within the individual charts, since each chart lies within a linear space to which the usual rules of calculus apply. If the charts are suitably compatible, then computations done in one chart are valid in any other differentiable chart.

This is a list of **contributors to the mathematical background for general relativity**. For ease of readability, the contributions are unlinked but can be found in the contributors' article.

In multilinear algebra, a **multivector**, sometimes called **Clifford number**, is an element of the exterior algebra Λ(*V*) of a vector space V. This algebra is graded, associative and alternating, and consists of linear combinations of **simple***k*-vectors of the form

A **rotor** is an object in the geometric algebra of a vector space that represents a rotation about the origin. The term originated with William Kingdon Clifford, in showing that the quaternion algebra is just a special case of Hermann Grassmann's "theory of extension" (Ausdehnungslehre). Hestenes defined a rotor to be any element of a geometric algebra that can be written as the product of an even number of unit vectors and satisfies , where is the "reverse" of —that is, the product of the same vectors, but in reverse order.

**Tensor software** is a class of mathematical software designed for manipulation and calculation with tensors.

**Robert C. Hermann** was an American mathematician and mathematical physicist. In the 1960s Hermann worked on elementary particle physics and quantum field theory, and published books which revealed the interconnections between vector bundles on Riemannian manifolds and gauge theory in physics, before these interconnections became "common knowledge" among physicists in the 1970s.

- Hermann Grassmann (2000)
*Extension Theory*, American Mathematical Society. Translation by Lloyd Kannenberg of the 1862*Ausdehnungslehre*. - Fleming, Wendell H. (1977). "Exterior algebra and differential calculus".
*Functions of several variables*(2nd ed.). New York: Springer-Verlag. ISBN 0-387-90206-6. OCLC 2401829. - Ricci-Curbastro, Gregorio; Levi-Civita, Tullio (1900), "Méthodes de calcul différentiel absolu et leurs applications",
*Mathematische Annalen*,**54**(1): 125–201, doi:10.1007/BF01454201, ISSN 1432-1807, S2CID 120009332 - Ronald Shaw (1983) "Multilinear algebra and group representations", volume 2 of
*Linear Algebra and Group Representations*, Academic Press ISBN 0-12-639202-1.

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