In mathematics, a **quadratic form** is a polynomial with terms all of degree two ("form" is another name for a homogeneous polynomial). For example,

- Introduction
- History
- Real quadratic forms
- Definitions
- Quadratic spaces
- Generalization
- Related concepts
- Equivalence of forms
- Geometric meaning
- Integral quadratic forms
- Historical use
- Universal quadratic forms
- See also
- Notes
- References
- Further reading
- External links

is a quadratic form in the variables x and y. The coefficients usually belong to a fixed field K, such as the real or complex numbers, and one speaks of a quadratic form over K. If , and the quadratic form takes zero only when all variables are simultaneously zero, then it is a definite quadratic form, otherwise it is an isotropic quadratic form.

Quadratic forms occupy a central place in various branches of mathematics, including number theory, linear algebra, group theory (orthogonal group), differential geometry (Riemannian metric, second fundamental form), differential topology (intersection forms of four-manifolds), and Lie theory (the Killing form).

Quadratic forms are not to be confused with a quadratic equation, which has only one variable and includes terms of degree two or less. A quadratic form is one case of the more general concept of homogeneous polynomials.

Quadratic forms are homogeneous quadratic polynomials in *n* variables. In the cases of one, two, and three variables they are called **unary**, ** binary **, and **ternary** and have the following explicit form:

where *a*, …, *f* are the **coefficients**.^{ [1] }

The notation is often used^{[ citation needed ]} for the quadratic form

The theory of quadratic forms and methods used in their study depend in a large measure on the nature of the coefficients, which may be real or complex numbers, rational numbers, or integers. In linear algebra, analytic geometry, and in the majority of applications of quadratic forms, the coefficients are real or complex numbers. In the algebraic theory of quadratic forms, the coefficients are elements of a certain field. In the arithmetic theory of quadratic forms, the coefficients belong to a fixed commutative ring, frequently the integers **Z** or the *p*-adic integers **Z**_{p}.^{ [2] } Binary quadratic forms have been extensively studied in number theory, in particular, in the theory of quadratic fields, continued fractions, and modular forms. The theory of integral quadratic forms in *n* variables has important applications to algebraic topology.

Using homogeneous coordinates, a non-zero quadratic form in *n* variables defines an (*n*−2)-dimensional quadric in the (*n*−1)-dimensional projective space. This is a basic construction in projective geometry. In this way one may visualize 3-dimensional real quadratic forms as conic sections. An example is given by the three-dimensional Euclidean space and the square of the Euclidean norm expressing the distance between a point with coordinates (*x*, *y*, *z*) and the origin:

A closely related notion with geometric overtones is a **quadratic space**, which is a pair (*V*, *q*), with *V* a vector space over a field *K*, and *q* : *V* → *K* a quadratic form on *V*.

The study of particular quadratic forms, in particular the question of whether a given integer can be the value of a quadratic form over the integers, dates back many centuries. One such case is Fermat's theorem on sums of two squares, which determines when an integer may be expressed in the form *x*^{2} + *y*^{2}, where *x*, *y* are integers. This problem is related to the problem of finding Pythagorean triples, which appeared in the second millennium B.C.^{ [3] }

In 628, the Indian mathematician Brahmagupta wrote * Brāhmasphuṭasiddhānta *, which includes, among many other things, a study of equations of the form *x*^{2} − *ny*^{2} = *c*. In particular he considered what is now called Pell's equation, *x*^{2} − *ny*^{2} = 1, and found a method for its solution.^{ [4] } In Europe this problem was studied by Brouncker, Euler and Lagrange.

In 1801 Gauss published * Disquisitiones Arithmeticae,* a major portion of which was devoted to a complete theory of binary quadratic forms over the integers. Since then, the concept has been generalized, and the connections with quadratic number fields, the modular group, and other areas of mathematics have been further elucidated.

Any *n*×*n* real symmetric matrix *A* determines a quadratic form *q*_{A} in *n* variables by the formula

Conversely, given a quadratic form in *n* variables, its coefficients can be arranged into an *n* × *n* symmetric matrix.

An important question in the theory of quadratic forms is how to simplify a quadratic form *q* by a homogeneous linear change of variables. A fundamental theorem due to Jacobi asserts that a real quadratic form *q* has an orthogonal diagonalization.^{ [5] }

so that the corresponding symmetric matrix is diagonal, and this is accomplished with a change of variables given by an orthogonal matrix – in this case the coefficients *λ*_{1}, *λ*_{2}, ..., *λ*_{n} are determined uniquely up to a permutation.

There always exists a change of variables given by an invertible matrix, not necessarily orthogonal, such that the coefficients *λ*_{i} are 0, 1, and −1. Sylvester's law of inertia states that the numbers of each 1 and −1 are invariants of the quadratic form, in the sense that any other diagonalization will contain the same number of each. The **signature** of the quadratic form is the triple (*n*_{0}, *n*_{+}, *n*_{−}), where *n*_{0} is the number of 0s and *n*_{±} is the number of ±1s. Sylvester's law of inertia shows that this is a well-defined quantity attached to the quadratic form. The case when all *λ*_{i} have the same sign is especially important: in this case the quadratic form is called ** positive definite ** (all 1) or **negative definite** (all −1). If none of the terms are 0, then the form is called **nondegenerate**; this includes positive definite, negative definite, and indefinite (a mix of 1 and −1); equivalently, a nondegenerate quadratic form is one whose associated symmetric form is a nondegenerate *bilinear* form. A real vector space with an indefinite nondegenerate quadratic form of index (*p*, *q*) (denoting *p* 1s and *q* −1s) is often denoted as **R**^{p,q} particularly in the physical theory of spacetime.

The discriminant of a quadratic form, concretely the class of the determinant of a representing matrix in *K*/(*K*^{×})^{2} (up to non-zero squares) can also be defined, and for a real quadratic form is a cruder invariant than signature, taking values of only “positive, zero, or negative”. Zero corresponds to degenerate, while for a non-degenerate form it is the parity of the number of negative coefficients,

These results are reformulated in a different way below.

Let *q* be a quadratic form defined on an *n*-dimensional real vector space. Let *A* be the matrix of the quadratic form *q* in a given basis. This means that *A* is a symmetric *n* × *n* matrix such that

where *x* is the column vector of coordinates of *v* in the chosen basis. Under a change of basis, the column *x* is multiplied on the left by an *n* × *n* invertible matrix *S*, and the symmetric square matrix *A* is transformed into another symmetric square matrix *B* of the same size according to the formula

Any symmetric matrix *A* can be transformed into a diagonal matrix

by a suitable choice of an orthogonal matrix *S*, and the diagonal entries of *B* are uniquely determined – this is Jacobi's theorem. If *S* is allowed to be any invertible matrix then *B* can be made to have only 0,1, and −1 on the diagonal, and the number of the entries of each type (*n*_{0} for 0, *n*_{+} for 1, and *n*_{−} for −1) depends only on *A*. This is one of the formulations of Sylvester's law of inertia and the numbers *n*_{+} and *n*_{−} are called the **positive** and **negative****indices of inertia**. Although their definition involved a choice of basis and consideration of the corresponding real symmetric matrix *A*, Sylvester's law of inertia means that they are invariants of the quadratic form *q*.

The quadratic form *q* is positive definite (resp., negative definite) if *q*(*v*) > 0 (resp., *q*(*v*) < 0) for every nonzero vector *v*.^{ [6] } When *q*(*v*) assumes both positive and negative values, *q* is an **indefinite** quadratic form. The theorems of Jacobi and Sylvester show that any positive definite quadratic form in *n* variables can be brought to the sum of *n* squares by a suitable invertible linear transformation: geometrically, there is only *one* positive definite real quadratic form of every dimension. Its isometry group is a * compact * orthogonal group O(*n*). This stands in contrast with the case of indefinite forms, when the corresponding group, the indefinite orthogonal group O(*p*, *q*), is non-compact. Further, the isometry groups of *Q* and −*Q* are the same (O(*p*, *q*) ≈ O(*q*, *p*)), but the associated Clifford algebras (and hence pin groups) are different.

A **quadratic form** over a field *K* is a map from a finite dimensional *K* vector space to *K* such that for all and the function is bilinear.

More concretely, an *n*-ary **quadratic form** over a field *K* is a homogeneous polynomial of degree 2 in *n* variables with coefficients in *K*:

This formula may be rewritten using matrices: let *x* be the column vector with components *x*_{1}, ..., *x*_{n} and *A* = (*a*_{ij}) be the *n*×*n* matrix over *K* whose entries are the coefficients of *q*. Then

A vector is a null vector if *q*(*v*) = 0.

Two *n*-ary quadratic forms *φ* and *ψ* over *K* are **equivalent** if there exists a nonsingular linear transformation *C* ∈ GL(*n*, *K*) such that

Let the characteristic of *K* be different from 2.^{ [7] } The coefficient matrix *A* of *q* may be replaced by the symmetric matrix (*A* + *A*^{T})/2 with the same quadratic form, so it may be assumed from the outset that *A* is symmetric. Moreover, a symmetric matrix *A* is uniquely determined by the corresponding quadratic form. Under an equivalence *C*, the symmetric matrix *A* of *φ* and the symmetric matrix *B* of *ψ* are related as follows:

The **associated bilinear form** of a quadratic form *q* is defined by

Thus, *b*_{q} is a symmetric bilinear form over *K* with matrix *A*. Conversely, any symmetric bilinear form *b* defines a quadratic form

and these two processes are the inverses of each other. As a consequence, over a field of characteristic not equal to 2, the theories of symmetric bilinear forms and of quadratic forms in *n* variables are essentially the same.

A quadratic form *q* in *n* variables over *K* induces a map from the *n*-dimensional coordinate space *K*^{n} into *K*:

The map *Q* is a homogeneous function of degree 2, which means that it has the property that, for all *a* in *K* and *v* in *V*:

When the characteristic of *K* is not 2, the bilinear map *B* : *V* × *V* → *K* over *K* is defined:

This bilinear form *B* is symmetric. That is, *B*(*x*, *y*) = *B*(*y*, *x*) for all *x*, *y* in *V*, and it determines *Q*: *Q*(*x*) = *B*(*x*, *x*) for all *x* in *V*.

When the characteristic of *K* is 2, so that 2 is not a unit, it is still possible to use a quadratic form to define a symmetric bilinear form *B*′(*x*, *y*) = *Q*(*x* + *y*) − *Q*(*x*) − *Q*(*y*). However, *Q*(*x*) can no longer be recovered from this *B*′ in the same way, since *B*′(*x*, *x*) = 0 for all *x* (and is thus alternating^{ [8] }). Alternatively, there always exists a bilinear form *B*″ (not in general either unique or symmetric) such that *B*″(*x*, *x*) = *Q*(*x*).

The pair (*V*, *Q*) consisting of a finite-dimensional vector space *V* over *K* and a quadratic map *Q* from *V* to *K* is called a **quadratic space**, and *B* as defined here is the associated symmetric bilinear form of *Q*. The notion of a quadratic space is a coordinate-free version of the notion of quadratic form. Sometimes, *Q* is also called a quadratic form.

Two *n*-dimensional quadratic spaces (*V*, *Q*) and (*V*′, *Q*′) are **isometric** if there exists an invertible linear transformation *T* : *V* → *V*′ (**isometry**) such that

The isometry classes of *n*-dimensional quadratic spaces over *K* correspond to the equivalence classes of *n*-ary quadratic forms over *K*.

Let *R* be a commutative ring, *M* be an *R*-module, and *b* : *M* × *M* → *R* be an *R*-bilinear form.^{ [9] } A mapping *q* : *M* → *R* : *v* ↦ *b*(*v*, *v*) is the *associated quadratic form* of *b*, and *B* : *M* × *M* → *R* : (*u*, *v*) ↦ *q*(*u* + *v*) − *q*(*u*) − *q*(*v*) is the *polar form* of *q*.

A quadratic form *q* : *M* → *R* may be characterized in the following equivalent ways:

- There exists an
*R*-bilinear form*b*:*M*×*M*→*R*such that*q*(*v*) is the associated quadratic form. *q*(*av*) =*a*^{2}*q*(*v*) for all*a*∈*R*and*v*∈*M*, and the polar form of*q*is*R*-bilinear.

Two elements *v* and *w* of *V* are called ** orthogonal ** if *B*(*v*, *w*) = 0. The **kernel** of a bilinear form *B* consists of the elements that are orthogonal to every element of *V*. *Q* is **non-singular** if the kernel of its associated bilinear form is {0}. If there exists a non-zero *v* in *V* such that *Q*(*v*) = 0, the quadratic form *Q* is ** isotropic **, otherwise it is **anisotropic**. This terminology also applies to vectors and subspaces of a quadratic space. If the restriction of *Q* to a subspace *U* of *V* is identically zero, then *U* is **totally singular**.

The orthogonal group of a non-singular quadratic form *Q* is the group of the linear automorphisms of *V* that preserve *Q*: that is, the group of isometries of (*V*, *Q*) into itself.

If a quadratic space (*A*, *Q*) has a product so that *A* is an algebra over a field, and satisfies

- then it is a composition algebra.

Every quadratic form *q* in *n* variables over a field of characteristic not equal to 2 is equivalent to a **diagonal form**

Such a diagonal form is often denoted by Classification of all quadratic forms up to equivalence can thus be reduced to the case of diagonal forms.

Using Cartesian coordinates in three dimensions, let , and let be a symmetric 3-by-3 matrix. Then the geometric nature of the solution set of the equation depends on the eigenvalues of the matrix .

If all eigenvalues of are non-zero, then the solution set is an ellipsoid or a hyperboloid ^{[ citation needed ]}. If all the eigenvalues are positive, then it is an ellipsoid; if all the eigenvalues are negative, then it is an *imaginary ellipsoid* (we get the equation of an ellipsoid but with imaginary radii); if some eigenvalues are positive and some are negative, then it is a hyperboloid.

If there exist one or more eigenvalues , then the shape depends on the corresponding . If the corresponding , then the solution set is a paraboloid (either elliptic or hyperbolic); if the corresponding , then the dimension degenerates and does not come into play, and the geometric meaning will be determined by other eigenvalues and other components of . When the solution set is a paraboloid, whether it is elliptic or hyperbolic is determined by whether all other non-zero eigenvalues are of the same sign: if they are, then it is elliptic; otherwise, it is hyperbolic.

Quadratic forms over the ring of integers are called **integral quadratic forms**, whereas the corresponding modules are **quadratic lattices** (sometimes, simply lattices). They play an important role in number theory and topology.

An integral quadratic form has integer coefficients, such as *x*^{2} + *xy* + *y*^{2}; equivalently, given a lattice Λ in a vector space *V* (over a field with characteristic 0, such as **Q** or **R**), a quadratic form *Q* is integral *with respect to* Λ if and only if it is integer-valued on Λ, meaning *Q*(*x*, *y*) ∈ **Z** if *x*, *y* ∈ Λ.

This is the current use of the term; in the past it was sometimes used differently, as detailed below.

Historically there was some confusion and controversy over whether the notion of **integral quadratic form** should mean:

*twos in*- the quadratic form associated to a symmetric matrix with integer coefficients
*twos out*- a polynomial with integer coefficients (so the associated symmetric matrix may have half-integer coefficients off the diagonal)

This debate was due to the confusion of quadratic forms (represented by polynomials) and symmetric bilinear forms (represented by matrices), and "twos out" is now the accepted convention; "twos in" is instead the theory of integral symmetric bilinear forms (integral symmetric matrices).

In "twos in", binary quadratic forms are of the form , represented by the symmetric matrix

this is the convention Gauss uses in * Disquisitiones Arithmeticae *.

In "twos out", binary quadratic forms are of the form , represented by the symmetric matrix

Several points of view mean that *twos out* has been adopted as the standard convention. Those include:

- better understanding of the 2-adic theory of quadratic forms, the 'local' source of the difficulty;
- the lattice point of view, which was generally adopted by the experts in the arithmetic of quadratic forms during the 1950s;
- the actual needs for integral quadratic form theory in topology for intersection theory;
- the Lie group and algebraic group aspects.

An integral quadratic form whose image consists of all the positive integers is sometimes called *universal*. Lagrange's four-square theorem shows that is universal. Ramanujan generalized this to and found 54 multisets {*a*, *b*, *c*, *d*} that can each generate all positive integers, namely,

- {1, 1, 1,
*d*}, 1 ≤*d*≤ 7 - {1, 1, 2,
*d*}, 2 ≤*d*≤ 14 - {1, 1, 3,
*d*}, 3 ≤*d*≤ 6 - {1, 2, 2,
*d*}, 2 ≤*d*≤ 7 - {1, 2, 3,
*d*}, 3 ≤*d*≤ 10 - {1, 2, 4,
*d*}, 4 ≤*d*≤ 14 - {1, 2, 5,
*d*}, 6 ≤*d*≤ 10

There are also forms whose image consists of all but one of the positive integers. For example, {1,2,5,5} has 15 as the exception. Recently, the 15 and 290 theorems have completely characterized universal integral quadratic forms: if all coefficients are integers, then it represents all positive integers if and only if it represents all integers up through 290; if it has an integral matrix, it represents all positive integers if and only if it represents all integers up through 15.

- ↑ A tradition going back to Gauss dictates the use of manifestly even coefficients for the products of distinct variables, that is, 2
*b*in place of*b*in binary forms and 2*b*, 2*d*, 2*f*in place of*b*,*d*,*f*in ternary forms. Both conventions occur in the literature. - ↑ away from 2, that is, if 2 is invertible in the ring, quadratic forms are equivalent to symmetric bilinear forms (by the polarization identities), but at 2 they are different concepts; this distinction is particularly important for quadratic forms over the integers.
- ↑ Babylonian Pythagoras
- ↑ Brahmagupta biography
- ↑ Maxime Bôcher (with E.P.R. DuVal)(1907)
*Introduction to Higher Algebra*, § 45 Reduction of a quadratic form to a sum of squares via HathiTrust - ↑ If a non-strict inequality (with ≥ or ≤) holds then the quadratic form
*q*is called semidefinite. - ↑ The theory of quadratic forms over a field of characteristic 2 has important differences and many definitions and theorems must be modified.
- ↑ This alternating form associated with a quadratic form in characteristic 2 is of interest related to the Arf invariant – Irving Kaplansky (1974),
*Linear Algebra and Geometry*, p. 27. - ↑ The bilinear form to which a quadratic form is associated is not restricted to being symmetric, which is of significance when 2 is not a unit in
*R*.

**Quadratic programming** (**QP**) is the process of solving certain mathematical optimization problems involving quadratic functions. Specifically, one seeks to optimize a multivariate quadratic function subject to linear constraints on the variables. Quadratic programming is a type of nonlinear programming.

In mathematics, a **Clifford algebra** is an algebra generated by a vector space with a quadratic form, and is a unital associative algebra. As *K*-algebras, they generalize the real numbers, complex numbers, quaternions and several other hypercomplex number systems. The theory of Clifford algebras is intimately connected with the theory of quadratic forms and orthogonal transformations. Clifford algebras have important applications in a variety of fields including geometry, theoretical physics and digital image processing. They are named after the English mathematician William Kingdon Clifford.

In mathematics, the **discriminant** of a polynomial is a quantity that depends on the coefficients and determines various properties of the roots. It is generally defined as a polynomial function of the coefficients of the original polynomial. The discriminant is widely used in polynomial factoring, number theory, and algebraic geometry. It is often denoted by the symbol .

In linear algebra, a **symmetric matrix** is a square matrix that is equal to its transpose. Formally,

In mathematics, the **orthogonal group** in dimension *n*, denoted O(*n*), is the group of distance-preserving transformations of a Euclidean space of dimension *n* that preserve a fixed point, where the group operation is given by composing transformations. The orthogonal group is sometimes called the **general orthogonal group**, by analogy with the general linear group. Equivalently, it is the group of *n*×*n* orthogonal matrices, where the group operation is given by matrix multiplication. The orthogonal group is an algebraic group and a Lie group. It is compact.

In mathematics, the **unitary group** of degree *n*, denoted U(*n*), is the group of *n* × *n* unitary matrices, with the group operation of matrix multiplication. The unitary group is a subgroup of the general linear group GL(*n*, **C**). **Hyperorthogonal group** is an archaic name for the unitary group, especially over finite fields. For the group of unitary matrices with determinant 1, see Special unitary group.

In mathematics, particularly in linear algebra, a **skew-symmetric****matrix** is a square matrix whose transpose equals its negative. That is, it satisfies the condition

In mathematics, an **algebra over a field** is a vector space equipped with a bilinear product. Thus, an algebra is an algebraic structure consisting of a set together with operations of multiplication and addition and scalar multiplication by elements of a field and satisfying the axioms implied by "vector space" and "bilinear".

In mathematics, a **bilinear form** on a vector space *V* is a bilinear map *V* × *V* → *K*, where *K* is the field of scalars. In other words, a bilinear form is a function *B* : *V* × *V* → *K* that is linear in each argument separately:

In mathematics, a **Casimir element** is a distinguished element of the center of the universal enveloping algebra of a Lie algebra. A prototypical example is the squared angular momentum operator, which is a Casimir element of the three-dimensional rotation group.

In mathematics, a **sesquilinear form** is a generalization of a bilinear form that, in turn, is a generalization of the concept of the dot product of Euclidean space. A bilinear form is linear in each of its arguments, but a sesquilinear form allows one of the arguments to be "twisted" in a semilinear manner, thus the name; which originates from the Latin numerical prefix *sesqui-* meaning "one and a half". The basic concept of the dot product – producing a scalar from a pair of vectors – can be generalized by allowing a broader range of scalar values and, perhaps simultaneously, by widening the definition of a vector.

**Sylvester's law of inertia** is a theorem in matrix algebra about certain properties of the coefficient matrix of a real quadratic form that remain invariant under a change of basis. Namely, if *A* is the symmetric matrix that defines the quadratic form, and *S* is any invertible matrix such that *D* = *SAS*^{T} is diagonal, then the number of negative elements in the diagonal of *D* is always the same, for all such *S*; and the same goes for the number of positive elements.

A **symmetric bilinear form** on a vector space is a bilinear map from two copies of the vector space to the field of scalars such that the order of the two vectors does not affect the value of the map. In other words, it is a bilinear function that maps every pair of elements of the vector space to the underlying field such that for every and in . They are also referred to more briefly as just **symmetric forms** when "bilinear" is understood.

In linear algebra, an **eigenvector** or **characteristic vector** of a linear transformation is a nonzero vector that changes at most by a scalar factor when that linear transformation is applied to it. The corresponding **eigenvalue**, often denoted by , is the factor by which the eigenvector is scaled.

In mathematics, the **spin representations** are particular projective representations of the orthogonal or special orthogonal groups in arbitrary dimension and signature. More precisely, they are representations of the spin groups, which are double covers of the special orthogonal groups. They are usually studied over the real or complex numbers, but they can be defined over other fields.

In mathematics, the **classical groups** are defined as the special linear groups over the reals **R**, the complex numbers **C** and the quaternions **H** together with special automorphism groups of symmetric or skew-symmetric bilinear forms and Hermitian or skew-Hermitian sesquilinear forms defined on real, complex and quaternionic finite-dimensional vector spaces. Of these, the **complex classical Lie groups** are four infinite families of Lie groups that together with the exceptional groups exhaust the classification of simple Lie groups. The **compact classical groups** are compact real forms of the complex classical groups. The finite analogues of the classical groups are the **classical groups of Lie type**. The term "classical group" was coined by Hermann Weyl, it being the title of his 1939 monograph *The Classical Groups*.

In mathematics, **Macdonald polynomials***P*_{λ}(*x*; *t*,*q*) are a family of orthogonal symmetric polynomials in several variables, introduced by Macdonald in 1987. He later introduced a non-symmetric generalization in 1995. Macdonald originally associated his polynomials with weights λ of finite root systems and used just one variable *t*, but later realized that it is more natural to associate them with affine root systems rather than finite root systems, in which case the variable *t* can be replaced by several different variables *t*=(*t*_{1},...,*t*_{k}), one for each of the *k* orbits of roots in the affine root system. The Macdonald polynomials are polynomials in *n* variables *x*=(*x*_{1},...,*x*_{n}), where *n* is the rank of the affine root system. They generalize many other families of orthogonal polynomials, such as Jack polynomials and Hall–Littlewood polynomials and Askey–Wilson polynomials, which in turn include most of the named 1-variable orthogonal polynomials as special cases. Koornwinder polynomials are Macdonald polynomials of certain non-reduced root systems. They have deep relationships with affine Hecke algebras and Hilbert schemes, which were used to prove several conjectures made by Macdonald about them.

In mathematics and theoretical physics, a **pseudo-Euclidean space** is a finite-dimensional real *n*-space together with a non-degenerate quadratic form *q*. Such a quadratic form can, given a suitable choice of basis (*e*_{1}, ..., *e*_{n}), be applied to a vector *x* = *x*_{1}*e*_{1} + ... + *x*_{n}*e*_{n}, giving

In mathematics, specifically the theory of quadratic forms, an ** ε-quadratic form** is a generalization of quadratic forms to skew-symmetric settings and to *-rings;

In mathematics, an **algebraic number field** is an extension field of the field of rational numbers such that the field extension has finite degree . Thus is a field that contains and has finite dimension when considered as a vector space over .

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