Quadratic form explained

In mathematics, a quadratic form is a polynomial with terms all of degree two ("form" is another name for a homogeneous polynomial). For example, $$4x^2\; +\; 2xy\; -\; 3y^2$$

is a quadratic form in the variables and . The coefficients usually belong to a fixed field, such as the real or complex numbers, and one speaks of a quadratic form over . If, and the quadratic form equals 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 groups), differential geometry (the Riemannian metric, the second fundamental form), differential topology (intersection forms of manifolds, especially four-manifolds), Lie theory (the Killing form), and statistics (where the exponent of a zero-mean multivariate normal distribution has the quadratic form

- x ^{T\boldsymbol\Sigma} ^{-1} x )

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.

Introduction

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

where, ..., are the coefficients.^[1]

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 or the -adic integers .^[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 variables has important applications to algebraic topology.

Using homogeneous coordinates, a non-zero quadratic form in variables defines an -dimensional quadric in the -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 and the origin:$$q(x,y,z)\; =\; d((x,y,z),\; (0,0,0))^2\; =\; \backslash left\backslash |(x,y,z)\backslash right\backslash |^2\; =\; x^2\; +\; y^2\; +\; z^2.$$

A closely related notion with geometric overtones is a quadratic space, which is a pair, with a vector space over a field, and a quadratic form on V. See below for the definition of a quadratic form on a vector space.

History

The study of 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, where, are integers. This problem is related to the problem of finding Pythagorean triples, which appeared in the second millennium BCE.^[3]

In 628, the Indian mathematician Brahmagupta wrote Brāhmasphuṭasiddhānta, which includes, among many other things, a study of equations of the form . He considered what is now called Pell's equation,, 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.

Associated symmetric matrix

Any matrix determines a quadratic form in variables by$$q\_A(x\_1,\backslash ldots,x\_n)\; =\; \backslash sum\_^\backslash sum\_^a\_\; =\; \backslash mathbf\; x^\backslash mathsf\; A\; \backslash mathbf\; x,$$where .

Example

Consider the case of quadratic forms in three variables . The matrix has the form

The above formula gives$$q\_A(x,y,z)=ax^2\; +\; ey^2\; +kz^2\; +\; (b+d)xy\; +\; (c+g)xz\; +\; (f+h)yz.$$

So, two different matrices define the same quadratic form if and only if they have the same elements on the diagonal and the same values for the sums, and . In particular, the quadratic form is defined by a unique symmetric matrix

This generalizes to any number of variables as follows.

General case

Given a quadratic form, defined by the matrix,the matrix$$B\; =\; \backslash left(\backslash frac\; 2\backslash right)\; =\; \backslash frac\; 2(A\; +\; A^\backslash text)$$is symmetric, defines the same quadratic form as, and is the unique symmetric matrix that defines .

So, over the real numbers (and, more generally, over a field of characteristic different from two), there is a one-to-one correspondence between quadratic forms and symmetric matrices that determine them.

Real quadratic forms

See also: Sylvester's law of inertia, Definite quadratic form and Isotropic quadratic form. A fundamental problem is the classification of real quadratic forms under a linear change of variables.

Jacobi proved that, for every real quadratic form, there is an orthogonal diagonalization; that is, an orthogonal change of variables that puts the quadratic form in a "diagonal form"$$\backslash lambda\_1\; \backslash tilde\; x\_1^2\; +\; \backslash lambda\_2\; \backslash tilde\; x\_2^2\; +\; \backslash cdots\; +\; \backslash lambda\_n\; \backslash tilde\; x\_n^2,$$where the associated symmetric matrix is diagonal. Moreover, the coefficients are determined uniquely up to a permutation.^[5]

If the change of variables is given by an invertible matrix that is not necessarily orthogonal, one can suppose that all coefficients are 0, 1, or −1. Sylvester's law of inertia states that the numbers of each 0, 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, where these components count the number of 0s, number of 1s, and the number of −1s, respectively. Sylvester's law of inertia shows that this is a well-defined quantity attached to the quadratic form.

The case when all 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 ; this includes positive definite, negative definite, and isotropic quadratic form (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 (denoting 1s and −1s) is often denoted as particularly in the physical theory of spacetime.

The discriminant of a quadratic form, concretely the class of the determinant of a representing matrix in (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 be a quadratic form defined on an -dimensional real vector space. Let be the matrix of the quadratic form in a given basis. This means that is a symmetric matrix such that$$q(v)\; =\; x^\backslash mathsf\; Ax,$$where x is the column vector of coordinates of in the chosen basis. Under a change of basis, the column is multiplied on the left by an invertible matrix, and the symmetric square matrix is transformed into another symmetric square matrix of the same size according to the formula$$A\backslash to\; B=S^\backslash mathsfAS.$$

Any symmetric matrix can be transformed into a diagonal matrixby a suitable choice of an orthogonal matrix, and the diagonal entries of are uniquely determined – this is Jacobi's theorem. If is allowed to be any invertible matrix then can be made to have only 0, 1, and −1 on the diagonal, and the number of the entries of each type (for 0, for 1, and for −1) depends only on . This is one of the formulations of Sylvester's law of inertia and the numbers and 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 , Sylvester's law of inertia means that they are invariants of the quadratic form .

The quadratic form is positive definite if (similarly, negative definite if) for every nonzero vector .^[6] When assumes both positive and negative values, is an isotropic quadratic form. The theorems of Jacobi and Sylvester show that any positive definite quadratic form in variables can be brought to the sum of 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 . This stands in contrast with the case of isotropic forms, when the corresponding group, the indefinite orthogonal group, is non-compact. Further, the isometry groups of and are the same (but the associated Clifford algebras (and hence pin groups) are different.

Definitions

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

More concretely, an -ary quadratic form over a field is a homogeneous polynomial of degree 2 in variables with coefficients in :$$q(x\_1,\backslash ldots,x\_n)\; =\; \backslash sum\_^\backslash sum\_^a\_,\; \backslash quad\; a\_\backslash in\; K.$$

This formula may be rewritten using matrices: let be the column vector with components, ..., and be the matrix over whose entries are the coefficients of . Then$$q(x)\; =\; x^\backslash mathsf\; A\; x.$$

A vector is a null vector if .

Two -ary quadratic forms and over are equivalent if there exists a nonsingular linear transformation such that$$\backslash psi(x)\; =\; \backslash varphi(Cx).$$

Let the characteristic of be different from 2. The coefficient matrix of may be replaced by the symmetric matrix with the same quadratic form, so it may be assumed from the outset that is symmetric. Moreover, a symmetric matrix is uniquely determined by the corresponding quadratic form. Under an equivalence, the symmetric matrix of and the symmetric matrix of are related as follows:$$B\; =\; C^\backslash mathsf\; A\; C.$$

The associated bilinear form of a quadratic form is defined by$$b\_q(x,y)=\backslash tfrac(q(x+y)-q(x)-q(y))\; =\; x^\backslash mathsfAy\; =\; y^\backslash mathsfAx.$$

Thus, is a symmetric bilinear form over with matrix . Conversely, any symmetric bilinear form defines a quadratic form$$q(x)=b(x,x),$$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 variables are essentially the same.

Quadratic space

Given an -dimensional vector space over a field, a quadratic form on is a function that has the following property: for some basis, the function that maps the coordinates of to is a quadratic form. In particular, if with its standard basis, one has $$q(v\_1,\backslash ldots,\; v\_n)=\; Q([v\_1,\backslash ldots,v\_n])\backslash quad\; \backslash text\; \backslash quad\; [v\_1,\backslash ldots,v\_n]\; \backslash in\; K^n.$$

The change of basis formulas show that the property of being a quadratic form does not depend on the choice of a specific basis in, although the quadratic form depends on the choice of the basis.

A finite-dimensional vector space with a quadratic form is called a quadratic space.

The map is a homogeneous function of degree 2, which means that it has the property that, for all in and in :$$Q(av)\; =\; a^2\; Q(v).$$

When the characteristic of is not 2, the bilinear map over is defined:$$B(v,w)=\; \backslash tfrac(Q(v+w)-Q(v)-Q(w)).$$This bilinear form is symmetric. That is, for all, in, and it determines : for all in .

When the characteristic of is 2, so that 2 is not a unit, it is still possible to use a quadratic form to define a symmetric bilinear form . However, can no longer be recovered from this in the same way, since for all (and is thus alternating).^[7] Alternatively, there always exists a bilinear form (not in general either unique or symmetric) such that .

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

Two -dimensional quadratic spaces and are isometric if there exists an invertible linear transformation (isometry) such that$$Q(v)\; =\; Q\text{'}(Tv)\; \backslash text\; v\backslash in\; V.$$

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

Generalization

Let be a commutative ring, be an -module, and be an -bilinear form. A mapping is the associated quadratic form of, and is the polar form of .

A quadratic form may be characterized in the following equivalent ways:

• There exists an -bilinear form such that is the associated quadratic form.
• for all and, and the polar form of is -bilinear.

Related concepts

See also: Isotropic quadratic form. Two elements and of are called orthogonal if . The kernel of a bilinear form consists of the elements that are orthogonal to every element of . is non-singular if the kernel of its associated bilinear form is . If there exists a non-zero in such that, the quadratic form is isotropic, otherwise it is definite. This terminology also applies to vectors and subspaces of a quadratic space. If the restriction of to a subspace of is identically zero, then is totally singular.

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

If a quadratic space has a product so that is an algebra over a field, and satisfies$$\backslash forall\; x,\; y\; \backslash isin\; A\; \backslash quad\; Q(x\; y)\; =\; Q(x)\; Q(y),$$ then it is a composition algebra.

Equivalence of forms

Every quadratic form in variables over a field of characteristic not equal to 2 is equivalent to a diagonal form$$q(x)=a\_1\; x\_1^2\; +\; a\_2\; x\_2^2+\; \backslash cdots\; +a\_n\; x\_n^2.$$

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.

Geometric meaning

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. 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.

Integral quadratic forms

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 ; equivalently, given a lattice in a vector space (over a field with characteristic 0, such as or), a quadratic form is integral with respect to if and only if it is integer-valued on, meaning if .

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

Historical use

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 matrixThis 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.

Universal quadratic forms

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 and found 54 multisets that can each generate all positive integers, namely,

There are also forms whose image consists of all but one of the positive integers. For example, 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.

See also

• ε-quadratic form
• Cubic form
• Discriminant of a quadratic form
• Hasse–Minkowski theorem
• Quadric
• Ramanujan's ternary quadratic form
• Square class
• Witt group
• Witt's theorem

References

• • • Book: Shafarevich . I. R. . Igor Shafarevich . Remizov . A. O. . 2012 . Linear Algebra and Geometry . . 978-3-642-30993-9 .

Further reading

• Book: Cassels, J.W.S. . J. W. S. Cassels . Rational Quadratic Forms . London Mathematical Society Monographs . 13 . . 1978 . 0-12-163260-1 . 0395.10029 .
• Book: Kitaoka, Yoshiyuki . Arithmetic of quadratic forms . Cambridge Tracts in Mathematics . 106 . Cambridge University Press . 1993 . 0-521-40475-4 . 0785.11021 .
• Book: Lam, Tsit-Yuen . Tsit Yuen Lam . 2005 . Introduction to Quadratic Forms over Fields . 67 . . . 0-8218-1095-2 . 1068.11023 . 2104929 .
• Book: J. . Milnor . John Milnor. D. . Husemoller . 1973 . Symmetric Bilinear Forms . . 73 . . 3-540-06009-X . 0292.10016 .
• Book: O'Meara, O.T. . O. Timothy O'Meara . 1973 . Introduction to quadratic forms . Die Grundlehren der mathematischen Wissenschaften . 117 . . 3-540-66564-1 . 0259.10018 .
• Book: Pfister, Albrecht . Albrecht Pfister (mathematician) . 1995 . Quadratic Forms with Applications to Algebraic Geometry and Topology . London Mathematical Society lecture note series . 217 . . 0-521-46755-1 . 0847.11014 . registration .

Notes and References

1. A tradition going back to Gauss dictates the use of manifestly even coefficients for the products of distinct variables, that is, in place of in binary forms and,, in place of,, in ternary forms. Both conventions occur in the literature.
2. [Localization of a ring#Terminology|away from 2]
3. http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Babylonian_Pythagoras.html Babylonian Pythagoras
4. http://www-groups.dcs.st-and.ac.uk/~history/Biographies/Brahmagupta.html Brahmagupta biography
5. [Maxime Bôcher]
6. If a non-strict inequality (with ≥ or ≤) holds then the quadratic form is called semidefinite.
7. This alternating form associated with a quadratic form in characteristic 2 is of interest related to the Arf invariant – .