“Quadratic form”的意思、由来-开放百科全书

In mathematics, a quadratic form is a polynomial with terms all of degree two. For example,

is a quadratic form in the variables x and y. This is not to be confused with a quadratic equation which is strictly univariate and includes terms of degree two or less. A quadratic form is one case of the more general concept of homogeneous polynomials.

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).

Introduction

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] Note that quadratic functions, such as {{nowrap|ax² + bx + c}} in the one variable case, are not quadratic forms, as they are typically not homogeneous (unless b and c are both 0).

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.

A closely related notion with geometric overtones is a quadratic space, which is a pair {{nowrap|(V, q)}}, with V a vector space over a field K, and {{nowrap|q : V → K}} a quadratic form on V. 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 {{nowrap|(x, y, z)}} and the origin:

History

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 {{nowrap|x² + y²}}, 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 {{nowrap|1=x² − ny² = c}}. In particular he considered what is now called Pell's equation, {{nowrap|1=x² − ny² = 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.

Real quadratic forms

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 {{nowrap|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 λ₁, λ₂, ..., 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 {{nowrap|(n₀, n₊, n₋)}}, where n₀ 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 {{visible anchor|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 {{nowrap|(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,

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 {{nowrap|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 {{nowrap|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

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 (n₀ 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 {{nowrap|q(v) > 0}} (resp., {{nowrap|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 ({{nowrap|1=O(p, q) ≈ O(q, p))}}, but the associated Clifford algebras (and hence pin groups) are different.

Definitions

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 x₁, ..., x_n and {{nowrap|1=A = (a_ij)}} be the n×n matrix over K whose entries are the coefficients of q. Then

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

Let the characteristic of K be different from 2.{{refn|The theory of quadratic forms over a field of characteristic 2 has important differences and many definitions and theorems must be modified.}} The coefficient matrix A of q may be replaced by the symmetric matrix {{nowrap|(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:

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 one another. 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.

Quadratic spaces

A quadratic form q in n variables over K induces a map from the n-dimensional coordinate space Kⁿ 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 {{nowrap|B : V × V → K}} over K below is defined:

This bilinear form B is symmetric, i.e. {{nowrap|1=B(x, y) = B(y, x)}} for all x, y in V, and it determines Q: {{nowrap|1=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 {{nowrap|1=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 {{nowrap|1=B′(x, x) = 0}} for all x (and is thus alternating^[7]). Alternately, there always exists a bilinear form B″ (not in general either unique or symmetric) such that {{nowrap|1=B″(x, x) = Q(x)}}.

The pair {{nowrap|(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 {{nowrap|(V, Q)}} and {{nowrap|(V′, Q′)}} are isometric if there exists an invertible linear transformation {{nowrap|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.

Generalization

Let R be a commutative ring, M be an R-module and {{nowrap|b : M × M → R}} be an R-bilinear form.{{refn|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.}} A mapping {{nowrap|Q : M → R : v ↦ b(v, v)}} is the associated quadratic form of b, and {{nowrap|B : M × M → R : (u, v) ↦ Q(u + v) − Q(u) − Q(v)}} is the polar form of Q.

Alternatively, a quadratic form {{nowrap|Q : M → R}} may be characterized as follows:

Related concepts

Two elements v and w of V are called orthogonal if {{nowrap|1=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 {{nowrap|1=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, 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, i.e. the group of isometries of {{nowrap|(V, Q)}} into itself.

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

Equivalence of forms

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

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{{Citation needed|date=February 2017}}. 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 get 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 {{nowrap|x² + xy + y²}}; 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 {{nowrap|Q(x, y) ∈ Z}} if {{nowrap|x, y ∈ Λ}}.

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:

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

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:

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 to

and found 54 multisets {a,b,c,d} 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, {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.