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In linear algebra, a linear functional or linear form (also called a one-form or covector) is a linear map from a vector space to its field of scalars. In n, if vectors are represented as column vectors, then linear functionals are represented as row vectors, and their action on vectors is given by the dot product, or the matrix product with the row vector on the left and the column vector on the right.  In general, if V is a vector space over a field k, then a linear functional f is a function from V to k that is linear:

for all
for all

The set of all linear functionals from V to k, Homk(V,k), forms a vector space over k with the addition of the operations of addition and scalar multiplication (defined pointwise).  This space is called the dual space of V, or sometimes the algebraic dual space, to distinguish it from the continuous dual space.  It is often written V, V′, or V when the field k is understood.

Contents

Continuous linear functionalsEdit

If V is a topological vector space, the space of continuous linear functionals — the continuous dual — is often simply called the dual space.  If V is a Banach space, then so is its (continuous) dual.  To distinguish the ordinary dual space from the continuous dual space, the former is sometimes called the algebraic dual space.  In finite dimensions, every linear functional is continuous, so the continuous dual is the same as the algebraic dual, but in infinite dimensions the continuous dual is a proper subspace of the algebraic dual.

Examples and applicationsEdit

Linear functionals in RnEdit

Suppose that vectors in the real coordinate space Rn are represented as column vectors

 

For each row vector [a1an] there is a linear functional f defined by

 

and each linear functional can be expressed in this form.

This can be interpreted as either the matrix product or the dot product of the row vector [a1 ... an] and the column vector  :

 

IntegrationEdit

Linear functionals first appeared in functional analysis, the study of vector spaces of functions.  A typical example of a linear functional is integration: the linear transformation defined by the Riemann integral

 

is a linear functional from the vector space C[ab] of continuous functions on the interval [ab] to the real numbers. The linearity of I follows from the standard facts about the integral:

 

EvaluationEdit

Let Pn denote the vector space of real-valued polynomial functions of degree ≤n defined on an interval [ab].  If c ∈ [ab], then let evc : PnR be the evaluation functional

 

The mapping f → f(c) is linear since

 

If x0, ..., xn are n + 1 distinct points in [a, b], then the evaluation functionals evxi, i = 0, 1, ..., n form a basis of the dual space of Pn.  (Lax (1996) proves this last fact using Lagrange interpolation.)

Application to quadratureEdit

The integration functional I defined above defines a linear functional on the subspace Pn of polynomials of degree n. If x0, ..., xn are n + 1 distinct points in [a, b], then there are coefficients a0, ..., an for which

 

for all fPn. This forms the foundation of the theory of numerical quadrature.

This follows from the fact that the linear functionals evxi : ff(xi) defined above form a basis of the dual space of Pn.[1]

Linear functionals in quantum mechanicsEdit

Linear functionals are particularly important in quantum mechanics.  Quantum mechanical systems are represented by Hilbert spaces, which are antiisomorphic to their own dual spaces.  A state of a quantum mechanical system can be identified with a linear functional.  For more information see bra–ket notation.

DistributionsEdit

In the theory of generalized functions, certain kinds of generalized functions called distributions can be realized as linear functionals on spaces of test functions.

PropertiesEdit

  • Any linear functional L is either trivial (equal to 0 everywhere) or surjective onto the scalar field.  Indeed, this follows since just as the image of a vector subspace under a linear transformation is a subspace, so is the image of V under L.  however, the only subspaces (i.e., k-subspaces) of k are {0} and k itself.
  • A linear functional is continuous if and only if its kernel is closed. [2]
  • Linear functionals with the same kernel are proportional.
  • The absolute value of any linear functional is a seminorm on its vector space.[clarification needed]

Visualizing linear functionalsEdit

 
Geometric interpretation of a 1-form α as a stack of hyperplanes of constant value, each corresponding to those vectors that α maps to a given scalar value shown next to it along with the "sense" of increase. The      zero plane is through the origin.

In finite dimensions, a linear functional can be visualized in terms of its level sets.  In three dimensions, the level sets of a linear functional are a family of mutually parallel planes; in higher dimensions, they are parallel hyperplanes.  This method of visualizing linear functionals is sometimes introduced in general relativity texts, such as Gravitation by Misner, Thorne & Wheeler (1973).

Dual vectors and bilinear formsEdit

 
Linear functionals (1-forms) α, β and their sum σ and vectors u, v, w, in 3d Euclidean space. The number of (1-form) hyperplanes intersected by a vector equals the inner product.[3]

Every non-degenerate bilinear form on a finite-dimensional vector space V induces an isomorphism VV : vv such that

 

where the bilinear form on V is denoted ⟨ , ⟩ (for instance, in Euclidean space v, w⟩ = vw is the dot product of v and w).

The inverse isomorphism is VV : vv, where v is the unique element of V such that

 

The above defined vector vV is said to be the dual vector of vV.

In an infinite dimensional Hilbert space, analogous results hold by the Riesz representation theorem.  There is a mapping VV into the continuous dual space V.  However, this mapping is antilinear rather than linear.

Bases in finite dimensionsEdit

Basis of the dual space in finite dimensionsEdit

Let the vector space V have a basis  , not necessarily orthogonal.  Then the dual space V* has a basis   called the dual basis defined by the special property that

 

Or, more succinctly,

 

where δ is the Kronecker delta.  Here the superscripts of the basis functionals are not exponents but are instead contravariant indices.

A linear functional   belonging to the dual space   can be expressed as a linear combination of basis functionals, with coefficients ("components") ui,

 

Then, applying the functional   to a basis vector ej yields

 

due to linearity of scalar multiples of functionals and pointwise linearity of sums of functionals.  Then

 

So each component of a linear functional can be extracted by applying the functional to the corresponding basis vector.

The dual basis and inner productEdit

When the space V carries an inner product, then it is possible to write explicitly a formula for the dual basis of a given basis.  Let V have (not necessarily orthogonal) basis  .  In three dimensions (n = 3), the dual basis can be written explicitly

 

for i = 1, 2, 3, where ε is the Levi-Civita symbol and   the inner product (or dot product) on V.

In higher dimensions, this generalizes as follows

 

where   is the Hodge star operator.

See alsoEdit

NotesEdit

  1. ^ Lax 1996
  2. ^ Rudin 1991, Theorem 1.18
  3. ^ J.A. Wheeler; C. Misner; K.S. Thorne (1973). Gravitation. W.H. Freeman & Co. p. 57. ISBN 0-7167-0344-0.

ReferencesEdit