Lp space
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- The correct title of this article is Lp space. It features superscript or subscript characters that are substituted or omitted because of technical limitations.
In mathematics, the Lp and spaces are spaces of p-power integrable functions, and corresponding sequence spaces. They form an important class of examples of Banach spaces in functional analysis, and of topological vector spaces.
Lp spaces have applications in statistics, finance, and the engineering field of finite element analysis.
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[edit] Motivation
Consider the real vector space Rn. The sum of vectors in Rn is given by
and the scalar action is given by
The length of a vector is usually given by
but this is by no means the only way of defining length. If p is a real number, p≥1, define
for any vector . It turns out that this definition indeed satisfies the properties of a "length function" (or norm), which are that only the zero vector has zero length, the length of the vector changes (modulus-)linearly when we multiply it by a scalar, and the length of the sum of two vectors is no larger than the sum of lengths of the vectors. For any p≥1, Rn together with the p-norm just defined becomes a Banach space.
[edit] spaces
The above p-norm can be extended to vectors having an infinite number of components, yielding the space . For an infinite sequence of real (or complex) numbers, define the vector sum to be
while the scalar action is given by
Define the p-norm
Here, a complication arises, that being that the series on the right is not always convergent, so for example, the sequence made up of only ones, will have an infinite p-norm (length), no matter what p is. The space is then defined as the set of all infinite sequences of real (or complex) numbers such that the p-norm is finite.
One can check that as p increases, the set grows larger. For example, the sequence
is not in , but it is in for p>1, as the series
diverges for p=1 (the harmonic series), but is convergent for p>1.
One also defines the ∞-norm as
and the corresponding space of all bounded sequences. It turns out that
if the right-hand side is finite, or the left-hand side is infinite. Thus, we will consider spaces for 1≤p≤∞.
The p-norm thus defined on is indeed a norm, and together with this norm is a Banach space. The fully general Lp space, is obtained, as seen below, when one considers vectors not only with several components or with a countably infinite many components, but rather, vectors with arbitrarily many components, in other words, functions. Instead of using a sum to define the p-norm, one will use an integral.
[edit] Properties of spaces
The space is the only of the spaces that is a Hilbert space, since any norm that is induced by an inner product should satisfy the parallelogram identity . Direct substitution with unit vectors results in a counter example.
The , 1 < p < ∞ spaces are reflexive: , where (1/p) + (1/q) = 1.
The dual of c0 is ; the dual of is . For the case of natural numbers index set, the and c0 are separable, with the sole exception of . Here, c0 is defined as the space of all sequences converging zero, with norm identical to ||x||∞.
The spaces can be embedded into many Banach spaces. The question of whether all Banach spaces have such an embedding was answered negatively by B. S. Tsirelson's construction of Tsirelson space in 1974.
Except for the trivial finite case, an unusual feature of is that it is not polynomially reflexive.
[edit] Lp spaces
Let p be a positive real number and let (S, μ) be a measure space. Consider the set of all measurable functions from S to C (or R) whose absolute value raised to the p-th power has a finite Lebesgue integral, or equivalently, that
The vector sum of two functions is given by
- (f + g)(x) = f(x) + g(x),
and one checks that the sum of two pth power integrable functions is again pth power integrable. The scalar action on a function is given by
- (λf)(x) = λf(x).
This vector space together with the function is a seminormed complete vector space denoted by . To make it into a Banach space one considers the Kolmogorov quotient of this space, a standard procedure for spaces which are not T0; one divides out the kernel of the norm. Thus we define . This means we are identifying two functions if they are equal almost everywhere. The space L∞(S), while related, is defined differently. We start with the set of all measurable functions from S to C (or R) which are, up to a set of measure zero, bounded. By identifying two such functions if they are equal almost everywhere, we get the set L∞(S). For f in L∞(S), we set
As before, we have
if f ∈ L∞(S) ∩ Lq(S) for some q < ∞.
This definition is generalized in the context of Bochner spaces.
[edit] Special cases
The most important case is when p = 2; like the space, the space L2 is the only Hilbert space of this class, having major applications to Fourier series and quantum mechanics, as well as other fields.
If we use complex-valued functions, the space L∞ is a commutative C*-algebra with pointwise multiplication and conjugation. For many measure spaces, including all sigma-finite ones, it is in fact a commutative von Neumann algebra, since any element of L∞ defines an operator on the Hilbert space L2 by pointwise multiplication.
[edit] Relation to spaces
The spaces (1≤p≤∞) are a special case of L p spaces, when the set S is the positive integers, and the measure used in the integration in the definition is a counting measure.
More generally, if one considers a countable set S with the counting measure, the obtained L p space is denoted . For example, the space is the space of all sequences indexed by the integers, and when defining the p-norm on such a space, one sums over all the integers. The space , where n is the set with n elements, is Rn with its p-norm as defined above.
[edit] Properties of Lp spaces
If 1 ≤ p ≤ ∞, then the Minkowski inequality, proved using Hölder's inequality, establishes the triangle inequality in L p(S). Using the convergence theorems for the Lebesgue integral, one can then show that L p(S) is complete and hence is a Banach space. (Here it is crucial that the Lebesgue integral is employed, and not the Riemann integral.)
[edit] Dual spaces
The dual space (the space of all continuous linear functionals) of Lp for has a natural isomorphism with Lq, where q is such that 1/p + 1/q = 1, which associates with the functional G defined by
(where means the complex conjugate). It is possible to show that any G can be expressed this way. Since the relationship 1/p + 1/q = 1 is symmetric, L p is reflexive for these values of p: the natural monomorphism from L p to (L p)** is onto, that is, it is an isomorphism of Banach spaces.
If the measure on S is sigma-finite, then the dual of L1(S) is isomorphic to L∞(S). However, except in rather trivial cases, the dual of L∞ is much bigger than L1. Elements of (L∞)* can be identified with bounded signed finitely additive measures on S in a construction similar to the ba space.
If 0 < p < 1, then Lp can be defined as above, but || · ||p does not satisfy the triangle inequality in this case, and hence it defines only a quasi-norm. However, we can still define a metric by setting d(f, g) = (||f − g||p)p. The resulting metric space is complete, and L p for 0 < p < 1 is the prototypical example of an F-space that is not locally convex.
[edit] Embeddings
Suppose that the domain S has finite measure, and that 1 ≤ p ≤ q ≤ ∞. Then the bound
follows from Hölder's inequality. Hence, the space Lp is embedded in Lq.
It follows that, without any restriction on S, the embedding
holds, where the space on the left consists of the locally integrable functions on S.
[edit] Weighted Lp spaces
As before, consider a measure space . Let be an absolutely integrable function, i.e. . The w-weighted Lp space is defined as , where means the measure ν defined by
or, in terms of the Radon-Nikodym derivative,
The norm for is explicitly
[edit] See also
[edit] Reference
- Adams, Robert A. (1975). Sobolev Spaces. New York: Academic Press. ISBN 0-12-044150-0.