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Birnbaum–Orlicz space : ウィキペディア英語版
Birnbaum–Orlicz space
In the mathematical analysis, and especially in real and harmonic analysis, a Birnbaum–Orlicz space is a type of function space which generalizes the Lp spaces. Like the Lp spaces, they are Banach spaces. The spaces are named for Władysław Orlicz and Zygmunt William Birnbaum, who first defined them in 1931.
Besides the ''L''p spaces, a variety of function spaces arising naturally in analysis are Birnbaum–Orlicz spaces. One such space ''L'' log+ ''L'', which arises in the study of Hardy–Littlewood maximal functions, consists of measurable functions ''f'' such that the integral
:\int_ |f(x)|\log^+ |f(x)|\,dx < \infty.
Here log+ is the positive part of the logarithm. Also included in the class of Birnbaum–Orlicz spaces are many of the most important Sobolev spaces.
==Formal definition==

Suppose that μ is a σ-finite measure on a set ''X'', and Φ : [0, ∞) → [0, ∞) is a Young function, i.e., a convex function such that
:\frac \to \infty,\quad\mathrmx\to \infty,
:\frac \to 0,\quad\mathrmx\to 0.
Let L^\dagger_\Phi be the set of measurable functions ''f'' : ''X'' → R such that the integral
:\int_X \Phi(|f|)\, d\mu
is finite, where, as usual, functions that agree almost everywhere are identified.
This ''may not be'' a vector space (it may fail to be closed under scalar multiplication). The vector space of functions spanned by L^\dagger_\Phi is the Birnbaum–Orlicz space, denoted L_\Phi.
To define a norm on L_\Phi, let Ψ be the Young complement of Φ; that is,
:\Psi(x) = \int_0^x (\Phi')^(t)\, dt.
Note that Young's inequality holds:
:ab\le \Phi(a) + \Psi(b).
The norm is then given by
:\|f\|_\Phi = \sup\left\.
Furthermore, the space L_\Phi is precisely the space of measurable functions for which this norm is finite.
An equivalent norm is defined on LΦ by
:\|f\|'_\Phi = \inf\left\,
and likewise LΦ(μ) is the space of all measurable functions for which this norm is finite.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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