These spaces are called Orlicz spaces by an overwhelming majority of mathematicians and by all monographies studying them, because Władysław Orlicz was the first who introduced them, in 1932.^[1] Some mathematicians, including Wojbor Woyczyński, Edwin Hewitt and Vladimir Mazya, include the name of Zygmunt Birnbaum as well, referring to his earlier joint work with Władysław Orlicz. However in the Birnbaum–Orlicz paper the Orlicz space is not introduced, neither explicitly nor implicitly, hence the name Orlicz space is preferred. By the same reasons this convention has been also openly criticized by another mathematician (and an expert in the history of Orlicz spaces), Lech Maligranda.^[2] Orlicz was confirmed as the person who introduced Orlicz spaces already by Stefan Banach in his 1932 monograph.^[3]

Setup edit

μ is a σ-finite measure on a set X,

${\displaystyle \Phi :[0,\infty )\to [0,\infty ]}$ , is a Young function, i.e. convex, lower semicontinuous, and non-trivial, in the sense that it is not the zero function ${\displaystyle x\mapsto 0}$  , and it is not the convex dual of the zero function ${\displaystyle x\mapsto {\begin{cases}0{\text{ if }}x=0,\\+\infty {\text{ else.}}\end{cases}}}$ 

Orlicz spaces edit

Let ${\displaystyle L_{\Phi }^{\dagger }}$  be the set of measurable functions f : X → R such that the integral

${\displaystyle \int _{X}\Phi (|f|)\,d\mu }$ 

is finite, where, as usual, functions that agree almost everywhere are identified.

This might not be a vector space (i.e., it might fail to be closed under scalar multiplication). The vector space of functions spanned by ${\displaystyle L_{\Phi }^{\dagger }}$  is the Orlicz space, denoted ${\displaystyle L_{\Phi }}$ . In other words, it is the smallest linear space containing ${\displaystyle L_{\Phi }^{\dagger }}$ . In other words,

Norm edit

To define a norm on ${\displaystyle L_{\Phi }}$ , let Ψ be the Young complement of Φ; that is,

${\displaystyle \Psi (x)=\int _{0}^{x}(\Phi ')^{-1}(t)\,dt.}$ 

Note that Young's inequality for products holds:

${\displaystyle ab\leq \Phi (a)+\Psi (b).}$ 

The norm is then given by

${\displaystyle \|f\|_{\Phi }=\sup \left\{\|fg\|_{1}\mid \int \Psi (|g|)\,d\mu \leq 1\right\}.}$ 

Furthermore, the space ${\displaystyle L_{\Phi }}$  is precisely the space of measurable functions for which this norm is finite.

An equivalent norm,^[4]: §3.3 called the Luxemburg norm, is defined on L_Φ by

${\displaystyle \|f\|'_{\Phi }=\inf \left\{k\in (0,\infty )\mid \int _{X}\Phi (|f|/k)\,d\mu \leq 1\right\},}$ 

and likewise ${\displaystyle L_{\Phi }(\mu )}$  is the space of all measurable functions for which this norm is finite.

Proposition.^[5]

• The two norms are equivalent in the sense that ${\displaystyle \|f\|_{\Phi }'\leq \|f\|_{\Phi }\leq 2\|f\|_{\Phi }'}$  for all measurable ${\displaystyle f}$ .
• By monotone convergence theorem, if ${\displaystyle 0<\|f\|_{\Phi }'<\infty }$ , then ${\displaystyle \int _{X}\Phi (|f|/\|f\|_{\Phi }')\,d\mu \leq 1}$ .

For any ${\displaystyle p\in [1,\infty ]}$ , the ${\displaystyle L^{p}}$  space is the Orlicz space with Orlicz function ${\displaystyle \Phi (t)=t^{p}}$ . Here ${\displaystyle t^{\infty }={\begin{cases}0&{\text{ if }}t\in [0,1],\\+\infty &{\text{ else.}}\end{cases}}}$ 

When ${\displaystyle 1<p<\infty }$ , the small and the large Orlicz spaces for ${\displaystyle \Phi (x)=x^{p}}$  are equal: ${\displaystyle M_{\Phi }\simeq L_{\Phi }}$ .

Example where ${\displaystyle L_{\Phi }^{\dagger }}$  is not a vector space, and is strictly smaller than ${\displaystyle L_{\Phi }}$ . Suppose that X is the open unit interval (0,1), Φ(x) = exp(x) – 1 – x, and f(x) = log(x). Then af is in the space ${\displaystyle L_{\Phi }}$  but is only in the set ${\displaystyle L_{\Phi }^{\dagger }}$  if |a| < 1.

Proposition. The Orlicz norm is a norm.

Proof. Since ${\displaystyle \Phi (x)>0}$  for some ${\displaystyle x>0}$ , we have ${\displaystyle \|f\|_{\Phi }=0\to f=0}$  a.e.. That ${\displaystyle \|kf\|_{\Phi }=|k|\|f\|_{\Phi }}$  is obvious by definition. For triangular inequality, we have:

Theorem.^[5] ${\displaystyle M_{\Phi },L_{\Phi ^{*}}}$  are topological dual Banach spaces.

In particular, if ${\displaystyle M_{\Phi }=L_{\Phi }}$ , then ${\displaystyle L_{\Phi ^{*}},L_{\Phi }}$  are topological dual spaces. In particular, ${\displaystyle L^{p},L^{q}}$  are dual Banach spaces when ${\displaystyle 1/p+1/q=1}$  and ${\displaystyle 1<p<\infty }$ .

Certain Sobolev spaces are embedded in Orlicz spaces: for ${\displaystyle n>1}$  and ${\displaystyle X\subseteq \mathbb {R} ^{n}}$  open and bounded with Lipschitz boundary ${\displaystyle \partial X}$ , we have

${\displaystyle W_{0}^{1,n}(X)\subseteq L^{\varphi }(X)}$ 

for

${\displaystyle \varphi (t):=\exp \left(|t|^{n/(n-1)}\right)-1.}$ 

This is the analytical content of the Trudinger inequality: For ${\displaystyle X\subseteq \mathbb {R} ^{n}}$  open and bounded with Lipschitz boundary ${\displaystyle \partial X}$ , consider the space ${\displaystyle W_{0}^{k,p}(X)}$  with ${\displaystyle kp=n}$  and ${\displaystyle p>1}$ . Then there exist constants ${\displaystyle C_{1},C_{2}>0}$  such that

${\displaystyle \int _{X}\exp \left(\left({\frac {|u(x)|}{C_{1}\|\mathrm {D} ^{k}u\|_{L^{p}(X)}}}\right)^{n/(n-k)}\right)\,\mathrm {d} x\leq C_{2}|X|.}$ 

Similarly, the Orlicz norm of a random variable characterizes it as follows:

${\displaystyle \|X\|_{\Psi }\triangleq \inf \left\{k\in (0,\infty )\mid \operatorname {E} [\Psi (|X|/k)]\leq 1\right\}.}$ 

This norm is homogeneous and is defined only when this set is non-empty.

When ${\displaystyle \Psi (x)=x^{p}}$ , this coincides with the p-th moment of the random variable. Other special cases in the exponential family are taken with respect to the functions ${\displaystyle \Psi _{q}(x)=\exp(x^{q})-1}$  (for ${\displaystyle q\geq 1}$ ). A random variable with finite ${\displaystyle \Psi _{2}}$  norm is said to be "sub-Gaussian" and a random variable with finite ${\displaystyle \Psi _{1}}$  norm is said to be "sub-exponential". Indeed, the boundedness of the ${\displaystyle \Psi _{p}}$  norm characterizes the limiting behavior of the probability distribution function:

${\displaystyle \|X\|_{\Psi _{p}}<\infty \iff {\mathbb {P}}(|X|\geq x)\leq Ke^{-K'x^{p}}\qquad {\rm {for\ some\ constants\ }}K,K'>0,}$ 

so that the tail of the probability distribution function is bounded above by ${\displaystyle O(e^{-K'x^{p}})}$ .

The ${\displaystyle \Psi _{1}}$  norm may be easily computed from a strictly monotonic moment-generating function. For example, the moment-generating function of a chi-squared random variable X with K degrees of freedom is ${\displaystyle M_{X}(t)=(1-2t)^{-K/2}}$ , so that the reciprocal of the ${\displaystyle \Psi _{1}}$  norm is related to the functional inverse of the moment-generating function:

${\displaystyle \|X\|_{\Psi _{1}}^{-1}=M_{X}^{-1}(2)=(1-4^{-1/K})/2.}$ 

• Krasnosel'skii, M.A.; Rutickii, Ya B. (1961-01-01). Convex Functions and Orlicz Spaces (1 ed.). Gordon & Breach. ISBN 978-0-677-20210-5. Contains most commonly used properties of Orlicz spaces over ${\displaystyle \mathbb {R} ^{n}}$  with the Lebesgue measure.
• Rao, M.M.; Ren, Z.D. (1991). Theory of Orlicz Spaces. Pure and Applied Mathematics. Marcel Dekker. ISBN 0-8247-8478-2. Contains properties of Orlicz spaces over general spaces with general measures, including many pathological examples.
• Rubshtein, Ben-Zion A.; Grabarnik, Genady Ya; Muratov, Mustafa A.; Pashkova, Yulia S. (2016-12-20). Foundations of Symmetric Spaces of Measurable Functions: Lorentz, Marcinkiewicz and Orlicz Spaces (1st ed.). New York, NY: Springer. ISBN 978-3-319-42756-0.
• Birnbaum, Z. W.; Orlicz, W. (1931), "Über die Verallgemeinerung des Begriffes der zueinander Konjugierten Potenzen" (PDF), Studia Mathematica, 3: 1–67, archived from the original (PDF) on 27 Sep 2011. The original paper.
• Bund, Iracema (1975), "Birnbaum–Orlicz spaces of functions on groups", Pacific Mathematics Journal, 58 (2): 351–359.
• Hewitt, Edwin; Stromberg, Karl, Real and abstract analysis, Springer-Verlag.
• Zygmund, Antoni, "Chapter IV: Classes of functions and Fourier series", Trigonometric Series, Volume 1 (3rd ed.), Cambridge University Press.
• Ledoux, Michel; Talagrand, Michel, Probability in Banach Spaces, Springer-Verlag.

• "Orlicz space", Encyclopedia of Mathematics, EMS Press, 2001 [1994]