Sharp L log α L inequalities for conjugate functions  [ Sur les inégalités L log α L exactes pour les fonctions conjuguées ]
Annales de l'Institut Fourier, Tome 52 (2002) no. 2, pp. 623-659.

Nous donnons une méthode pour la construction des fonctions φ et ψ telles que H(x,y)=φ(x)-ψ(y) aî t une minorante sousharmonique spécifiée. D’après un théorème de B. Cole, ce procédé établit des inégalités d’intégrales pour les fonctions conjuguées. Nous appliquons cette méthode pour déduire des inégalités optimales pour les conjuguées des fonctions de la classe Llog α L, pour -1α<. En particulier, le cas α=1 procure une amélioration de la version de Pichorides de l’inégalité classique de Zygmund pour les conjuguées des fonctions de LlogL. Nous appliquons aussi cette méthode pour obtenir une nouvelle preuve de l’inégalité de M. Riesz pour les fonctions de L p (1<p<2), avec meilleure constante. Toutes ces inégalités sont des cas spéciaux d’une inégalité générale et optimale pour les fonctions conjuguées (cf. Théorème 6).

We give a method for constructing functions φ and ψ for which H(x,y)=φ(x)-ψ(y) has a specified subharmonic minorant h(x,y). By a theorem of B. Cole, this procedure establishes integral mean inequalities for conjugate functions. We apply this method to deduce sharp inequalities for conjugates of functions in the class Llog α L, for -1α<. In particular, the case α=1 yields an improvement of Pichorides’ form of Zygmund’s classical inequality for the conjugates of functions in LlogL. We also apply the method to produce a new proof of the M. Riesz’s inequality for functions in L p , (1<p<2), also with sharp constant. All these inequalities are special cases of a general sharp inequality for conjugate functions (cf. Theorem 6).

DOI : https://doi.org/10.5802/aif.1896
Classification : 42A50,  30D55,  31A15
Mots clés : fonctions conjuguées, estimation des normes, effilement minimal
@article{AIF_2002__52_2_623_0,
     author = {Ess\'en, Matts and Shea, Daniel F. and Stanton, Charles S.},
     title = {Sharp $L\;{\rm log}^\alpha L$ inequalities for conjugate functions},
     journal = {Annales de l'Institut Fourier},
     pages = {623--659},
     publisher = {Association des Annales de l'institut Fourier},
     volume = {52},
     number = {2},
     year = {2002},
     doi = {10.5802/aif.1896},
     zbl = {1053.42012},
     language = {en},
     url = {archive.numdam.org/item/AIF_2002__52_2_623_0/}
}
Essén, Matts; Shea, Daniel F.; Stanton, Charles S. Sharp $L\;{\rm log}^\alpha L$ inequalities for conjugate functions. Annales de l'Institut Fourier, Tome 52 (2002) no. 2, pp. 623-659. doi : 10.5802/aif.1896. http://archive.numdam.org/item/AIF_2002__52_2_623_0/

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