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image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Journal d Analyse Ma...arrow_drop_down
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Journal d Analyse Mathématique
Article . 2003 . Peer-reviewed
License: Springer TDM
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image/svg+xml Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao Closed Access logo, derived from PLoS Open Access logo. This version with transparent background. http://commons.wikimedia.org/wiki/File:Closed_Access_logo_transparent.svg Jakob Voss, based on art designer at PLoS, modified by Wikipedia users Nina and Beao
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Article . 2003
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Almost holomorphic extensions of ultradifferentiable functions

Authors: Andersson, Mats; Berndtsson, Bo;

Almost holomorphic extensions of ultradifferentiable functions

Abstract

For a function \(f\) on the real line (or the unit circle), the existence of extension to the complex \(F\) such that \(\overline\partial F\) tends to zero in a prescribed manner when approaching the real line (respectively the unit circle) (this \(F\) is called an almost holomorphic extensions) is related to regularity properties of the function \(f\). The authors extends this sort of result in the case of several variable. To achieve this, they review, in the proper setting, the results of \textit{A. Beurling} [Acta Math. 128, 153--182 (1972; Zbl 0235.30003) and Lecture Notes, Stanford 1961] and \textit{E. M. Dyn'kin} [Math. Sb., n. Ser. 89(131), 182--190 (1972; Zbl 0251.30033), J. Anal. Math. 60, 45--70 (1993; Zbl 0795.30034)]. More precisely, let \(H\) the class of continuous functions on \(\mathbb{R}\), nonnegative and increasing on \(\mathbb{R}^+\) and concave and decreasing on \(\mathbb{R}\) -- and tending to \(\infty\) for \(t\to\pm\infty\), \({\mathcal A}_h= \{f\in{\mathcal C}(\mathbb{R})(\int|\widehat f| e^h0\}}(h(t)- ty)\) and for \(g\) nonnegative decreasing on \(\mathbb{R}^+\) and increasing on \(\mathbb{R}^-\), \(g^b(t)= \inf_{\{y,yt>0\}}(g(y) +ty)\). One has that \(f^{\#b}\) is the smallest concave majorant of \(h\) and \(g^{b,\#}\), is the largest convex minorant of \(g\). If \(h\in H\) then \(h= h^{\# b}\); if \(g\in G\), \(g= g^{b\#}\). A precise version of Dyn'kin result is proved: Let \(h\in H\), \(f\in{\mathcal C}(\mathbb{R})\). Then \(f\) can be extended to a function \(F\) in \({\mathcal C}(\mathbb{C})\) such that \[ \sup_n \int_{\mathbb{R}} \Biggl|{\partial F(x\sin)\over\partial\overline z}\Biggr|\, e^{g(-y)} dy\leq {1\over 2} \int_{\mathbb{R}}|\widehat f(t)|\;e^{h(t)}\,dt \] with \(g= h^\#\) (and if \(h\in {\mathcal C}^2\) an uniform estimate of \(|{\partial F\over\partial\overline z}| e^{g(-y)}\) in terms of \(|\widehat f|\) and \(e^h\) is given) and also a partial converse which states that if \(f\) has an extension \(\overline\partial F\) small near the real line, the \(f\) must decrease rapidly at infinity. Moreover, if \(g\in G\) is of class \(C^2\), one can control the support of the extension. Let now \(\Omega\) be a domain in \(\mathbb{C}\) such that \(\Omega\cap \mathbb{R}\neq\emptyset\) and \(\omega\) a holomorphic \((1,0)\) form on \(\Omega\setminus\mathbb{R}\) satisfying \(\int_\Omega |\omega| e^{-g} 0\) for any \(a\in S^{n-1}\). Now \((k^a)^\#\) and \((k^a)^b\) make sense. For \(h\) any nonnegative function on \(\mathbb{R}\{0\}\) define now \[ g^b(t)= \sup_{a\in S^{n-1}} (g^a)^b(a\cdot t),\;h^\#(y)= \inf_{a\in S^{n-1}} (h^a)^\#(a\cdot y). \] The authors are able to prove: Let \(h\) a weight function concave on rays, and \(g= h^\#\). If \(f\in S'(\mathbb{R})\) such that \(\int_{\mathbb{R}^n} |\widehat f(t)|\,e^{h(t)}\,dt 1)\) and \(\omega\) a \(\overline\partial\)-closed \((n, n 1)\) form in \(\Omega\setminus\mathbb{R}^n\) such that \(\int|\omega|\, e^{-g(-y)}< \infty\), where \(g\) satisfies the nonquasianalyticity condition and \((***)\) the following are equivalent: (i) \(\overline\partial\omega= 0\) across \(\mathbb{R}^n\). (ii) For any \(\Omega'\subset\subset\Omega\) , there is a \((n, n- 2)\) form in \(\Omega'\setminus\mathbb{R}^n\) such that \(\overline\partial u= \omega\) and \(\int| u|\, e^{-g'(-y)}< \infty\) for some weight function \(g'\) that satisfies \((**)\). (iii) For any \(\Omega'\subset\subset\Omega\) there is a \((n, n- 2)\) form in \(\Omega'\setminus \mathbb{R}^n\) with \(\overline\partial u= w\). This is proved using Cauchy-Fantappiê kernels. As a corollary it follows that if \(E\) is the space of exact forms in \(\Omega\setminus \mathbb{R}^n\), and \(g(y)= g(| y|)\) then \(E\) is closed in \(L^1_{\text{loc}}(e^{- g})\) iff \(g\) satisfy \((**)\).

Keywords

Several complex variables and analytic spaces, Quasi-analytic and other classes of functions of one complex variable, almost holomorphic extension, \(C^\infty\)-functions, quasi-analytic functions, Legendre transform, 0251.30033

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selected citations
These citations are derived from selected sources.
This is an alternative to the "Influence" indicator, which also reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Citations provided by BIP!
popularity
This indicator reflects the "current" impact/attention (the "hype") of an article in the research community at large, based on the underlying citation network.
BIP!Popularity provided by BIP!
influence
This indicator reflects the overall/total impact of an article in the research community at large, based on the underlying citation network (diachronically).
BIP!Influence provided by BIP!
impulse
This indicator reflects the initial momentum of an article directly after its publication, based on the underlying citation network.
BIP!Impulse provided by BIP!
2
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