{{Short description|Mathematical transform}} {{multiple| {{dispute about|substantially different definitions in the literature|FBI vs Gabor transform|date=March 2018}} {{no footnotes|date=October 2012}} }} In mathematics, the '''FBI transform''' or '''Fourier–Bros–Iagolnitzer transform''' is a generalization of the Fourier transform developed by the French mathematical physicists Jacques Bros and Daniel Iagolnitzer in order to characterise the local analyticity of functions (or distributions) on '''R'''<sup>''n''</sup>. The transform provides an alternative approach to analytic wave front sets of distributions, developed independently by the Japanese mathematicians Mikio Sato, Masaki Kashiwara and Takahiro Kawai in their approach to microlocal analysis. It can also be used to prove the analyticity of solutions of analytic elliptic partial differential equations as well as a version of the classical uniqueness theorem, strengthening the Cauchy–Kowalevski theorem, due to the Swedish mathematician Erik Albert Holmgren (1872–1943).
==Definitions== The Fourier transform of a Schwartz function ''f'' in ''S''('''R'''<sup>''n''</sup>) is defined by
:<math> ({\mathcal F}f)(t) = (2\pi)^{-n / 2} \int_{{\mathbf R}^n}f(x) e^{-ix \cdot t}\, dx.</math>
The '''FBI transform''' of ''f'' is defined for ''a'' ≥ 0 by
:<math> ({\mathcal F}_a f)(t,y) = (2\pi)^{-n / 2} \int_{{\mathbf R}^n}f(x)e^{-a |x-y|^2/2} e^{-ix \cdot t}\, dx.</math>
Thus, when ''a'' = 0, it essentially coincides with the Fourier transform.
The same formulas can be used to define the Fourier and FBI transforms of tempered distributions in ''S'''('''R'''<sup>''n''</sup>).
==Inversion formula== The Fourier inversion formula
:<math> f(x)= {\mathcal F}^2 f(-x)</math>
allows a function ''f'' to be recovered from its Fourier transform.
In particular
:<math>f(x)= (2\pi)^{-n/2}\int_{{\mathbf R}^n} e^{i t \cdot x}{\mathcal F}(f)(t) \, dt </math>
Similarly, at a positive value of ''a'', ''f''(0) can be recovered from the FBI transform of ''f''(''x'') by the inversion formula
:<math>f(x)= (2\pi)^{-n/2}\int_{{\mathbf R}^n} e^{i t \cdot x}e^{a|x-y|^2/2}{\mathcal F}_a (f)(t,y) \, dt </math>
==Criterion for local analyticity== Bros and Iagolnitzer showed that a distribution ''f'' is locally equal to a real analytic function at ''y'', in the direction ''ξ'' if and only if its FBI transform satisfies an inequality of the form
:<math>|({\mathcal F}_{|\xi|}f)(\xi,y)| \le C e^{-\varepsilon |\xi|},</math>
for ''|ξ|'' sufficiently large.
==Holmgren's uniqueness theorem== A simple consequence of the Bros and Iagolnitzer characterisation of local analyticity is the following regularity result of Lars Hörmander and Mikio Sato.<ref name=":0">{{citation |last=Sjöstrand |first=Johannes |title=Singularités analytiques microlocales. [Microlocal analytic singularities] |journal=Astérisque |volume=95 |pages=1–166 |year=1982}}</ref>
'''Theorem.''' Let ''P'' be an elliptic partial differential operator with analytic coefficients defined on an open subset ''X'' of '''R'''<sup>''n''</sup>. If ''Pf'' is analytic in ''X'', then so too is ''f''.
When "analytic" is replaced by "smooth" in this theorem, the result is just Hermann Weyl's classical lemma on elliptic regularity, usually proved using Sobolev spaces.<ref>{{Cite book |last=Warner |first=Frank |title=Foundations of differential geometry and Lie groups |date=1983 |publisher=Springer-Verlag |isbn=0-387-90894-3 |volume=94}}</ref> It is a special case of more general results involving the analytic wave front set (see below), which imply Holmgren's classical strengthening of the Cauchy–Kowalevski theorem on linear partial differential equations with real analytic coefficients. In modern language, Holmgren's uniqueness theorem states that any distributional solution of such a system of equations must be analytic and therefore unique, by the Cauchy–Kowalevski theorem.
==The analytic wave front set== The '''analytic wave front set''' or '''singular spectrum''' WF<sub>A</sub>(''f'') of a distribution ''f'' (or more generally of a hyperfunction) can be defined in terms of the FBI transform<ref name=":1">{{Citation |last=Hörmander |first=Lars |title=The analysis of linear partial differential operators. 1: Distribution theory and Fourier analysis |date=1983 |issue=256 |access-date= |series=Die Grundlehren der mathematischen Wissenschaften in Einzeldarstellungen |place=Berlin Heidelberg |publisher=Springer |isbn=978-3-540-12104-6}}</ref> as the complement of the conical set of points (''x'', λ ξ) (λ > 0) such that the FBI transform satisfies the Bros–Iagolnitzer inequality
:<math>|({\mathcal F}_{|\xi|}f)(\xi,y)| \le C e^{-\varepsilon |\xi|},</math>
for ''y'' the point at which one would like to test for analyticity, and |''ξ''| sufficiently large and pointing in the direction one would like to look for the wave front, that is, the direction at which the singularity at ''y'', if it exists, propagates. J.M. Bony<ref name=":0" /><ref name=":1" /> proved that this definition coincided with other definitions introduced independently by Sato, Kashiwara and Kawai and by Hörmander. If ''P'' is an ''m''th order linear differential operator having analytic coefficients
:<math> P =\sum_{|\alpha|\le m} a_\alpha(x) D^\alpha,</math>
with '''''principal symbol'''''
:<math> \sigma_P(x,\xi) = \sum_{|\alpha|= m} a_\alpha(x) \xi^\alpha,</math>
and '''''characteristic variety'''''
:<math>{\rm char}\, P =\{(x,\xi): \xi\ne0, \, \sigma_P(x,\xi) =0\},</math>
then
*<math> \operatorname{WF}_A(Pf) \subseteq \operatorname{WF}_A(f)</math> *<math> \operatorname{WF}_A(f) \subseteq \operatorname{WF}_A(Pf) \cup \operatorname{char} P.</math>
In particular, when ''P'' is elliptic, char ''P'' = ø, so that
:WF<sub>A</sub>(''Pf'') = WF<sub>A</sub>(''f'').
This is a strengthening of the analytic version of elliptic regularity mentioned above.
==References== <references /> *{{citation|first=Gerald B.|last=Folland|title=Harmonic Analysis in Phase Space|series= Annals of Mathematics Studies|publisher=Princeton University Press|volume=122|year=1989|isbn=0-691-08528-5}} *{{citation|first=Lars|last=Gårding|author-link=Lars Gårding|title=Mathematics and Mathematicians: Mathematics in Sweden Before 1950|publisher=American Mathematical Society|year=1998|isbn=0-8218-0612-2|url-access=registration|url=https://archive.org/details/mathematicsmathe0013gard}} *{{citation|first=Daniel|last=Iagolnitzer|title=Microlocal essential support of a distribution and local decompositions – an introduction. In Hyperfunctions and theoretical physics|series= Lecture Notes in Mathematics|volume= 449|pages=121–132|year=1975|publisher=Springer-Verlag}} *{{citation|first=Steven|last=Krantz|author1-link=Steven G. Krantz|last2=Parks|first2=Harold R.|author2-link=Harold R. Parks|title=A Primer of Real Analytic Functions| year=1992|publisher=Birkhäuser|isbn = 0-8176-4264-1}}. 2nd ed., Birkhäuser (2002), {{ISBN|0-8176-4264-1}}. *{{citation|first=François|last=Trèves|title=Hypo-analytic structures: Local theory|series= Princeton Mathematical Series|volume= 40| publisher=Princeton University Press|year= 1992|isbn=0-691-08744-X}} (Chapter 9, FBI Transform in a Hypo-Analytic Manifold.)
{{DEFAULTSORT:Fourier-Bros-Iagolnitzer transform}} Category:Fourier analysis Category:Transforms Category:Generalized functions Category:Mathematical physics