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m (moved Balian–Low theorem to Balian-Low theorem: ascii title)
m (AUTOMATIC EDIT (latexlist): Replaced 52 formulas out of 52 by TEX code with an average confidence of 2.0 and a minimal confidence of 2.0.)
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A theorem dealing with the representation of arbitrary functions in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200301.png" /> as a sum of time-frequency atoms, or Gabor functions (cf. also [[Gabor transform|Gabor transform]]), of the form
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<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200302.png" /></td> </tr></table>
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Out of 52 formulas, 52 were replaced by TEX code.-->
  
where <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200303.png" /> is a fixed window function and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200304.png" /> are fixed lattice parameters. The goal is to write an arbitrary function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200305.png" /> in a series of the form
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A theorem dealing with the representation of arbitrary functions in $L ^ { 2 } ( \mathbf{R} )$ as a sum of time-frequency atoms, or Gabor functions (cf. also [[Gabor transform|Gabor transform]]), of the form
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200306.png" /></td> <td valign="top" style="width:5%;text-align:right;">(a1)</td></tr></table>
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\begin{equation*} \{ e ^ { 2 \pi i m b x } g ( x - n a ) : n , m \in \mathbf{Z} \} = \{ g _ { n , m} : n , m \in \mathbf{Z} \}, \end{equation*}
  
where the coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200307.png" /> depend linearly on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200308.png" />. One requires that the collection <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b1200309.png" /> forms a frame for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003010.png" />, that is, that there exist constants <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003011.png" /> such that for any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003012.png" />,
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where $g \in L ^ { 2 } ( \mathbf{R} )$ is a fixed window function and $a , b &gt; 0$ are fixed lattice parameters. The goal is to write an arbitrary function $f \in L ^ { 2 } ( \mathbf{R} )$ in a series of the form
  
<table class="eq" style="width:100%;"> <tr><td valign="top" style="width:94%;text-align:center;"><img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003013.png" /></td> <td valign="top" style="width:5%;text-align:right;">(a2)</td></tr></table>
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\begin{equation} \tag{a1} f ( x ) = \sum _ { n \in \mathbf{Z} } \sum _ { m \in \mathbf{Z} } c _ { n , m } (\, f ) g _ { n , m } ( x ), \end{equation}
  
Inequality (a2) implies the existence of coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003014.png" /> satisfying (a1) and the inequality <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003015.png" />. This inequality can be interpreted as expressing the continuous dependence of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003016.png" /> on the coefficients <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003017.png" /> and the continuous dependence of these coefficients on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003018.png" />. Whether or not an arbitrary collection of Gabor functions <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003019.png" /> forms a frame for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003020.png" /> depends on the window function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003021.png" /> and on the lattice density <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003022.png" />. The lattice density <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003023.png" /> is referred to as the critical density, for the following reason. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003024.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003025.png" /> forms a frame, then that frame is non-redundant, i.e., it is a [[Riesz basis|Riesz basis]]. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003026.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003027.png" /> forms a frame, then that frame is redundant, i.e., the representation (a1) is not unique. If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003028.png" />, then for any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003029.png" />, the collection <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003030.png" /> is incomplete. See [[#References|[a9]]], [[#References|[a10]]].
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where the coefficients $\{ c _ { n ,m}  ( f ) : n , m \in \mathbf{Z} \}$ depend linearly on $f$. One requires that the collection $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$, that is, that there exist constants $A , B &gt; 0$ such that for any $f \in L ^ { 2 } ( \mathbf{R} )$,
  
The time-frequency atom <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003031.png" /> is said to be localized at time <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003032.png" /> and frequency <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003033.png" /> since the [[Fourier transform|Fourier transform]] of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003034.png" /> is given by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003035.png" />. A window function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003036.png" /> is said to have  "good localization"  in time and frequency if both <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003037.png" /> and its Fourier transform <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003038.png" /> decay rapidly at infinity. Good localization can be measured in various ways. One way is to require that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003039.png" />. This is related to the classical uncertainty principle inequality, which asserts that any function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003040.png" /> satisfies <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003041.png" />.
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\begin{equation} \tag{a2} A \| f \| _ { 2 } ^ { 2 } \leq \sum _ { n \in \mathbf Z } \sum _ { m \in \mathbf Z } |\langle f , g _ { n  , m} \rangle | ^ { 2 } \leq B \| f \| _ { 2 } ^ { 2 }. \end{equation}
  
The Balian–Low theorem asserts that if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003042.png" /> and if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003043.png" /> forms a frame for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003044.png" />, then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003045.png" /> cannot have good localization. Specifically: If <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003046.png" /> and if <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003047.png" /> forms a frame for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003048.png" />, then <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003049.png" />, i.e., <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003050.png" /> maximizes the uncertainty principle inequality.
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Inequality (a2) implies the existence of coefficients $\{ c _ { n ,m}  ( f ) : n , m \in \mathbf{Z} \}$ satisfying (a1) and the inequality $B ^ { - 1 } \| f \| _ { 2 } ^ { 2 } \leq \sum _ { n , m \in \mathbf{Z} } | c _ { n , m } ( f ) | ^ { 2 } \leq A ^ { - 1 } \| f \| _ { 2 } ^ { 2 }$. This inequality can be interpreted as expressing the continuous dependence of $f$ on the coefficients $\{ c _ { n ,m}  ( f ) : n , m \in \mathbf{Z} \}$ and the continuous dependence of these coefficients on $f$. Whether or not an arbitrary collection of Gabor functions $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$ depends on the window function $g$ and on the lattice density $( a b ) ^ { - 1 }$. The lattice density $( a b ) ^ { - 1 } = 1$ is referred to as the critical density, for the following reason. If $( a b ) ^ { - 1 } = 1$ and $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ forms a frame, then that frame is non-redundant, i.e., it is a [[Riesz basis|Riesz basis]]. If $( a b ) ^ { - 1 } &gt; 1$ and $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ forms a frame, then that frame is redundant, i.e., the representation (a1) is not unique. If $( a b ) ^ { - 1 } &lt; 1$, then for any $g \in L ^ { 2 } ( \mathbf{R} )$, the collection $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ is incomplete. See [[#References|[a9]]], [[#References|[a10]]].
 +
 
 +
The time-frequency atom $g_{n,m}$ is said to be localized at time $na$ and frequency $m b$ since the [[Fourier transform|Fourier transform]] of $g_{n,m}$ is given by $e ^ { 2 \pi i m n a b } e ^ { 2 \pi i m b x }\hat{ g} ( \gamma - m b )$. A window function $g$ is said to have  "good localization" in time and frequency if both $g$ and its Fourier transform $\hat{g}$ decay rapidly at infinity. Good localization can be measured in various ways. One way is to require that $\| tg ( t ) \| _ { 2 } \| \gamma \hat{g} ( \gamma ) \| _ { 2 } &lt; \infty$. This is related to the classical uncertainty principle inequality, which asserts that any function $g \in L ^ { 2 } ( \mathbf{R} )$ satisfies $\| \operatorname { tg } ( t ) \| _ { 2 } \| \gamma \hat{g} ( \gamma ) \| _ { 2 } \geq ( 4 \pi ) ^ { - 1 } \| g \| _ { 2 } ^ { 2 }$.
 +
 
 +
The Balian–Low theorem asserts that if $( a b ) ^ { - 1 } = 1$ and if $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$, then $g$ cannot have good localization. Specifically: If $( a b ) ^ { - 1 } = 1$ and if $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$, then $\| t g ( t ) \| _ { 2 } \| \gamma \hat{g} ( \gamma ) \| _ { 2 } = \infty$, i.e., $g$ maximizes the uncertainty principle inequality.
  
 
More generally, the term  "Balian–Low theorem"  or  "Balian–Low-type theorem"  can refer to any theorem which asserts time and frequency localization restrictions on the elements of a Riesz basis. Such theorems include, for example, [[#References|[a4]]], Thm. 3.2, in which a different criterion for  "good localization"  for the elements of a Gabor system is used, [[#References|[a7]]], Thm. 4.4, in which more general time-frequency lattices for Gabor systems are considered, and [[#References|[a3]]], which asserts a time-frequency restriction on bases of wavelets (cf. also [[Wavelet analysis|Wavelet analysis]]).
 
More generally, the term  "Balian–Low theorem"  or  "Balian–Low-type theorem"  can refer to any theorem which asserts time and frequency localization restrictions on the elements of a Riesz basis. Such theorems include, for example, [[#References|[a4]]], Thm. 3.2, in which a different criterion for  "good localization"  for the elements of a Gabor system is used, [[#References|[a7]]], Thm. 4.4, in which more general time-frequency lattices for Gabor systems are considered, and [[#References|[a3]]], which asserts a time-frequency restriction on bases of wavelets (cf. also [[Wavelet analysis|Wavelet analysis]]).
  
The Balian–Low theorem was originally stated and proved by R. Balian [[#References|[a1]]] and independently by F. Low [[#References|[a8]]] under the stronger assumption that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003051.png" /> forms an orthonormal basis (cf. also [[Orthonormal system|Orthonormal system]]) for <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/b/b120/b120030/b12003052.png" />, and an extension of their argument to frames was given by I. Daubechies, R.R. Coifmann and S. Semmes [[#References|[a5]]]. An elegant and entirely new proof of the theorem for orthonormal bases using the classical uncertainty principle inequality was given by G. Battle [[#References|[a2]]], and an extension of this argument to frames was given by Daubechies and A.J.E.M. Janssen [[#References|[a6]]]. Proofs of the Balian–Low theorem for frames use the differentiability properties of the Zak transform in an essential way.
+
The Balian–Low theorem was originally stated and proved by R. Balian [[#References|[a1]]] and independently by F. Low [[#References|[a8]]] under the stronger assumption that $\{ g _ { n  , m} : n , m \in \mathbf{Z} \}$ forms an orthonormal basis (cf. also [[Orthonormal system|Orthonormal system]]) for $L ^ { 2 } ( \mathbf{R} )$, and an extension of their argument to frames was given by I. Daubechies, R.R. Coifmann and S. Semmes [[#References|[a5]]]. An elegant and entirely new proof of the theorem for orthonormal bases using the classical uncertainty principle inequality was given by G. Battle [[#References|[a2]]], and an extension of this argument to frames was given by Daubechies and A.J.E.M. Janssen [[#References|[a6]]]. Proofs of the Balian–Low theorem for frames use the differentiability properties of the Zak transform in an essential way.
  
 
====References====
 
====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  R. Balian,  "Un principe d'incertitude fort en théorie du signal ou en mécanique quantique"  ''C.R. Acad. Sci. Paris'' , '''292'''  (1981)  pp. 1357–1362</TD></TR><TR><TD valign="top">[a2]</TD> <TD valign="top">  G. Battle,  "Heisenberg proof of the Balian–Low theorem"  ''Lett. Math. Phys.'' , '''15'''  (1988)  pp. 175–177</TD></TR><TR><TD valign="top">[a3]</TD> <TD valign="top">  G. Battle,  "Phase space localization theorem for ondelettes"  ''J. Math. Phys.'' , '''30'''  (1989)  pp. 2195–2196</TD></TR><TR><TD valign="top">[a4]</TD> <TD valign="top">  J. Benedetto,  C. Heil,  D. Walnut,  "Differentiation and the Balian–Low Theorem"  ''J. Fourier Anal. Appl.'' , '''1'''  (1995)  pp. 355–402</TD></TR><TR><TD valign="top">[a5]</TD> <TD valign="top">  I. Daubechies,  "The wavelet transform, time-frequency localization and signal analysis"  ''IEEE Trans. Inform. Th.'' , '''39'''  (1990)  pp. 961–1005</TD></TR><TR><TD valign="top">[a6]</TD> <TD valign="top">  I. Daubechies,  A.J.E.M. Janssen,  "Two theorems on lattice expansions"  ''IEEE Trans. Inform. Th.'' , '''39'''  (1993)  pp. 3–6</TD></TR><TR><TD valign="top">[a7]</TD> <TD valign="top">  H. Feichtinger,  K. Gröchenig,  "Gabor frames and time—frequency distributions"  ''J. Funct. Anal.'' , '''146'''  (1997)  pp. 464–495</TD></TR><TR><TD valign="top">[a8]</TD> <TD valign="top">  F. Low,  "Complete sets of wave packets"  C. DeTar (ed.)  et al. (ed.) , ''A Passion for Physics: Essays in Honor of Geoffrey Chew'' , World Sci.  (1985)  pp. 17–22</TD></TR><TR><TD valign="top">[a9]</TD> <TD valign="top">  M. Rieffel,  "Von Neumann algebras associated with pairs of lattices in Lie groups"  ''Math. Ann.'' , '''257'''  (1981)  pp. 403–418</TD></TR><TR><TD valign="top">[a10]</TD> <TD valign="top">  J. Ramanathan,  T. Steger,  "Incompleteness of Sparse Coherent States"  ''Appl. Comput. Harm. Anal.'' , '''2'''  (1995)  pp. 148–153</TD></TR></table>
+
<table><tr><td valign="top">[a1]</td> <td valign="top">  R. Balian,  "Un principe d'incertitude fort en théorie du signal ou en mécanique quantique"  ''C.R. Acad. Sci. Paris'' , '''292'''  (1981)  pp. 1357–1362</td></tr><tr><td valign="top">[a2]</td> <td valign="top">  G. Battle,  "Heisenberg proof of the Balian–Low theorem"  ''Lett. Math. Phys.'' , '''15'''  (1988)  pp. 175–177</td></tr><tr><td valign="top">[a3]</td> <td valign="top">  G. Battle,  "Phase space localization theorem for ondelettes"  ''J. Math. Phys.'' , '''30'''  (1989)  pp. 2195–2196</td></tr><tr><td valign="top">[a4]</td> <td valign="top">  J. Benedetto,  C. Heil,  D. Walnut,  "Differentiation and the Balian–Low Theorem"  ''J. Fourier Anal. Appl.'' , '''1'''  (1995)  pp. 355–402</td></tr><tr><td valign="top">[a5]</td> <td valign="top">  I. Daubechies,  "The wavelet transform, time-frequency localization and signal analysis"  ''IEEE Trans. Inform. Th.'' , '''39'''  (1990)  pp. 961–1005</td></tr><tr><td valign="top">[a6]</td> <td valign="top">  I. Daubechies,  A.J.E.M. Janssen,  "Two theorems on lattice expansions"  ''IEEE Trans. Inform. Th.'' , '''39'''  (1993)  pp. 3–6</td></tr><tr><td valign="top">[a7]</td> <td valign="top">  H. Feichtinger,  K. Gröchenig,  "Gabor frames and time—frequency distributions"  ''J. Funct. Anal.'' , '''146'''  (1997)  pp. 464–495</td></tr><tr><td valign="top">[a8]</td> <td valign="top">  F. Low,  "Complete sets of wave packets"  C. DeTar (ed.)  et al. (ed.) , ''A Passion for Physics: Essays in Honor of Geoffrey Chew'' , World Sci.  (1985)  pp. 17–22</td></tr><tr><td valign="top">[a9]</td> <td valign="top">  M. Rieffel,  "Von Neumann algebras associated with pairs of lattices in Lie groups"  ''Math. Ann.'' , '''257'''  (1981)  pp. 403–418</td></tr><tr><td valign="top">[a10]</td> <td valign="top">  J. Ramanathan,  T. Steger,  "Incompleteness of Sparse Coherent States"  ''Appl. Comput. Harm. Anal.'' , '''2'''  (1995)  pp. 148–153</td></tr></table>

Revision as of 16:46, 1 July 2020

A theorem dealing with the representation of arbitrary functions in $L ^ { 2 } ( \mathbf{R} )$ as a sum of time-frequency atoms, or Gabor functions (cf. also Gabor transform), of the form

\begin{equation*} \{ e ^ { 2 \pi i m b x } g ( x - n a ) : n , m \in \mathbf{Z} \} = \{ g _ { n , m} : n , m \in \mathbf{Z} \}, \end{equation*}

where $g \in L ^ { 2 } ( \mathbf{R} )$ is a fixed window function and $a , b > 0$ are fixed lattice parameters. The goal is to write an arbitrary function $f \in L ^ { 2 } ( \mathbf{R} )$ in a series of the form

\begin{equation} \tag{a1} f ( x ) = \sum _ { n \in \mathbf{Z} } \sum _ { m \in \mathbf{Z} } c _ { n , m } (\, f ) g _ { n , m } ( x ), \end{equation}

where the coefficients $\{ c _ { n ,m} ( f ) : n , m \in \mathbf{Z} \}$ depend linearly on $f$. One requires that the collection $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$, that is, that there exist constants $A , B > 0$ such that for any $f \in L ^ { 2 } ( \mathbf{R} )$,

\begin{equation} \tag{a2} A \| f \| _ { 2 } ^ { 2 } \leq \sum _ { n \in \mathbf Z } \sum _ { m \in \mathbf Z } |\langle f , g _ { n , m} \rangle | ^ { 2 } \leq B \| f \| _ { 2 } ^ { 2 }. \end{equation}

Inequality (a2) implies the existence of coefficients $\{ c _ { n ,m} ( f ) : n , m \in \mathbf{Z} \}$ satisfying (a1) and the inequality $B ^ { - 1 } \| f \| _ { 2 } ^ { 2 } \leq \sum _ { n , m \in \mathbf{Z} } | c _ { n , m } ( f ) | ^ { 2 } \leq A ^ { - 1 } \| f \| _ { 2 } ^ { 2 }$. This inequality can be interpreted as expressing the continuous dependence of $f$ on the coefficients $\{ c _ { n ,m} ( f ) : n , m \in \mathbf{Z} \}$ and the continuous dependence of these coefficients on $f$. Whether or not an arbitrary collection of Gabor functions $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$ depends on the window function $g$ and on the lattice density $( a b ) ^ { - 1 }$. The lattice density $( a b ) ^ { - 1 } = 1$ is referred to as the critical density, for the following reason. If $( a b ) ^ { - 1 } = 1$ and $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ forms a frame, then that frame is non-redundant, i.e., it is a Riesz basis. If $( a b ) ^ { - 1 } > 1$ and $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ forms a frame, then that frame is redundant, i.e., the representation (a1) is not unique. If $( a b ) ^ { - 1 } < 1$, then for any $g \in L ^ { 2 } ( \mathbf{R} )$, the collection $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ is incomplete. See [a9], [a10].

The time-frequency atom $g_{n,m}$ is said to be localized at time $na$ and frequency $m b$ since the Fourier transform of $g_{n,m}$ is given by $e ^ { 2 \pi i m n a b } e ^ { 2 \pi i m b x }\hat{ g} ( \gamma - m b )$. A window function $g$ is said to have "good localization" in time and frequency if both $g$ and its Fourier transform $\hat{g}$ decay rapidly at infinity. Good localization can be measured in various ways. One way is to require that $\| tg ( t ) \| _ { 2 } \| \gamma \hat{g} ( \gamma ) \| _ { 2 } < \infty$. This is related to the classical uncertainty principle inequality, which asserts that any function $g \in L ^ { 2 } ( \mathbf{R} )$ satisfies $\| \operatorname { tg } ( t ) \| _ { 2 } \| \gamma \hat{g} ( \gamma ) \| _ { 2 } \geq ( 4 \pi ) ^ { - 1 } \| g \| _ { 2 } ^ { 2 }$.

The Balian–Low theorem asserts that if $( a b ) ^ { - 1 } = 1$ and if $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$, then $g$ cannot have good localization. Specifically: If $( a b ) ^ { - 1 } = 1$ and if $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ forms a frame for $L ^ { 2 } ( \mathbf{R} )$, then $\| t g ( t ) \| _ { 2 } \| \gamma \hat{g} ( \gamma ) \| _ { 2 } = \infty$, i.e., $g$ maximizes the uncertainty principle inequality.

More generally, the term "Balian–Low theorem" or "Balian–Low-type theorem" can refer to any theorem which asserts time and frequency localization restrictions on the elements of a Riesz basis. Such theorems include, for example, [a4], Thm. 3.2, in which a different criterion for "good localization" for the elements of a Gabor system is used, [a7], Thm. 4.4, in which more general time-frequency lattices for Gabor systems are considered, and [a3], which asserts a time-frequency restriction on bases of wavelets (cf. also Wavelet analysis).

The Balian–Low theorem was originally stated and proved by R. Balian [a1] and independently by F. Low [a8] under the stronger assumption that $\{ g _ { n , m} : n , m \in \mathbf{Z} \}$ forms an orthonormal basis (cf. also Orthonormal system) for $L ^ { 2 } ( \mathbf{R} )$, and an extension of their argument to frames was given by I. Daubechies, R.R. Coifmann and S. Semmes [a5]. An elegant and entirely new proof of the theorem for orthonormal bases using the classical uncertainty principle inequality was given by G. Battle [a2], and an extension of this argument to frames was given by Daubechies and A.J.E.M. Janssen [a6]. Proofs of the Balian–Low theorem for frames use the differentiability properties of the Zak transform in an essential way.

References

[a1] R. Balian, "Un principe d'incertitude fort en théorie du signal ou en mécanique quantique" C.R. Acad. Sci. Paris , 292 (1981) pp. 1357–1362
[a2] G. Battle, "Heisenberg proof of the Balian–Low theorem" Lett. Math. Phys. , 15 (1988) pp. 175–177
[a3] G. Battle, "Phase space localization theorem for ondelettes" J. Math. Phys. , 30 (1989) pp. 2195–2196
[a4] J. Benedetto, C. Heil, D. Walnut, "Differentiation and the Balian–Low Theorem" J. Fourier Anal. Appl. , 1 (1995) pp. 355–402
[a5] I. Daubechies, "The wavelet transform, time-frequency localization and signal analysis" IEEE Trans. Inform. Th. , 39 (1990) pp. 961–1005
[a6] I. Daubechies, A.J.E.M. Janssen, "Two theorems on lattice expansions" IEEE Trans. Inform. Th. , 39 (1993) pp. 3–6
[a7] H. Feichtinger, K. Gröchenig, "Gabor frames and time—frequency distributions" J. Funct. Anal. , 146 (1997) pp. 464–495
[a8] F. Low, "Complete sets of wave packets" C. DeTar (ed.) et al. (ed.) , A Passion for Physics: Essays in Honor of Geoffrey Chew , World Sci. (1985) pp. 17–22
[a9] M. Rieffel, "Von Neumann algebras associated with pairs of lattices in Lie groups" Math. Ann. , 257 (1981) pp. 403–418
[a10] J. Ramanathan, T. Steger, "Incompleteness of Sparse Coherent States" Appl. Comput. Harm. Anal. , 2 (1995) pp. 148–153
How to Cite This Entry:
Balian-Low theorem. Encyclopedia of Mathematics. URL: http://encyclopediaofmath.org/index.php?title=Balian-Low_theorem&oldid=50014
This article was adapted from an original article by D. Walnut (originator), which appeared in Encyclopedia of Mathematics - ISBN 1402006098. See original article