MEMOIRS of the American Mathematical Society Volume 233 • Number 1097 (third of 6 numbers) • January 2015 R2 Self-Affine Scaling Sets in Xiaoye Fu Jean-Pierre Gabardo ISSN 0065-9266 (print) ISSN 1947-6221 (online) American Mathematical Society MEMOIRS of the American Mathematical Society Volume 233 • Number 1097 (third of 6 numbers) • January 2015 R2 Self-Affine Scaling Sets in Xiaoye Fu Jean-Pierre Gabardo ISSN 0065-9266 (print) ISSN 1947-6221 (online) American Mathematical Society Providence, Rhode Island Library of Congress Cataloging-in-Publication Data Fu,Xiaoye,1979- Self-affinescalingsetsinR2 /XiaoyeFu,Jean-PierreGabardo. pages cm. – (Memoirs of the AmericanMathematicalSociety, ISSN 0065-9266; volume 233, number1097) Includesbibliographicalreferencesandindex. ISBN978-1-4704-1091-9(alk. paper) 1. Scaling laws (Statistical physics) 2. Wavelets (Mathematics) 3. R (Computer program language) I.Gabardo,Jean-Pierre,1958- II.Title. 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(cid:2)c 2014bytheAmericanMathematicalSociety. Allrightsreserved. Copyrightofindividualarticlesmayreverttothepublicdomain28years afterpublication. ContacttheAMSforcopyrightstatusofindividualarticles. (cid:2) ThispublicationisindexedinMathematicalReviewsR,Zentralblatt MATH,ScienceCitation Index(cid:2)R,ScienceCitation IndexTM-Expanded,ISI Alerting ServicesSM,SciSearch(cid:2)R,Research (cid:2) (cid:2) (cid:2) AlertR,CompuMathCitation IndexR,Current ContentsR/Physical, Chemical& Earth Sciences. ThispublicationisarchivedinPortico andCLOCKSS. PrintedintheUnitedStatesofAmerica. (cid:2)∞ Thepaperusedinthisbookisacid-freeandfallswithintheguidelines establishedtoensurepermanenceanddurability. VisittheAMShomepageathttp://www.ams.org/ 10987654321 201918171615 Contents Chapter 1. Introduction 1 1.1. Wavelets and Wavelet Sets 1 1.2. Scaling Sets 2 1.3. Self-Affine Tiles 3 1.4. Main Results 5 Chapter 2. Preliminary Results 9 Chapter 3. A sufficient condition for a self-affine tile to be an MRA scaling set 15 Chapter 4. Characterization of the inclusion K ⊂BK 19 Chapter 5. Self-affine scaling sets in R2: the case 0∈D 29 5.1. The case A=C 31 1 5.2. The case A=C 33 2 5.3. The case A=C 34 3 5.4. The case A=−C 35 3 5.5. The case A=C 36 4 5.6. The case A=−C 50 4 Chapter 6. Self-affine scaling sets in R2: the case D ={d ,d }⊂R2 53 1 2 6.1. The case A=C 53 1 6.2. The case A=C 57 2 6.3. The case A=C ,−C ,C ,−C 59 3 3 4 4 Chapter 7. Conclusion 81 Bibliography 83 Index 85 iii Abstract There exist results on the connection between the theory of wavelets and the theory of integral self-affine tiles and in particular, on the construction of wavelet bases using integral self-affine tiles. However, there are many non-integral self- affine tiles which can also yield wavelet basis. In this work, we give a complete characterizationof all one and two dimensional A(cid:2)-dilation scaling setsK such that K is a self-affine tile satisfying BK = (K +d ) (K +d ) for some d ,d ∈ R2, 1 2 1 2 where A is a 2×2 integral expansive matrix with |detA|=2 and B =At. ReceivedbytheeditorOctober11,2012. ArticleelectronicallypublishedonMay19,2014. DOI:http://dx.doi.org/10.1090/memo/1097 2010 MathematicsSubjectClassification. Primary42C15;Secondary52C20. Key wordsand phrases. (MRA)scalingset,self-affinetile,(MRA)waveletset. Affiliations at time of publication: Xiaoye Fu, Department of Mathematics, The Chinese University of Hong Kong, Hong Kong, email: [email protected]; and Jean-Pierre Gabardo, Department of Mathematics and Statistics, McMaster University, Hamilton, Ontario, L8S 4K1, Canada,email: [email protected]. (cid:3)c2014 American Mathematical Society v CHAPTER 1 Introduction Our main goal in this work is to characterize all two dimensional self-affine scalingsetswithasetofassociateddigitsoftheform{d ,d },whered ,d ∈R2but 1 2 1 2 not necessarily in Z2, for an integral expansive matrix A with |detA| = 2. Before starting the discussion of this topic, we will first introduce some basic concepts that wil(cid:3)l be used frequently later. The Fourier transform of any function f ∈ L1(Rn) L2(Rn) is defined by (cid:4) (1.1) F(f)(ξ)=fˆ(ξ)= e−2πix·ξ f(x) dx, Rn where x·ξ is the standard inner product of the vectors x,ξ ∈ Rn. The inverse Fourier transform will be denotedby F−1. For two me(cid:2)asurable sets E,F ⊆Rn, we write E ∼=F if their symmetric difference i.e. (F \E) (E\F) has zero Lebesgue measure. Two(cid:3)measurable sets E,F ⊆ Rn are essentially disjoint if the Lebesgue measure of E F is zero. The Lebesgue measure of a measurable set K ⊂ Rn is denoted by |K|. 1.1. Wavelets and Wavelet Sets Asarelativelynewsubject, waveletanalysiswasintroducedintheearly1980s. It has attracted much attention of researchers from many different fields including mathematics, physics, engineering and so on because of its wide range of applica- tions in signal analysis, image analysis, communications systems and differential equations. An important problem in wavelet analysis is the construction of var- ious kinds of wavelet bases obtained by applying dilations and translations to a particular wavelet. Suppose A is an n×n real expansive matrix, i.e. a matrix with real entries whose eigenvalues are all of modulus greater than one. An A-dilation wavelet is a measurable function ψ ∈L2(Rn) such that the set (cid:5) (cid:6) (1.2) |detA|2jψ(Aj ·−k): j ∈Z, k ∈Zn forms an orthonormal basis for L2(Rn). Around 1987, Mallat [27] introduced the idea of Multiresolution Analysis (MRA) with associated dilation A = 2 in dimen- sion one that provides a tool for the construction of wavelets. The construction of anorthonormalwaveletbasis inL2(R)wasreducedtotheconstructionofascaling function in a corresponding MRA. Later, Daubechies [9] used this result to con- struct compactly supported wavelet of arbitrary smoothness. Y. Meyer [28] gave a similar generalizationofMRAinhigherdimension byconsideringmatrixdilations. If an A-dilation wavelet ψ ∈ L2(Rn) is associated with an MRA, then ψ is called an A-dilation MRA wavelet. 1 2 XIAOYEFUandJEAN-PIERREGABARDO However, not every A-dilation wavelet is associated with an MRA. It has been known[1,17]that|detA|=2isnecessaryandsufficientfortheexistenceofanMRA wavelet. Dai, Larson and Speegle [7] proved the existence of a single wavelet of theform F−1(χ ) for some measurable set Q⊆Rn and thisfor any real expansive Q matrix A, where χ is the characteristic function of Q. A measurable set Q⊆Rn Q is called an A-dilation wavelet set if F−1(χ ) is an A-dilation wavelet. If an A- Q dilationwaveletF−1(χ )isassociatedwithanMRA,thenthesetQ⊆Rn iscalled Q an A-dilation MRA wavelet set. Moreover, Dai, Larson and Speegle [7] provided a criterion for a measurable set Q⊆Rn to be an A-dilation wavelet set. Proposition 1.1. A measurable set Q ⊆ Rn is an A-dilation wavelet set if and only if Q satisfies the following two conditions: (cid:7) (cid:8) (i) BjQ∼=Rn and BiQ BjQ∼=∅ for i(cid:8)=j, j∈(cid:7)Z (cid:8) (ii) (Q+k)∼=Rn and Q (Q+(cid:5))∼=∅ for 0(cid:8)=(cid:5)∈Zn. k∈Zn The condition (ii) of Proposition 1.1 is equivalent to the following condition (cid:9) χ (ξ+k)=1 for a.e. ξ ∈Rn. Q k∈Zn A measurable subset Q of Rn is called a Zn-tiling set if it satisfies the criterion (ii). The wavelets of form F−1(χ ) are a particular class of minimally supported Q frequency(MSF) wavelets (see [10,18]). They are analogs of Shannon wavelet and lack smoothness in the Fourier domain. However, this defect is compensated by good localization properties in the frequency domain and these wavelets may have good potential for applications just like Shannon wavelet or Haar wavelet. Many researchersincludingBaggett,MedinaandMerrill[2], Dai, LarsonandSpeegle[8], Benedetto and Leon [3,5], Gabardo and Yu [13] and so on (e.g. see [4,20,30]) have constructed concrete examples of wavelet sets in Rn. 1.2. Scaling Sets Suppose A is an n×n real expansive matrix and let B =At. The relationship betweenscalingfunctionandwaveletfunctionreflectstheconnectionbetweenscal- ing sets and wavelet sets. Bownik, Rzeszotnik and Speegle [6] clarified the relation between scaling sets and wavelet sets. A measurable set K is called an A-dilation scaling set (resp. MRA scaling set) if Q = BK \K is an A-dilation wavelet set (resp. MRA wavelet set). The A-dilation scaling set introduced by Bownik, Rzes- zotnik and Speegle [6], can be defined in several equivalent ways. The following result, see ([6], Proposition 3.2), can also serve as a definition. Lemma 1.2. A measurable set K ⊆Rn is an A-dilation scaling set if and only (cid:2)∞ if K = B−jQ for some A-dilation wavelet set Q. j=1 SinceQ=BK\K, ifK andQareasinthepreviouslemma, itfollowsthatan A-dilation scaling set can be associated with a unique A-dilation wavelet set and vice-versa. The following lemmas can be easily obtained from [6]. 1. INTRODUCTION 3 Lemma 1.3. A wavelet set Q is an A-dilation MRA wavelet set if and only if (cid:7)∞ the set K := B−jQ is a Zn-tiling set. Such a set K is called an A-dilation j=1 MRA scaling set. Lemma 1.4. Let A be an n×n integral expansive matrix with |detA| = 2. A measurable set K ⊂Rn is an A-dilation MRA scaling set if and only if (i) K is a Zn-tiling set, (ii) K ⊂BK, (iii) lim χ (B−mξ)=1 for a.e. ξ ∈Rn. K m→∞ 1.3. Self-Affine Tiles A self-affine tile in Rn is a measurable(cid:7)set K with positive Lebesgue measure satisfying the set-valued equation BK = (K +d), where B is an n×n real d∈D expanding matrix with |detB| = m is an integer and D = {d ,d ,...,d } ⊆ Rn 1 2 m i(cid:7)s a set of m digits. It has been shown [19] that the set valued equation BK = (K+d) has a unique solution among all compact sets, given by d∈D (cid:5)(cid:9)∞ (cid:6) K =K(B,D)= A−jd , d ∈D . j j j=1 In the case, where B is an n×n integral expanding matrix with |detB|=m and a set of m digits D={d ,d ,...,d }⊆Zn, the set K is called an integral self-affine 1 2 m tile in Rn. The theory of self-affine tiles has been studied by many authors. Particularly, Lagarias and Wang [22–24] studied in detail the structure and tiling properties of self-affinetiles. Theseproblemsareconnectedwiththeconstructionoforthonormal wavelet basis in Rn. Gr¨ochenig and Madych [15] revealed an interesting connec- tion between the theory of compactly supported wavelet bases and the theory of integral self-affine tiles. They showed that Haar-type wavelet bases which can be constructed from an MRA always have an associated scaling function which is the characteristic function of an integral self-affine tile K and that such a tile gives a scaling function if and only if K is a Zn-tiling set in Rn. Gr¨ochenig and Hass [16], LagariasandWang[21]proved, forexample, thatanydilationAyieldsHaar- type wavelets in dimension 1 and dimension 2 respectively using Gro¨chenig and Madych’s result [15]. We should mention that these authors only consider integral self-affine tiles in their work, i.e. the matrix A is an integral expansive matrix and the associated digits are in Zn. Gabardo and Yu [13] also contributed their work to construct wavelet sets using integral self-affine tiles. To our knowledge, until now, there is no result in the literature on the con- nection between the theory of wavelets and the theory of non-integral self-affine tiles. However, there are many wavelets which come from non-integral self-affine tiles. One interesting example is the well-known Shannon wavelet whose corre- sponding scaling set is K = [−1,1] with associated dilation factor A = 2. Then (cid:2) 2 2 BK = (K − 1) (K + 1), where B = At = 2, which shows that K is a self- 2 2 affine tile but not an integral self-affine tile. Gu and Han [17] showed that there
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