多重调和Bergman空间上多重调和符号的Topelitz算子
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第13卷第3期 2011年9月 应用泛函分析学报 ACTA ANALYSIS FUNCT10NALIS APPLICATA Vol_l3.NO.3 Sept.,2011 DOI:10.3724/SP.J.1160.2011.00292 文章编号:1009—1327(2011)03—0292—11 Toeplitz Operators with Pluriharmonic Symbols on the P luriharmonic B ergman Space LU Yufeng .ZHOU Xiaoyang2 1 School 0l Mathematical Sciences Dalian University of Technology,Dalian 116024。China 2.School of Science,Dalian Nationalities University,Dalian 1 16600,China Abstract:In this paper,we study Toeplitz operators on the pluriharmonic Bergman space of the unit bal1.For pluriharmonic symbols,we obtain a necessary condition for the product of two Toeplitz operators to be a Toeplitz operator.And we also give a characterization of some pluriharmonic symbols which induce commuting Toeplitz operators and zero semi—commutators, respectively. 'Keywords:function space;product;commuting;Toeplitz operator;pluriharmonic Bergman space CLC number:0177.1 Document COde:A 1 Basic Concepts of Function Spaces Hardy space and Bergman space are main two function spaces. Hardy space The Hardy space H consists of all holomorphic functions on the open unit disk D such that 1 ll/IIH一0<r<1\( I,T ’ ds( )) <+。。 sup…where T stands for the unit circle and ds is the arc length measure,normalized SO that the measure of T equals 1.In terms of Taylor coefifcients,the norm takes a more appealing form:if f(z)=En an “, then 1 _I,l1日z=(∑1。 Bergman space The normalized area measure on D is denoted by dA.Thus d : frd d More generally,for any-I<Ol<∞ write dA ( )=(1+ )(1一 。) dA(z) The weighted Bergman spaces (D)=工, (D,dA。)n日(D) Here H(D)is the space of all analytic functions in日(D).Write AP(]D)=Ag(D). Assume that,( )=E 。n ∈日(D),if∑ lanl。< ,then,∈H ;if∑ 告<。。,then /∈A。 ). Theory of Bergman spaces The Theory of Bergman spaces consists of two main parts: operator theory and function theory.Both theories have counterparts in and are motivated and Received date:2011一O6—15 Foundation item:Supported by the National Science Foundation of China(1097l020) 第3期 LU Yufeng.et al:RToeplitz Operators with Pluriharmonic Symbols Oil 293 inspired by the more classical theory of Hardy.The main tools and methods in the theory of Bergman spaces are:the Bergman kernel,the Bergman projection,the Bergman metric and the function theory. 2 Operators on Function Spaces Toeplitz operator and Hankel operator and Dual Torplitz operators Let P be the Bergman projection from L (D,dA)onto the Bergman space A ( ).Given a function ∈Lo。(D), we define the Toeplitz operator on A (D)with symbol by ,=P( ,),f∈A。( ) The Hankel operator with symbol from A (D)to(A。(D))上by defined, -,=(I—P)( ,),f∈A。 ) The dual Toeplitz operator with symbol on(A (D))上by deifned l厂=(I—P)( ,),f∈(A ( ))上 Connection between these operators It is well known that Toeplitz operator Hankel operator and dual Toeplitz operator are closely related to each other.Given a function ∈ (D), we define the multiplication operator A with symbo1 on (D,dA)by ,= ,,f∈L。(D,dA) Under the decomposition L2( ,dA)= 。(D)0(A (D))上,for ∈ (D),the operator A is represented as 294 应用泛函分析学报 第13卷 2.Toeplitz operators on the Bergman space A (D)difer in many ways from Toeplitz operators on Hardy space H2,for example,Boundedness,Compactness,spectrum,commutativity and algebra properties,etc 3.Connections with function theory. 4.Connection with Analysis,Geometry and Topology,Algebra,and C 一algebra・etc 5.With applications in various fields. 3 Pluriharmonic Bergman Spaces and Toeplitz Operators Let B礼be the open unit ball of the complex space Cn. For P≥1,we let Lp=Lp( ,dv) denote the usual Lebesgue space of the open unit ball Bn in the complex plane.Here,dv denotes the normalized area measure o12 Bn.The pluriharmonic Bergman space b2(Bn)is the space of the Lebesgue s ace L2 consisting of au complex-valued pluriharmonic functions on Bn・ One can check that b2(B )=A (B )+A。(B ) where A2(Bn)denotes the holomorphic Bergman space on Bn-As is well known,the pluriharmonic Bergman space b2(B札)is a closed subspace of L2 and hence is a Hilbert space.Each point evaluati。n is easily veriifed to be a bounded linear functiona1。n b2(Bn)・Hence,f0r each ∈Bn,there exists a unique function Rz∈b2( ),called the pluriharmonic Bergman kernel, which has the reproducing property u(z) :厂 Js ofr every u∈b2(B ).Since b2(B )=A。(B )+A (Bn), there is a simple relation between Rz and the Bergman kernel Kz: Rz:Kz+Kz一 Thus,the explicit formula of R is given by R:= l 1 + 一1, z∈Bn and the orthogonal projection Q:L。(Bn,dv) 6 (Bn)admits the integral representation ( + (1一( ,叫))“+ 1) ( )山( ),z∈Bn for ufnction ∈L2.See[2】for more information and related facts. oFr u∈L2,the Toeplitz operator咒with symbol is defined by f=Q(uf) for function f∈6 (Bn).The operator is densely defined and not bounded in general・ TOeplitz operators on the pluriharmonic Bergman space b2( )difer from Toeplitz operators on the holomorphic Bergman space A2(Bn). For example,in[3】Lee and Zhu obtained a necessarY and sutticient conditi0n for commuting Toeplitz operators with holomorphic symbols on b2( ),but 8uch T0eDlitz operators always commute on the holomorphic Bergman space・In recent years,manY mathematicians Day much attention to Toeplitz operators on the harmonic Bergman space of the unit disk or on the DlurihalrmOnie Bergman space of the unit ball and extend these properties paralleling those of Toeplitz operators OD the Bergman space or Hardy space,for example,see【4-8]・ In the 8etting of the classical Hardy space,Brown and Halmos【 J gave a complete characterization flor the Droduct of two Toeplitz operators to be a Toeplitz operator.On the Bergman space of the unit disk.Ahern and eu6kovid[9]and Ahem[ 。]obtained a similar characterization ofr Toeplitz operators 第3期 LU Yufeng,et al:RToeplitz Operators with Pluriharmonic Symbols on 295 Toeplitz operators on the Bergman space of the。unit ball in C with pluriharminic symbols.The ifrst author characterized commuting Toeplitz operators on the bidisk with pluriharmonic symbols[ 引. Choe,Koo and Lee obtained characterizations of(essentially)commuting Toeplitz operators with pluriharmonic symbols on the Bergman space of the polydisk[ 1. On the harmonic Bergman space of the unit disk,commuting Toeplitz operators were investigated in【5 7】_On the pluriharmonic Bergman space of several complex variables,Lee and Zhu[。]and Lee and Chonnam[引studied commuting Toeplitz operators. Knowing commutativity of two Toeplitz operators often helps to give an idea of what these oper— ators look like;trying to determine commutativity of two Toeplitz operators often leads to interesting problems in analysis.Characterizing commuting Toeplitz operators is still an open problem,whether on the Bergman space or on the harmonic Bergman space.In Section 5 and 6,we will study semi— commuting and commuting Toeplitz operators on the pluriharmonic Bergman space of the unit ball, respectively. 4 Product of Toeplitz Operators on Pluriharmonic Bergman Spaces Before stating our results,we recall some notation.Let P denote the Bergman projection of L onto A。(B ).Since R2=Kz+ 一1,and )( d for function U∈L ,the projection Q can be represented by Q( )=P(u)+P(瓦)一P( )(0) ofr function U∈L2. For any multi-index =(OL1,…,Ozn),where each OLk is a nonnegative integer,we will write I= 1+…+ n, !=Ol1 1…Odn We will also write Z。=Zl … ” ofr Z=(Zl,…,Zn)∈ . oFr two multi—indexes =(Oq,…,Oln)and =( 1,…, ),the notation means that OLi≥ , i=1,…,n 296 应用泛函分析学报 第13卷 Wle also define 一 =( 1一 1,…, 一 ) for a .Moreover, 一 I=I I—I We begin with some simple properties of the Bergman projection,which are taken from[。】and quite useful in this paper. The following lemma is Lemma 5 in[3】. Lemma 4.1 Let f∈A (Bn)and assume f(z):∑a A。Z is the power series representation off.Then P( ,)( )=∑ A t筹 几十 1 ofr all Z∈B and 1≤k≤n.If in addition f(O)=0,then P(wkf)=P(面 .厂)(0) ofr all 1≤k≤n The following lemma is Lemma 10 in[3]. Lemma 4.2 Let f,g∈A (Bn)and h be bounded.Then the following statements hold (a)P(7P( 9))( )=P(7 g)(z); (b)P(fP(hg)))(0):P(f蚵)(0). The following Theorem is a necessary condition for the product of two Toeplitz operators with pluriharmonic symbols to be a Toeplitz operator on b2(玩). Theorem 4.1 Let Gr∈L and assume U=f ̄h,V=_9+ for f,g,h, ∈A (Bn).If = , then a nontrivial linear combination of f and g is constant and a nontrivial linear combination of h and k is constant. Proof Without loss of generality,we may assume f(o)=h(0)=g(0)=k(0)=0. By Lemma 4.1,for each J=1,…,n, ( )=P( ̄-#7g)+P(wj ̄)一P(面 )(0)=P( ̄-Tg) (巧)=Q(,P(巧9))=fP( ̄'Tg) and ( )=Q( P(砀一9))=P(h-P( ̄Tg))+P(h—P(— ̄-Tjg)) —P(hP( ̄-f-g))(O) By Lemma 4.2 (巧)=P(hwig)+P(hP( ̄-fg))一P(hw#g)(O) Since Q(kwj) =kw#,we have (巧)= (kwj)=P(fkw#)+P(fkw#)一P(fkwj)(O) Note that (巧)=hkw# It fo・llows that (巧)=咒 (巧)=,P(面-9)+P(hw#g)+P(^P(面-9))一P(hwjg)(O) 第3期LU Yufeng,et al:RToeplitz Operators with Plurihaxmonic Symbols on… 297 +P(fkwj)+P(fkwj)一P(fkwj)(O)+hkwj Similarly,we have ( )= (巧)=kP(- ̄h)+P(-g ̄-h)+P(夕P( ))一P(f- ̄h)(O) +P(k—fw—j)+P(-kfwj)一P(k—fw—j)(O)+—gf—wj T ̄(w3)= ( )=fgwj+P(fP(@Tk))+P(fwjk)一P(fwjk)(O) +P(wjgh)+JF)(, 面 )一P(wjgh)(O)+hP( ̄Tk) ( )=马 ( )= +P(k 丽)+P(一kwjf)一P(kwjf)(O) +P(wjh ̄)+P(g—wj—h)一P(wjh- ̄)(O)+ Note that for each J=1,・一,n, 正 (瓦 )=P( )+P(awj)一P( 酉)(0) T.(wj)=P(awj)+P( 巧)一P(awj)(O) and 咧。)=fO" d = 巧 =P(瓦 )(0) Bn SO we have (巧)= (叫j) Similarly, (巧):T ̄(wj) Thus P(巧 )+P(kfwj)+P(g 丽)=P(f 丽)+P(wjg-h)+ P(巧 ) (1) fP( ̄Tg)+P(fkwj)+P(hP(@Tg))=P( P(瓦 I厂))+P(“0, )+gP( ̄-j]f) (2) Since (0)P(:瓦 ^)(0)= (0)P(瓦 )(0)=0 we can take the holomorphic part on both sides of(1)to obtain kP( ̄-j-h):hP(@Tk).By Theorem 8 in[3]j h and k are linearly dependent. It follows from the equation(2)that fP(w'Tg)=9P(巧,).So f and g are linearly dependent. The proof is complete. 5 Semi-commutators of Toeplitz Operators Pluriharmonic Bergman Spaces In[3,Lemma i4],Lee and Zhu only gave a necessary condition ofr zero semi-commutators as follows. Lemma 5.1 Let ,V∈b2(Bn)and assume U=,+ ,V=g+ for holomorphic.厂,g,h,k.If "= on b2(Bn),then at least one of,and g is constant,and at least one of h and k is constant. Similar to the proof of Theorem 3.1 in[6】 jwe also get the following Lemma. Llemma 5.2 Let f,g∈A。(Bn).Then the following conditions are equivalent. 1) =T ̄Ts; 2)Tro=T- ̄Ts; 3) 虿= 巧. 298 应用泛函分析学报 第l3卷 On holomorphic Berginan space,for f,g∈A , = 虿always holds.Returning to tile pluriharInonic Bergman space,we have not obtained the complete characterization fox‘the abow ̄ equations,but we do have the following result if we put SOIIle restrictions on tile symbols. Theorem 5.1 Let f∈A0(Bn),and :c+aw。,where a,c are constant,the multi—imtex“≠0. Then = ,i on b2( )if and only if either f or is constant. Proof We only need to prove the necessity.Without loss of generality,we may asslnne f(O) k(0)=0.For any multi—index , ( )=fP(kw )+P(,P( ))4-P(fw' ̄k)一P(fw )(O)一fP(kw )(0) rs ̄(w )=P(fw )+P(fw )一P(fw )(0) Since (“, )= ( ) fP(kw )一fP(kw )(0)+P( P( ))=P(fw ) By the assumption that 0≠0,we will consider two cases as follows. Case 1 Assume that there exists an integer i with 1≤i≤凡such that(Ii=0. We can choose a multi—index-y such that oL and ≠oL.Then p(k- ̄T)=0,P( 叫 )(0) and therefore_厂P( 。)=P(, ).We can write ,=∑6 卢 0>.o with b(0,….0)=0,then .厂P( 叫 ) :n 4-l71)!(一y— ) 卢+ 一 ∑瓦 (8 o Since P(fw 尼1 =∑瓦 (佗4-l l -I4 I)!( 4-7一 )! 臼 o ( 4- )!(他4-l l十I l—I 1) we have ( namely 一 ≠0,then =一) If there exists a nonzero multi—index such that 1(n -l47l—l I)! ( 4-7)!(n -I4 {4-l7I—1 1) (n -I471)!( — ) (n 4l- l -l4 })!( + 一 )! ll +l7l—j I)!(n+l71) n l( 4- )!( 一 )! f∥l+l 1)!(几+l l—l 1) o),where e i)=l and e orF any i=1,2,.一,礼,let e(。)=(0,.-. 0,1 1( + 一 )! …fn (3) 0 whencve ̄‘j≠i =,,And let 7=ct+/e( )and = +(f+1)e( ,where l≥1.It is easy to see that I By the equation(3), I7l+1=l l+f+l 7 1( 4- 一 )!( +7 )!( 一 )! (n -l4 }-l4 I—l“1)!(n -l4,yI)!(n -I4 l+I 1)!(n+l—y I—l 1) ( 4- )!( 一 )!,y !( + 一 )! (n 4{-∥l -l4—yI)!(几十I l—I 1)!(n+l I+l l—I 1)!(n -I4 {) that is, + t+l+1)(1+1) (礼4-I l+} l+1+1)(n+f+1) +l -14)( +1+1) (n十l I+2+1)(n+l l+f+1) Since =0,then l+1) 1+11 第3期 LU Yufeng,et ah RToeplitz Operators with Pluriharmonic Symbols on >0.Note that the function 299 For oz, ≠0,l l>0, ,= for >0.then 一 oL (! I+ )(I i+ ) (n+I J+1 l+1+1)(他+1+1) (他+f l+l+1)(n+l l+1+1) It is a contradiction.Thus a=0 or be=0 for any . <O Case 2.Assume 1 and let = 一e( )+e( )=( 1,ot2,…,od 一1,.一,olm+1,…,oLn) with m≠i.It is easy to see that for any with ≠0,we have +7 oL.Write ,=∑bez eLo with b(o,…,0) 0,then P(fw )= ∑ +一y 0 瓦6 __ } + 一。 一 =fP(kw )一fP(kw )(0)+P(1厂P( )) 0 =Thus a=0 or be=0 for any with fli≠0. If a=0.then k=0. is stated above,for any i= If a≠0,then f=∑ :0 bez ,i.e.f is independent of zi.As 1,2,・-.,佗,f is independent of zi.So f=0.The proof is complete. Theorem 5.2 Let f=f+h,u=9+ ,where f,i9,h, ∈A。(B ). Suppose that one of f,9,h, is the snnl of a constant and a monomia1.Then COIlstant. = 口on b2( ) if and only if either u or v is Proof The sufficiency is trivia1.For the proof of necessity,we may assuIne that either g or is tile sum of a constant and a monomial,since two cases as follows = 而.By Lemma 5.1,we need only to consider Case 1 If is holomorphic and"is co—holomorphie.then“=f.1,= and k is the surrl of a constant and a monomial,and .By Theorem 5.1,the proof is complete. Case 2 If札is co—holomorphic and is holomorphic.then u= ."=9 is tile sum of a constant and a monomial,and 瓦・By Lemma 5.2, = .Then死 = 可by taking adjoints.So the proof is complete by Theorem 5.1. 6 Commuting Toeplitz Operators on Pluriharmonic Bergman Spaces hl this section,our goal is to obtain sortie characterizations of commuting Toeplitz operators with pluriharmolifC symbols on b2(Bn). Theorem 6.1 Let札, ∈D。(B )and assulne“=.厂+ , ,=g+ for f,g,h,k∈A。(Bn).If = , on b2(Bn),then a nontrivial linear combination of,and g is constant and a nontrivial 1inear combinatio ̄l of h and is constant. Proof Without loss of generality,we let f(0) h(0) g(o)=k(o)=o.For each J (巧)=。厂尸(巧9)+P(h,wjg)+P(hP(b- ̄jg))一P(hwjg)(0) 应用泛函分析学报 第13卷 +e(f- ̄j)+P(Tk ̄j)一 P(fkwi)(O)+一hkwj (巧)=gP( ̄jjf)+e(k ̄df)+P( P(巧,)) P(- ̄wjf)(O) P(g )(0)+hkwj By死 (砑)=TvTu( ̄),we have ,P(面.夕)+P(f'kwj)+P( P(巧9)) :9P(巧,)+P h j)+ P(kP( ̄Tf)) since,(0)P( )(0): 尸(巧,)(0)=0,we can take the h。l。moTphic p2Lrt。nb。 h s des。f h。 above equation to get fP( ̄jjg1=gP( ̄-Tf) Bv Theorem 8 in[4],,and g are linearly dependent 0n the。ther hand,by死 ( )= ( ),and we take the c。。holoHl()rphi p g。 P(瓦 )= P( h) So九aIld are linearly dependent.The proof is complete・ . In the f0ll。wing,we will describe several specia1 symb。1s which induce commuting上b。plif onerators. c。r。llary 6.1 Let u, be n。nc。nstant re valued pluriharm。nic f11ncti。ns・Then L and c0mnmte on b2(Bn)if and only if = u+ ,where , ∈Ⅱ ‘ Pr。。f We。nly need t。pr。ve the necessity.Since札, are nOnc。璐talnt reaLvalued plunharm。 c functions,we can write u:,+了, :-9+ ,where,,9∈A2( ),andb。也,and g a nonconstant. …If and c。mmute。n b2(Bn),then by The。rem 6.1,,= g+ ,where Q, 。c。n ant・vve n ea8ily see that TS+7Tg+ ̄=rg+ ̄Ts+7 imp1ie that ((9/一 )( 马一谒)=0 Since口is nonconstant, and c叽n0t c。mmute by Lemma 5.2 and The。rem 17 n[3】_s。 :西, i,e,Q∈ .Thus :f+7:ag+ +7+可= + where :,y+可∈ .The proof is complete‘ I 『3]£ee d zhu。btaiI坨d the characterizatiOn。f c。mmuting Toeplitz。peratOrs w h hO1。mOr Lemma 6.1 Let,1 g∈ ( )be n。ncOnstant functi。ns.Then and c。m n 。n b'Z(Bn) if and only if,=Ag+ for some c0n8tam七s入, ・ Under some conditions,commuting Toelpitz operators are c㈣pletely descr be d. The。rem 6.2 Let u∈bU(B礼),and 9 be the sum。f a c。nstant and a m。n。m al・ l he .=TgT ̄on b2(Bn)if and only if a nontrivia1 linear combina tionofuandgisconstant.Proof 2rearefuncti。ns,, 、W_e only need to prove the necessity. 上 If∈∈口 6(B’n,’¨ ll¨ ),thenthe……… in 。(Bn)such that钆:.厂+元.By The。rem 6.1,a nontr ial linear c。Ⅱ ina i。n。f,and夕 c。n nt‘ If口:口(01,0. +g is constant.’ If ≠口f01,then there are constants n and c sUCh that,=。9+c・s。 = mphes曲龇 T- ̄Tg: .By Lemma 5.2 and The。rem 5.1,^is c。nstant・Then ∈ (Bn),and a n。nt Ⅳ linear c0mbination of and夕is constant by Lemma 6・1・The Proof is complete. 第3期LU Yufeng,et ah RToeplitz Operators with Pluriharmonic Symbols on… 301 Theorem 6.3 Let u=/+ ,v=g+ ,where/,g,h,k∈A (B ).Suppose that one of f,g,h, is the sum of a constant and a monomia1.Then linear combination of“and v is constant. = on b2( )if and only if a nontrivial Proof We only need to prove the necessity.Since assume that either g or = by taking adjoints,we may is the sum of a constant and a monomial,and nonconstant.By Theorem 6.2.we can also assume that both g and k are nonconstant.By Theorem 6.1,there are constants 01,02,c1,52 such that f=alg+Cl,h=a2k+c2.Note that =a1 and +瓦 +(c1+瓦) +a1 +瓦 4-(c1+瓦) =al Since = ,we have -瓦 4-(Cl4 4-瓦) 4-aIT ̄G -4 -(4Cl 4- ) . (al一 )( It follows from Theorem 5.1 that al=瓦.Thus =一 )=0 f -h=al4g -Cl4 4-a2k 4-瓦=al(g+ )4-c1 4-瓦=alv+c1 4-瓦 The proof is complete. References: Brown A,Halmos P R.Algebraic properties of Toeplitz operators[a].J Reine Angew Math,1964,213: 89-102. Axler S.Bourdon P,Ramey W.Harmonic Function Theory[M】.New York:Spring-Verlag,1992. Lee Y J,and Zhu K.Some diferential and integral equations with applications to Toeplitz operators[J]. Integr Equ Oper Theory.2002.44:446—479. Guo K and Zheng D.Toeplitz algebra and Hankel algebra on the harmonic Bergman space[J].J Math Anal Appl,2002,276:213—230. Choe B R,and Lee Y J.Commutants of analytic Toeplitz operators on the harmonic Bergman space[J]. Integr Equ Oper Theory,2004,50:559—564. Ding X.A question of Toeplitz operators on the harmonic Bergman space[J].J Math Anal Appl,2008, 344=367-372. Choe B R,Lee Y J.Commuting Toeplitz operators on the harmonic Bergman space[J].Michigan Math J,1999,46:163-174. Lee Y J,Chonnam.Commuting Toeplitz operators on the pluriharmonic Bergman space[J].Czechoslovak Mathematical Journal,2004,54(129):535 544. Ahern P and Cu ̄kovid Z.A theorem of Brown—Halmos type for Bergman space Toeplitz operators[J].J Funct Anal,2001,187:200—210. Ahern P.On the range of the Berezin transform[J].J Funct Anal,2004,215:206—216. Axler S,Cu ̄kovid Z.Commuting Toeplitz operators with harmonic symbols[J].Integral Equations Oper— ator Theory,1991,14:1-12. Stroethoff K.Essentially commuting Toeplitz oprtators with harmonic symbols[J].Can J Math,1993,45: 1080-1093. Axler S,CuSkovid Z,Rao N V.Commuting of analytic Toeplitz operators on the Bergman space[J].Proc Amer Math Soc,2000,128:1951-1953. Zheng D.Commuting Toeplitz operators with pluriharmonic symbo|s[J1.Trans Amer Math Soc,1998, 350:1595-1618. Lu Y F.Commuting of Toeplitz operators on the Bergman space of the bidisc[J].Bull Austral Math Sco, 2002,66:345-351. 302 应用泛两分析学报 第13卷 [16】Choe B R,Koo H,Lee Y J.Commuting Toeplitz operators on the polydisk[J].rIYalis Amer Math Soe 2004,356:1727 1749. 多重调和Bergman空间上多重调和符号的Topelitz算子 卢玉峰 ,周晓阳。 1.大连理工大学数学科学学院,大连116023 2.大连民族学院理学院,大连116600 摘要:研究多重调和Bergman空间上的Topelitz算子.对多重调和符号的Topelitz算子,给出了乘 积性质、交换性质的符号描述. 关键词:函数空间;乘积;Toeplitz算子;多重调和Bergman空间