连通区域的Dirichlet空间上的复合算子

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第7卷 2008侄 第2期 4月 广州大学学报(自然科学版) Journal of Guangzhou University(Natural Science Edition) Vo1.7 Apr. No.2 2008 文章编号:1671-4229(2008)0243018-04 Compositi,,perat ̄ of Dirichletomposition operators of Dirichlet spaces on connected domains xlA lin.WANG Xiao—feng (School of Mathematics and Information Sciences,Guangzhou University,Guangzhou 510006,China) Abstract:In this paper,the authors discuss how a composition operator from one Dirichlet space 0n connected domains in complex plane to another Diriehlet space on connected domains in comp1ex plane be invertible or Fredholm operator.The authors attain the results that the compositi0n 0Derat0r is invertible if and only if the inducing mapping of the composition operator is an injective,and the range of the inducing mapping satisfies some kind of boundary condition.Moreover.the composition operator is Fredholm operator if and only if the inducing mapping of the composition operator is an in— jective,and the range of the inducing mapping satisfies some kind of boundary condition. Key words:connected domain;composition operator;Fredholm composition operator CLC number:0 177 Document code:A Let M,Ⅳbe two open.connected.non—empty subsets in the complex plane,they are said to be connected domains. Dirichlet spaces D(M)are the spaces which consist of the complex—value.measurable.analytic functions defined on M whose derivatives are square integrable with respect to the area nteasure on M.We know that D(M)are Hilben spaces when they are mode by the complex number spaces C.In fact,if f ∈D(M),thenfQ-Lj(M),which are Bergman spaces on M. is the topological boundary of M.a point A∈蝴is said to be removable with respect to D(M)if there exists an open neighborhood V of such that every.function in D(M) can be extended to an analytic function defined on M U The set of all points of 0M which are removable with respect to D (M)is denoted by Od-rM.Similarly.the set of all points of which are removable with respect to Lj(M)is denoted by 一, M.Dirichlet essential boundary of M denoted by M is the set of all points of 0M which are not removable with respect to D(M);so cga_ ̄M=OM—cg,l_rM. If M is finitely connected.that is,there age finitely holes in M,then the Dirichlet removable boundary of M is just the set of isolated points of OM.The concept of essential boundary is first induced by Axler S,Conway J B and Mcdonald G lJ_ In Section 2 of this paper,we will give characterization of invertible and Fredholm composition operators. Invertibility of composition opera- tors The research on invertibility oi composition operators on Hardy spaces H2(D)was begun in Ref.[2].Hatori O , Cao G F and Sun S Hl 』proved an analogue of the result for the case of//2(B ),where B is the unite ball of C .For composition operators on other spaces,similar results were proved by Bourdonl’j and Singh and Veluchamyl .Hiroyuki Takagi 7j considered the Fredholm weighted composition oper- ators on C(X)or LP(X),where X is a set with some regular prope ̄y like intervals or balls in R .In Ref.[8],Ioana Mi— haila discussed the invertibility of composition operators on Ri— emann surfaces. Received date:2007—03—23;Revised date:2007—06—15 Foundation items:Supposed by Chinese National Natural Science Foundation Math Tianyuan Foundation(10526040) Biography:XIA Jin(1973一),female,instructor,master,mainly researches on functional analysis.

 维普资讯 http://www.cqvip.com 第2期 XIA Jin,et al:C。mp。siti。n。perat。rs。f Dirichlet sPaces on e0nnected doraains l 9 Proposition 1 Suppose M.N are connected domains in C,p: Ⅳis an analytic map such that lP 。P } dA。P ≤c(L4 Oil N,where C is a complex constant.Then c口is a bounded operator from D(N)to D(M). Proof By the identity J I/I Ip 。P J dA。P~= L I/(p)J dA(V,∈D(Ⅳ)), one can see easily that Cp is bounded・ Theorem 2 Suppose M,N are connected domains in C, P:M一'N is a nonconstant analytic map such that J(P ) 。P l dA。P≤CdA on M and satisfy Proposition 1. Then C。is an invertible operator from D(N)to D(M)if and only if (i)p is injective; (ii)N-p(M)C 一 (p(M)). Proof Assume C。is an invertible operator from D(Ⅳ) to D( ),we first prove that P is injective.Let ( )and (w)denote respectively the reproducing kernels of D( ) and D(N),it is routine to check that for any E-M,we have C:Kz=K 1. If there are I, ∈M with I≠ 2 such that P( )= p(z2),then c ( l— 2)=0.It is obvious that l— 2≠ 0,this contradicts the invertibility of .Hence P must be in— jective.(i)is complete. Since p(M)CN,we see that D(N)CD(p(M)).cD is also an operator from D(p(M))to D(N).We write the oper— ator by is a bounded operator from D(p(M))to D(M)by Proposition l,This shows that c 。。ep is a bounded operator from D(p(M))to D(N).For any g∈D(p(M)),we have 。。 gE-D(N).Write h= 。 gE-D(N), aJ1d :cp—I is the composition operator from R(cp)to D(p (M)).Then TpCph:TpCpg=g. F0r any w∈p(M),it is easy to see that( )(P w) =g(w).Hence every function in D(p(M))may be extended to a function in D(N).This implies(ii). Conversely,if(i)and(ii)in the Theorem hold,then cD: D(p(M))-- ̄D(M)is invertible with inverse Cp Since N— p(M)C0d_r(p( )),we see that D=(P( ))=D(Ⅳ), This shows that cp is indeed invertible. Corollary Suppose M and N are connected domains in C,p: Ⅳis an analytic map such that 0d,(p( )): ・ Then Cp is an invertible operator from D(fv)。nt。D(M)if and only if p is inveaible. 2 Fredholmness of composition 0p- erators we know that on almost all spaces of analytic functions, the invertibility of a composition operator is equivalent to it be— ing Fredholm.It is natural to consider the Fredholmness of composition operators on connected domains. Lemma 3 For any connected domain M in C,the area measure of OdrM is zero. Proof Let K be a compact subset of 0d,M.Suppose that K had positive area measure.Then there would exist a non—c0nstant bounded analytic function h defined on C—K (Garnett J ).In particular,hIM∈D(M)CL:(M),and SO h would extend to a non—constant bounded analytic function defined on all of C(Axler S,Conway J B and McDonald G【 ).This contradicts Liouville’S Theorem,and so K must have zero area measure.Since every compact subset of G ̄drM has zero measure,we can conclude that M has zero area measure. Theorem 4 Suppose M.N are bounded connected do- main in C,p:M_N is a nonconstant analytic map such that l(p ) 。pl dA。P≤CdA on M and satisfy proposition 1. Then C。is a Fredholm operator from D(N)to D(M)if and only if (i)p is injective; (ii)N-p(M)C0d一,(p( )). Proof By Theorem 2,we have only to prove the necessi— ty.Assume cD is Fredholm operator,we first prove that P is injective.In fact,if z1,z2∈M with z1≠z2 such thatp(z1): p(z2),then there exists open subsets U1,U2 of M such that Z ∈U and Ul nU2= .Write w =p( )。SinceP is an open map,both P(U1)and P(U2)are open sets which contain w. Thus P(U1)n P( )is a non—empty open subset of N. Choose a sequence{w }inP(U1)Mp(U2)with w ≠ ,( ≠ ,)’then there are sequences ’}CUI and ’}CU2 such thatp( ): (i=1,2).Set = } )一 } ),then { }is a linear independent sequence in D(M)since