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beurling's theorem and invariant subspaces for the shift on Hardy spaces

Science China Mathematics 2008年第1期51卷 131-142页

作者： QIU ZhiJian School of Economic Mathematics, Southwestern University of Finance and Economics, Chengdu 610074, China School of Economic Mathematics Southwestern University of Finance and Economics Chengdu China

Let G be a bounded open subset in the complex plane and let H 2(G) denote the Hardy space on G. We call a bounded simply connected domain W perfectly connected if the boundary value function of the inverse of the Riemann map from W onto the unit disk D is almost 1–1 with respect to the Lebesgue measure on ?D and if the Riemann map belongs to the weak-star closure of the polynomials in H ∞(W). Our main theorem states: in order that for each M ∈ Lat (M z ), there exist u ∈ H ∞(G) such that M = ∨{uνH 2(G)}, it is necessary and sufficient that the following hold:each component of G is a perfectly connected domainthe harmonic measures of the components of G are mutually singularthe set of polynomials is weak-star dense in H ∞(G).Moreover, if G satisfies these conditions, then every M ∈ Lat (M z ) is of the form uH 2(G), where u ∈ H ∞(G) and the restriction of u to each of the components of G is either an inner function or zero.

关键词： Hardy space invariant subspace shift operator 47b20 30H05 30E10 46E15

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Science China Mathematics 2007年第3期50卷 305-312页

作者： Zhi-jian QIU Department of Economic Mathematics, Southwestern University of Finance and Economics, Chengdu 610074, China Department of Economic Mathematics Southwestern University of Finance and Economics Chengdu China

For a compact subset K in the complex plane, let Rat(K) denote the set of the rational functions with poles off K. Given a finite positive measure with support contained in K, let R2(K,v) denote the closure of Rat(K) in L2(v) and let Sv denote the operator of multiplication by the independent variable z on R2(K, v), that is, Svf = zf for every f∈R2(K, v). SupposeΩis a bounded open subset in the complex plane whose complement has finitely many components and suppose Rat(Ω) is dense in the Hardy space H2(Ω). Letσdenote a harmonic measure forΩ. In this work, we characterize all subnormal operators quasi-similar to Sσ, the operators of the multiplication by z on R2(Ω,σ). We show that for a given v supported onΩ, Sv is quasi-similar to Sσif and only if v/■Ω■σ and log(dv/dσ)∈L1(σ). Our result extends a well-known result of Clary on the unit disk.

关键词： subnormal operator quasi-similarity harmonic measure Hardy space 47b20 30H05 30E10 46E15

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