
Let \(\mathbb{D}\) be the open unit disk in the complex plane. The Bergman space \(B^2\) is the subspace of \(L^2\) consisting of the holomorphic functions on \(\mathbb{D}\). For a bounded function \(u\) on \(\mathbb{D}\), the Toeplitz operator \(T_u\) defined on \(B^2\) is given by \(T_uf= P(uf)\), where \(P: L^2\to B^2\) is the orthogonal projection. It is easy to see that if \(\overline f\) or \(g\) is holomorphic, then \(T_fT_g= T_{fg}\). A. Brown and P. Halmos showed that in the Hardy space case the converse is true: if \(T_fT_g= T_h\), then one of the two symbols \(\overline f\) or \(g\) must be holomorphic and in this case \(h= fg\). In the previous paper by the author and the reviewer [J.\ Funct.\ Anal.\ 187, No.~1, 200--210 (2001; Zbl 0996.47037)], it was shown that, in general, the Brown-Halmos theorem is not true for Bergman space Toeplitz operators. It was also shown that there is a theorem of the Brown-Halmos type if we assume that \(f\) and \(g\) are bounded harmonic functions and that \(h\) is a bounded \(C^2\) function whose invariant Laplacian \(\widetilde\Delta h\) is bounded on \(\mathbb{D}\). One of the key ingredients in the proof of this result is a result about the range of the Berezin transform. For any integrable function \(f\) on \(\mathbb{D}\), the Berezin transform is defined by \(Bf(z)= (1-|z|^2)^2 \int_{\mathbb{D}}{f(\zeta)\over|1-\overline\zeta|^4}\,dA(\zeta)\). In the present paper, the author continues this line of investigation. At first, he characterizes all triples \((f,g,u)\) where \(f\) and \(g\) are nonconstant holomorphic functions on \(\mathbb{D}\) and \(u\) is integrable on \(\mathbb{D}\) such that \(Bu= f\overline g\). He starts with an example: \(Bu(z)= z\overline z^2\) where \(u(\zeta)= 2\overline\zeta-{1\over\zeta}\). The main result is very interesting: if \(f\) and \(g\) are holomorphic in \(\mathbb{D}\) and neither is constant and \(Bu= f\overline g\) for some \(u\in L^1(\mathbb{D})\), then there are nonconstant polynomials \(p\) and \(q\) with \(\deg(pq)\leq 3\) and an \(a\in\mathbb{D}\) such that \(f= p\circ\phi_a\) and \(g= q\circ\phi_a\), where \(\phi_a(z)= {a-z\over 1-\overline az}\). The proof is very nontrivial. This result again has consequences for products of Toeplitz operators on \(B^2\). As a special case, the author shows that if \(f\) and \(g\) are bounded harmonic and \(h\in L^1(\mathbb{D})\) is locally bounded such that \(T_fT_g= T_h\), then \(\overline f\) or \(g\) is holomorphic. This gives an improvement of the above mentioned theorem in the previous paper.
Berezin transform, Toeplitz operator, Banach spaces of continuous, differentiable or analytic functions, Brown-Halmos theorem, Toeplitz operators, Hankel operators, Wiener-Hopf operators, Bergman space, Special integral transforms (Legendre, Hilbert, etc.), Analysis, products of Toeplitz operators
Berezin transform, Toeplitz operator, Banach spaces of continuous, differentiable or analytic functions, Brown-Halmos theorem, Toeplitz operators, Hankel operators, Wiener-Hopf operators, Bergman space, Special integral transforms (Legendre, Hilbert, etc.), Analysis, products of Toeplitz operators
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