Research Article | Open Access
R. A. Hibschweiler, "Weighted Composition Operators between the Fractional Cauchy Spaces and the Bloch-Type Spaces", Journal of Complex Analysis, vol. 2017, Article ID 9486907, 5 pages, 2017. https://doi.org/10.1155/2017/9486907
Weighted Composition Operators between the Fractional Cauchy Spaces and the Bloch-Type Spaces
We characterize boundedness and compactness of weighted composition operators mapping the families of fractional Cauchy transforms into the Bloch-type spaces. Corollaries are obtained about composition operators and multiplication operators.
Let denote the open unit disc in the complex plane and let denote the space of functions analytic in . Let denote the Banach space of complex-valued Borel measures on , endowed with the total variation norm. For , the space of fractional Cauchy transforms is the collection of functions of the form where . The principal branch of the logarithm is used here. The space is a Banach space, with norm given by where varies over all measures in for which (1) holds. The families have been studied extensively [1, 2].
Let . The Bloch-type space is the Banach space of functions analytic in such that , with norm
The integral representation (1) implies that and there is a constant depending only on such that for .
It is known that any univalent belongs to for any . MacGregor  constructed a univalent function such that . Let denote the normalized function . Then . Since , the classical family of schlicht functions, the Distortion Theorem  yields .
Let be an analytic self-map of and let . The weighted composition operator is defined for by If , then the operator reduces to the composition operator defined by . If is the identity function, then is the multiplication operator defined by .
This paper characterizes and for which is bounded or compact. Corollaries are obtained for the operators and .
We follow the convention that denotes a positive constant, which may vary from one appearance to the next.
Theorem 1. Fix and . Let and let be an analytic self-map of . Ifthen is bounded.
Lemma 2. Fix and let . Let for . Then and .
Lemma 3. Fix and let . If , then and there is a positive constant independent of such that
Lemma 4. Let . If and , then and
Lemma 5 will be used to develop test functions needed for the proof of the converse.
Lemma 5. Fix . Let and define Then and there is a constant independent of such that for all .
Proof. First assume and fix generic . A calculation shows that is in the Hardy space and . Since the inclusion is bounded, this case is complete.
Fix . Then By the case for and Lemma 2, is the product of a function in and a function in . By Lemma 4, and there is a constant independent of such that for all .
Finally fix . By the previous case, and for all . By Lemma 3, and . The proof is complete.
Theorem 6. Fix and . Let and let be an analytic self-map of . Assume that is bounded. Then
Proof. Fix , , , and as described. By assumption there is a constant independent of such that for all .
The argument will first establish that . Let and define By Lemmas 2 and 5, there is a constant such that for all . Therefore for all . Since it follows that In particular, (17) yields .
To obtain the second condition in the theorem, let and define By Lemma 5, there is a constant independent of such that . Relation (13) yields for all . Since it follows that for all .
First consider with . By the triangle inequality, relation (21) yieldsfor such . By relation (17), it follows thatFinally consider with . Let in relation (13). Thus and for all . Therefore for all . ThereforeRelations (23) and (26) yield and the proof is complete.
Corollary 7. Fix . Let and let be an analytic self-map.
Xiao  characterized the self-maps for which is bounded for .
Corollary 8. Fix , and as above.
Proof. The equivalence of the first two conditions follows from Corollary 7. The equivalence of the second and third conditions is due to Xiao.
Let and let . The function is a multiplier of into if for every . By the Closed Graph Theorem, it follows that is bounded. The collection of all such multipliers is denoted . In , Ohno et al. characterized .
Corollary 9. Fix and let .
A characterization is given for functions for which is compact.
Lemma 10. Fix and let . Define by Then and there is a constant such that for all .
Proof. First fix and let . A particular case of Lemma 5 provides a constant independent of such that for all . Since Lemma 4 now implies that and for all .
When , By the previous case and Lemma 2, is the product of a function in and a function in . By Lemma 4, and for all .
Fix . By the previous cases and for all . Lemma 3 shows that and . A similar argument applies when . The proof is complete.
Lemma 11 is the standard sequential criterion for compactness.
Lemma 11. Fix . The operator is compact if and only if as for any sequence in with and uniformly on compact subsets of as .
Theorem 12. Fix . Assume that is bounded. The operator is compact if and only if
Proof. Fix and assume that is bounded.
First assume the limit conditions (35) and (36). Corollary 7 implies that is bounded and it now follows as in  that is compact. Suppose that is a sequence in such that for all and uniformly on compact subsets. By relation (4), and thus as . By Lemma 11, is compact.
Now assume that is compact. We may assume that . Let be any sequence in with as . For define By Lemma 5, for all . Also uniformly on compact subsets of as . Thus as and as . Calculations yield as .
The argument will first establish that as . As in , define the test functions where and . Then uniformly on compact subsets as . By Lemmas 10 and 5, there is a constant with for all . It now follows thatIn particular, as . relation (40) is established. Since is a generic sequence with as , relation (35) holds.
To complete the proof note that relations (39) and (40) yield as . Since , as . Condition (36) follows and the proof is complete.
Corollary 13. Fix and assume that is bounded.
Proof. The hypothesis and Corollary 7 yield that is bounded.
Assume that is compact. Since the inclusion is bounded, it follows that is compact.
Assume that is compact. By Theorem 12, conditions (35) and (36) hold. These conditions are sufficient to imply that the bounded operator is compact .
Let and assume that is bounded. In , Xiao provided additional conditions on necessary and sufficient for to be compact.
Corollary 14. Fix and assume that is bounded. The following are equivalent: (1) is compact.(2) is compact.(3).
Proof. Corollary 13 yields the equivalence of the first and second conditions.
Since is bounded, Corollary 8 yields that is bounded. Under this hypothesis, Xiao  proved the equivalence of the second and third conditions.
Fix . In , Ohno et al. characterized for which the bounded operator is compact.
Let and let . Recall that is in the little Bloch space if
Corollary 15. Fix and assume is bounded. (1)Assume . is compact .(2)Assume . is compact .
Conflicts of Interest
The author declares that there are no conflicts of interest regarding the publication of this paper.
- J. A. Cima, A. L. Matheson, and W. T. Ross, The Cauchy transform, Mathematical Surveys and Monographs, American Mathematical Society, Providence, RI, USA, 2006.
- R. A. Hibschweiler and T. H. MacGregor, Fractional Cauchy transforms, CRC Press, Boca Raton, Fla, USA, 2006.
- T. H. MacGregor, “Analytic and univalent functions with integral representations involving complex measures,” Indiana University Mathematics Journal, vol. 36, no. 1, pp. 109–130, 1987.
- P. L. Duren, Univalent Functions, Springer-Verlag, New York, NY, USA, 1983.
- S. Ohno, K. Stroethoff, and R. Zhao, “Weighted composition operators between Bloch-type spaces,” Rocky Mountain Journal of Mathematics, vol. 33, no. 1, pp. 191–215, 2003.
- R. Hibschweiler and E. Nordgren, “Cauchy transforms of measures and weighted shift operators on the disc algebra,” Rocky Mountain Journal of Mathematics, vol. 26, no. 2, pp. 627–654, 1996.
- J. Xiao, “Composition operators associated with Bloch-type spaces,” Complex Variables, Theory and Application, vol. 46, no. 2, pp. 109–121, 2001.
Copyright © 2017 R. A. Hibschweiler. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.