On the Growth of the Cauchy-Szegő Transform in the Unit Ball

The growth of analytic and harmonic functions in the unit ball Bn represented by the Cauchy-Stieltjes or Poisson-Stieltjes integral is studied. A description of the growth is given in terms of smoothness of the Stieltjes measure. Изучен рост аналитических и гармонических функций в единичном шаре, пр...

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Published in:Журнал математической физики, анализа, геометрии
Date:2015
Main Authors: Chyzhykov, I., Voitovych, M.
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Published: Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України 2015
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spelling Chyzhykov, I.
Voitovych, M.
2017-05-28T18:46:07Z
2017-05-28T18:46:07Z
2015
On the Growth of the Cauchy-Szegő Transform in the Unit Ball / I. Chyzhykov, M. Voitovych // Журнал математической физики, анализа, геометрии. — 2015. — Т. 11, № 3. — С. 236-244. — Бібліогр.: 11 назв. — англ.
1812-9471
DOI: 10.15407/mag11.03.236
MSC2000: 32A26 (primary); 32A25 (secondary)
https://nasplib.isofts.kiev.ua/handle/123456789/118150
The growth of analytic and harmonic functions in the unit ball Bn represented by the Cauchy-Stieltjes or Poisson-Stieltjes integral is studied. A description of the growth is given in terms of smoothness of the Stieltjes measure.
Изучен рост аналитических и гармонических функций в единичном шаре, представленых интегралом Коши-Стилтьеса или Пуассона-Стилтьеса. Описание роста дается в терминах гладкости меры Стилтьеса.
en
Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
Журнал математической физики, анализа, геометрии
On the Growth of the Cauchy-Szegő Transform in the Unit Ball
Article
published earlier
institution Digital Library of Periodicals of National Academy of Sciences of Ukraine
collection DSpace DC
title On the Growth of the Cauchy-Szegő Transform in the Unit Ball
spellingShingle On the Growth of the Cauchy-Szegő Transform in the Unit Ball
Chyzhykov, I.
Voitovych, M.
title_short On the Growth of the Cauchy-Szegő Transform in the Unit Ball
title_full On the Growth of the Cauchy-Szegő Transform in the Unit Ball
title_fullStr On the Growth of the Cauchy-Szegő Transform in the Unit Ball
title_full_unstemmed On the Growth of the Cauchy-Szegő Transform in the Unit Ball
title_sort on the growth of the cauchy-szegő transform in the unit ball
author Chyzhykov, I.
Voitovych, M.
author_facet Chyzhykov, I.
Voitovych, M.
publishDate 2015
language English
container_title Журнал математической физики, анализа, геометрии
publisher Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України
format Article
description The growth of analytic and harmonic functions in the unit ball Bn represented by the Cauchy-Stieltjes or Poisson-Stieltjes integral is studied. A description of the growth is given in terms of smoothness of the Stieltjes measure. Изучен рост аналитических и гармонических функций в единичном шаре, представленых интегралом Коши-Стилтьеса или Пуассона-Стилтьеса. Описание роста дается в терминах гладкости меры Стилтьеса.
issn 1812-9471
url https://nasplib.isofts.kiev.ua/handle/123456789/118150
citation_txt On the Growth of the Cauchy-Szegő Transform in the Unit Ball / I. Chyzhykov, M. Voitovych // Журнал математической физики, анализа, геометрии. — 2015. — Т. 11, № 3. — С. 236-244. — Бібліогр.: 11 назв. — англ.
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AT voitovychm onthegrowthofthecauchyszegotransformintheunitball
first_indexed 2025-11-25T22:46:31Z
last_indexed 2025-11-25T22:46:31Z
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fulltext Journal of Mathematical Physics, Analysis, Geometry 2015, vol. 11, No. 3, pp. 236–244 On the Growth of the Cauchy–Szegő Transform in the Unit Ball I. Chyzhykov and M. Voitovych Faculty of Mechanics and Mathematics, Lviv Ivan Franko National University 1 Universytetska Str., Lviv 79000, Ukraine E-mail: chyzhykov@yahoo.com urkevych@gmail.com Received September 22, 2014, revised January 14, 2015 The growth of analytic and harmonic functions in the unit ball Bn rep- resented by the Cauchy–Stieltjes or Poisson–Stieltjes integral is studied. A description of the growth is given in terms of smoothness of the Stieltjes measure. Key words: holomorphic function, Cauchy–Szegő transform, modulus of continuity, Lipschitz class, Poisson integral, Cauchy integral, Cauchy– Stieltjes integral, Poisson–Stielstjes integral, unit ball. Mathematics Subject Classification 2010: 32A26 (primary); 32A25 (se- condary). 1. Introduction It is reasonable to expect for a function analytic in a bounded domain and continuous up to the boundary to be smooth on the boundary if its derivative grows slowly, and conversely. For the unit disk this was established by Hardy– Littlewood ([8], [6, Chap. 5]). We say that a complex-valued function f(eiθ), θ ∈ R is of the class Λ∗α (0 < α ≤ 1) if ω∗(t) = O(tα) as t → 0 where ω∗(t) is the modulus of continuity of f(eiθ), i.e., ω∗(t) = sup |eiθ1−eiθ2 |≤t |f(eiθ1)− f(eiθ2)|. Theorem A ([6, Theorem 5.1]). Let f(z) be a function analytic in D = {z ∈ C : |z| < 1}. Then f(z) is continuous in D and f(eiθ) ∈ Λ∗α (0 < α ≤ 1) if and only if f ′(z) = O ( 1 (1− |z|)1−α ) . c© I. Chyzhykov and M. Voitovych, 2015 On the Growth of the Cauchy–Szegő Transform in the Unit Ball Let z, w ∈ Cn, n ∈ N, 〈z, w〉 = ∑n i=1 ziwi, |z| = 〈z, z〉 1 2 . We denote by Bn = {z ∈ Cn : |z| < 1} the unit ball in Cn and by Sn = {z ∈ Cn : |z| = 1} the unit sphere. For a complex-valued function f on Sn and a Borel measure µ on Sn, we denote the Cauchy integral C[f ](z) = ∫ Sn f(ξ)dm2n−1(ξ) (1− 〈z, ξ〉)n , z ∈ Bn, where m2n−1 is the normalized Lebesgue measure on Sn, m2n−1(Sn) = 1, and C[µ](z) = ∫ Sn dµ(ξ) (1− 〈z, ξ〉)n , z ∈ Bn (1) the Cauchy–Stieltjes integral. Similarly, we denote by P [f ](z) = ∫ Sn (1− |z|2)n |1− 〈z, ξ〉|2n f(ξ)dm2n−1(ξ), z ∈ Bn, P [µ](z) = ∫ Sn (1− |z|2)n |1− 〈z, ξ〉|2n dµ(ξ), z ∈ Bn the Poisson and Poisson–Stieltjes integrals, respectively. Let f be a holomorphic function in Bn and f = ∑∞ k=0 Fk be the homogeneous decomposition of f , then (Rf)(z) = ∑∞ k=0 kFk(z), z ∈ Bn is the radial derivative. The following theorems were proved by W. Rudin for several complex variables. Theorem B ([10]). Let 0 < α < 1 and f be a measurable complex func- tion such that |f | is integrable with respect to the measure m2n−1 on Sn. Then |f(eiθξ)− f(eitξ)| ≤ |eiθ − eit|α, ξ ∈ Sn, θ, t ∈ R, implies that |(RC[f ])(z)| ≤ Aα(1− |z|)α−1, z ∈ Bn. Theorem C ([10]). Let 0 < α < 1 and f be holomorphic in Bn. Then |(RC[f ])(z)| ≤ (1− |z|)α−1, z ∈ Bn, implies that f has a continuous extension to Bn which satisfies the Lipschitz condition of order α. Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3 237 I. Chyzhykov and M. Voitovych Some results in this direction that concern the unit polydisk can be found in [4, 5, 7]. In particular, necessary and sufficient conditions of the growth of Poisson– Stieltjes integral in terms of Stieltjes measure were described in [3, 4]. Some properties of harmonic functions of Lipschitz type spaces and their generalizations are described in [1, 2]. In particular, a multidimensional counterpart of Theorem A for harmonic functions in Bn was proved by S. Krantz in [9]. Note that, in general, the functions represented by the Poisson–Stieltjes integral or the Cauchy– Stieltjes integral can not be represented by the Poisson integral or the Cauchy integral, respectively. We are interested in the description of the growth of analytic and harmonic functions in the unit ball Bn represented by the Cauchy–Stieltjes or the Poisson– Stieltjes integral. The case of differentiable measures (with respect to m2n−1) is well known (see, e.g., [10, Chap. 3] and [11, Chap. 7]). We find sharp estimates for the growth of the Cauchy integral in the unit ball in Cn in terms of smoothness of the Stieltjes measure. Denote by d(z, ζ) = √ |1− 〈z, ζ〉|, z, ζ ∈ Bn, the anisotropic metric on Sn ([10, Sec. 5.1]) and by ω(δ, µ) = sup z0∈Sn |µ|({ξ ∈ Sn : d(ξ, z0) ≤ δ}) the modulus of continuity, where |µ| is the total variation of a complex-valued Borel measure µ on Sn. Theorem 1. Let µ be a complex-valued Borel measure on Sn, p ∈ (0, n]. Then ∃c > 0 ω(δ, µ) ≤ cδ2(n−p), 0 < δ ≤ √ 2, implies that C[µ](z) = O ( 1 (1− |z|)p ) , z ∈ Bn. The examples in Sec. 3 show that the estimate is sharp up to a constant factor. In order to prove Theorem 1, we use the standard approach [10]. The same method allows us to prove a criterion for the Poisson integral. Theorem 2. Let µ be a positive Borel measure on Sn, p ∈ (0, n). Then ∃c > 0 ω(δ, µ) ≤ cδ2(n−p), 0 < δ < 1 ⇔ P [µ](z) = O ( 1 (1− |z|)p ) , z ∈ Bn. 238 Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3 On the Growth of the Cauchy–Szegő Transform in the Unit Ball R e m a r k. Theorems 1 and 2 could be easily generalized for the integrals with kernels of the form 1 (1− 〈z, ξ〉)n+s , 1 |1− 〈z, ξ〉|n+s , s ∈ R, respectively, for an appropriate choice of p. 2. Proofs of the Theorems P r o o f of Theorem 1. Denote Ek(z) = { ξ ∈ Sn : ∣∣∣∣1− 〈 z |z| , ξ〉 ∣∣∣∣ < 2k+1(1− |z|) } for z ∈ Bn \ {0} and k ∈ {0, 1, 2, . . .}. Then (E−1(z) := ∅) ∞⋃ k=0 (Ek(z) \ Ek−1(z)) = Sn. Since ∀k ∈ N ∀ξ ∈ Ek(z) \ Ek−1(z) ∀z : 1 > |z| > 3 4 |1− 〈z, ξ〉| ≥ ||1− |z|| − ||z| − 〈z, ξ〉|| = |z| ∣∣∣∣1− 〈 z |z| , ξ 〉∣∣∣∣− (1− |z|) ≥ |z|2k(1− |z|)− (1− |z|) = (|z|2k − 1)(1− |z|) and ∀ξ ∈ E0(z): |1− 〈z, ξ〉| ≥ 1− |〈z, ξ〉| ≥ 1− |z|, we have |C[µ](z)| = ∣∣∣∣∣∣ ∫ Sn dµ(ξ) (1− 〈z, ξ〉)n ∣∣∣∣∣∣ = ∣∣∣∣∣∣∣ ∞∑ k=1 ∫ Ek(z)\Ek−1(z) dµ(ξ) (1− 〈z, ξ〉)n + ∫ E0(z) dµ(ξ) (1− 〈z, ξ〉)n ∣∣∣∣∣∣∣ ≤ ∞∑ k=1 ∫ Ek(z)\Ek−1(z) |dµ(ξ)| (|z|2k − 1)n(1− |z|)n + ∫ E0(z) |dµ(ξ)| (1− |z|)n ≤ (1− |z|)−n ∞∑ k=1 (|z|2k − 1)−n|µ|(Ek(z)) + (1− |z|)−n|µ|(E0(z)) ≤ (1− |z|)−n ∞∑ k=1 ω( √ 2k+1(1− |z|), µ) (|z|2k − 1)n + (1− |z|)−nω( √ 2(1− |z|) , µ) Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3 239 I. Chyzhykov and M. Voitovych ≤ (1− |z|)−n ∞∑ k=1 ( 3 4 2k − 1 )−n c(2k+1(1− |z|))n−p + 2n−pc(1− |z|)−p < c (1− |z|)p ( 2n−p ∞∑ k=1 2k(n−p) (3 · 2k−2 − 1)n + 2n−p ) , 3 4 < |z| < 1. Since the last series is convergent, we get the desired result. Now, let |z| ≤ 3 4 . Since d(1, z) ≤ √ 2 for all z ∈ Bn, |µ|(Sn) ≤ ω( √ 2, µ) ≤ c2n−p. Then |C[µ](z)| ≤ ∫ Sn ∣∣∣∣ dµ(ξ) (1− 〈z, ξ〉)n ∣∣∣∣ ≤ |µ|(Sn) (1− |z|)n ≤ c2n−p (1− |z|)p ( 1 4 )n−p ≤ c8n−p (1− |z|)p . P r o o f of Theorem 2.(⇐) For all ξ ∈ E1(z), |1− 〈z, ξ〉| ≤ ∣∣∣∣1− 〈 z |z| , ξ 〉∣∣∣∣ + ∣∣∣∣ 〈 z |z| − z, ξ 〉∣∣∣∣ ≤ 4(1− |z|) + ∣∣∣∣ z |z| − z ∣∣∣∣ = 5(1− |z|). By the assumption ∃c > 0 such that c (1− |z|)p ≥ ∣∣∣∣∣∣ ∫ Sn (1− |z|2)n |1− 〈z, ξ〉|2n dµ(ξ) ∣∣∣∣∣∣ ≥ ∣∣∣∣∣∣∣ ∫ E1(z) (1− |z|2)n |1− 〈z, ξ〉|2n dµ(ξ) ∣∣∣∣∣∣∣ ≥ (1 + |z|)n 52n(1− |z|)n µ(E1(z)). (2) Since for d(z, ξ) < √ 3(1− |z|) implies ∣∣∣∣1− 〈 z |z| , ξ 〉∣∣∣∣ ≤ |1− 〈z, ξ〉|+ ∣∣∣∣〈z, ξ〉 − 〈 z |z| , ξ 〉∣∣∣∣ < 3(1− |z|) + ∣∣∣∣z − z |z| ∣∣∣∣ = 4(1− |z|), we get E1(z) ⊃ {ξ ∈ Sn : d(z, ξ) < √ 3(1− |z|)}. 240 Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3 On the Growth of the Cauchy–Szegő Transform in the Unit Ball From inequality (2) and the last inclusion it follows that µ(E1(z)) ≤ c1(1− |z|)n−p, z ∈ Bn, ω( √ 3(1− |z|), µ) ≤ c1(1− |z|)n−p, z ∈ Bn, ω(δ, µ) ≤ c1δ 2(n−p)3p−n, 0 < δ ≤ √ 3, where c1 ≥ 52nc/2n. (⇒) Using the arguments similar to those of the proof of Theorem 1, we get |P [µ](z)| = ∫ Sn (1− |z|2)ndµ(ξ) |1− 〈z, ξ〉|2n = ∞∑ k=1 ∫ Ek(z)\Ek−1(z) (1− |z|2)ndµ(ξ) |1− 〈z, ξ〉|2n + ∫ E0(z) (1− |z|2)ndµ(ξ) |1− 〈z, ξ〉|2n ≤ ∞∑ k=1 ∫ Ek(z)\Ek−1(z) (1− |z|2)ndµ(ξ) (|z|2k − 1)2n(1− |z|)2n + ∫ E0(z) (1− |z|2)ndµ(ξ) (1− |z|)n ≤ (1− |z|)−n ∞∑ k=1 (1 + |z|)nµ(Ek(z)) (|z|2k − 1)2n + (1− |z|)−n(1 + |z|)nµ(E0(z)) ≤ (1− |z|)−n ∞∑ k=1 2nω( √ 2k+1(1− |z|), µ) (|z|2k − 1)2n + (1− |z|)−n2nω( √ 2(1− |z|), µ) ≤ (1− |z|)−n ∞∑ k=1 ( 3 4 2k − 1 )−2n 2nc(2k+1(1− |z|))n−p + 2n−p2nc(1− |z|)−p ≤ c (1− |z|)p ( 22n−p ∞∑ k=1 2k(n−p) (3 · 2k−2 − 1)2n + 22n−p ) . The convergence of the last series implies the required inequality. If |z| ≤ 3 4 , using the arguments similar to those of the proof of Theorem 1, we can obtain |P [µ](z)| ≤ ∫ Sn ∣∣∣∣ (1− |z|2)ndµ(ξ) |1− 〈z, ξ〉|2n ∣∣∣∣ ≤ 2n|µ|(Sn) (1− |z|)n ≤ c2n−p2n (1− |z|)p ( 1 4 )n−p ≤ c24n−3p (1− |z|)p . 3. Examples 1. Let in Theorem 1 µ be the Lebesgue measure m2n−1 on Sn. Note that Qδ = {ξ ∈ Sn : d(ξ, z0) < δ} is a “ball” on Sn and m2n−1(Qδ) ³ δ2n, δ → 0 Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3 241 I. Chyzhykov and M. Voitovych ([10, Ch. 5]). Since “the center” z0 is of no particular importance, the modulus of continuity ω(δ,m2n−1) = sup z0∈Sn m2n−1({ξ ∈ Sn : d(ξ, z0) < δ}) ³ δ2n, δ → 0, so ([10, Prop. 1.4.10]) ∫ Sn dm2n−1(ξ) |1− 〈z, ξ〉|n ³ ln 1 1− |z| , |z| ↑ 1. Hence the statement of Theorem 1 is not true for the case p = 0. 2. Let µ = δξ0 , i.e., µ(A) = { c, A 3 ξ0; 0, otherwise, where A ⊂ Sn, ξ0 ∈ Sn. Then ω(δ, µ) = c ∈ C, p = n and C[µ](tξ0) = ∫ Sn dµ(ξ) (1− 〈tξ0, ξ〉)n = c 1 (1− t)n , 0 < t < 1. This example shows the sharpness of Theorem 1 for p = n. 3. Let µ be a Borel measure on S2 ⊂ C2 and µ(ξ) = { k−l, ξ = (1− k−q, √ 1− (1− k−q)2) 0, otherwise, where 1 < l < 2q + 1, k = 0, 1, . . . . Then ω(δ, µ) ³ δ 2 l−1 q , i.e., µ ∈ Λ l−1 q and max |z|=r |C[µ](z)| ≥ c ( 1 (1−r) 2− l−1 q ) . Indeed, ω(δ, µ) = ∫ |1−ξ1|<δ2 dµ(ξ1, ξ2) = ∑ k−q<δ2 k−l = ∑ k>δ − 2 q k−l ³ δ 2 q (l−1) . Let z = re1, where e1 = (1, 0) ∈ S2, r ∈ (0, 1), C[µ](re1) = ∫ S2 dµ(ξ) (1− rξ1)2 = ∞∑ k=1 k−l (1− r(1− k−q))2 242 Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3 On the Growth of the Cauchy–Szegő Transform in the Unit Ball ≥ 2 [ 1 (1−r)1/q ] ∑ k= [ 1 (1−r)1/q ] 1 kl(1− r + r kq )2 ≥ 2 [ 1 (1−r)1/q ] ∑ k= [ 1 (1−r)1/q ] (1− r) l q 2l 1 ((1− r) + r(1− r))2 ≥ 1 2(1− r) 1 q (1− r) l q 2l 1 4(1− r)2 = 1 2l+3 (1− r) l−1 q −2 . 4. The Operator Theory Point of View Equality (1) is often considered as the Cauchy–Szegő operator acting from the space M(Sn) (M+(Sn)) of Borel (positive) measures on Sn into the class of analytic functions on Bn. Denote by Hp q (Bn), 1 ≤ p ≤ ∞, q ≥ 0, the class of analytic functions f on Bn with the norm ‖f‖p q = sup 0<r<1 (1− r)q   ∫ Sn ‖f(rξ)‖pdm2n−1(ξ)   1/p , ‖f‖∞q = sup 0<r<1 (1− r)q max |ξ|=r |f(ξ)|. Also denote by hp q the class of harmonic functions with the same norm. It is known that there exist the measures µ ∈ M(Sn) such that C[µ] /∈ H1 0 (Bn). If we denote by Λα(Sn) (Λ+ α (Sn)) the class of (positive) measures on Sn such that ‖µ‖α = sup 0<δ≤√2 ω(δ,µ) δ2α < +∞, it then follows from Theorems 1 and 2 that C[µ] and P [µ] are bounded operators from Λα(Sn) ⊂ M(Sn) and Λ+ α (Sn) ⊂ M+(Sn) into H∞ n−α(Bn) and h∞n−α(Bn), respectively. Moreover, ‖C‖ = sup |µ|6=0 ‖Cµ‖∞n−α ‖µ‖α ≤ max { 2n−p ∞∑ k=1 2k(n−p) (3 · 2k−2 − 1)n + 2n−p, 8n−p } ≤ max { 2n−p ∞∑ k=1 2k(n−p) 2(k−2)n + 2n−p, 8n−p } ≤ max { 23n 2p − 1 + 2n−p, 8n−p } = 23n 2p − 1 + 2n−p Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3 243 I. Chyzhykov and M. Voitovych and ‖P‖ = sup |µ|6=0 ‖Pµ‖∞n−α ‖µ‖α ≤ max { 22n−p ∞∑ k=1 2k(n−p) (3 · 2k−2 − 1)2n + 22n−p, 24n−3p } ≤ max { 27n 2n+p − 1 + 22n−p, 24n−3p } = 27n 2n+p − 1 + 22n−p. References [1] Sh. Chen, A. Rasila, and X. Wang, Radial Growth, Lipschitz and Dirichlet Spaces on Solutions to the Nonhomogenous Yukawa Equation. — Israel J. Math. 204 (2014), No. 1, 261–282. [2] Sh. Chen, M. Mateljevic, S. Ponnusamy, and X. Wang, Lipschitz Type Spaces and Landau–Bloch Type Theorems for Harmonic Functions and Poisson Equations. 2014, arXiv preprint arXiv:1407.7179. [3] I.E. Chyzhykov, Growth and Representation of Analytic and Harmonic Functions in the Unit Disc. — Ukrainian Math. Bull. 3 (2006), No. 1, 31–44. [4] I.E. Chyzhykov and O.A. Zolota, Sharp Estimates of the Growth of the Poisson– Stieltjes Integral in the Polydisc. — Mat. Stud. (2010), No. 2, 193–196. [5] I.E. Chyzhykov and O.A. Zolota, Growth of the Poisson–Stieltjes Integral in the Polydisc. — J. Math. Phys. Anal. Geom. 7 (2011), No. 2, 141–157. [6] P. Duren, Theory of Hp Spaces. Academic Press, New York, 1970. [7] K.T. Hahn and J. Mitchell, Representation of Linear Functionals in Hp-spaces over Bounded Symmetric Domains in Cn. — J. Math. Anal. Appl. 56 (1976), No. 22, 379–396. [8] G.H. Hardy and J.E. Littlewood, A Convergence Criterion for Fourier Series. — Math. Z. 28 (1928), No. 4, 612–634. [9] S.G. Kranz, Lipschitz Spaces, Smoothness of Functions, and Approximation Theory. — Expo. Math. 3 (1983), 193–260. [10] W. Rudin, Function Theory in the Unit ball of Cn. Springer Verlag, Berlin– Heidelberg, New York, 1980. [11] M. Stoll, Invariant Potential Theory in the Unit Ball of Cn. Cambridge Univ. Press, Cambridge, 1994. 244 Journal of Mathematical Physics, Analysis, Geometry, 2015, vol. 11, No. 3