Abstract
It is known that if a harmonic function u on the unit disk \({\mathbb {D}}\) in \({\mathbb {C}}\) has angular limits on a measurable set E of the unit circle \(\partial {\mathbb {D}}\), then its conjugate harmonic function v in \({\mathbb {D}}\) also has angular limits a.e. on E and both boundary functions are finite a.e. and measurable on E. This result is extended to arbitrary Jordan domains with rectifiable boundaries in terms of the natural parameter. On this basis, we study various Stieltjes integrals as Poisson-Stieltjes, conjugate Poisson-Stieltjes, Schwartz-Stieltjes and Cauchy-Stieltjes and prove theorems on the existence of their finite angular limits a.e. in terms of the Hilbert-Stieltjes integral. These results hold for arbitrary bounded integrands that are differentiable a.e. and, in particular, for integrands of the class \(\mathcal{{CBV}}\) (countably bounded variation).
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Communicated by Daniel Aron Alpay.
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Ryazanov, V. On the Theory of the Boundary Behavior of Conjugate Harmonic Functions. Complex Anal. Oper. Theory 13, 2899–2915 (2019). https://doi.org/10.1007/s11785-018-0861-y
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DOI: https://doi.org/10.1007/s11785-018-0861-y