CONTENTS Introduction..........................................................................................................................................................................5 Preliminaries. Complex harmonic functions..........................................................................................................................7 I. Spectral values and eigenvalues of a Jordan curve........................................................................................................19 1.1. On a boundary integral..............................................................................................................................................20 1.2. The generalized Cauchy singular integral operator $C_𝕍$.......................................................................................23 1.3. The Hilbert transformation $T_Ω$.............................................................................................................................28 1.4. The boundary space Ḣ²(∂Ω)......................................................................................................................................31 1.5. The generalized Neumann-Poincaré operator $N_𝕍$...............................................................................................36 II. Quasisymmetric automorphisms of the unit circle...........................................................................................................41 2.1. The Douady-Earle extension $E_γ$..........................................................................................................................42 2.2. On an approximation of the Hersch-Pfluger distortion function $Φ_K$......................................................................46 2.3. On the maximal dilatation of the Douady-Earle extension..........................................................................................48 2.4. The Hilbert space H...................................................................................................................................................54 2.5. The linear operator $B_γ$.........................................................................................................................................60 III. The generalized harmonic conjugation operator............................................................................................................64 3.1. The generalized harmonic conjugation operator $A_γ$.............................................................................................64 3.2. Spectral values and eigenvalues of a quasisymmetric automorphism of the unit circle..............................................73 3.3. The smallest positive eigenvalue of a quasisymmetric automorphism of the unit circle..............................................80 3.4. Limiting properties of spectral values and eigenvalues of a quasisymmetric automorphism of the unit circle............84 IV. Spectral values of a quasicircle.....................................................................................................................................90 4.1. Characterizations of the boundary space Ḣ²(∂Ω).......................................................................................................91 4.2. Spaces symmetric with respect to a Jordan curve.....................................................................................................93 4.3. Plemelj's formula for a quasicircle..............................................................................................................................96 4.4. The main spectral theorem for quasicircles.............................................................................................................103 4.5. Spectral values and eigenvalues of a quasicircle....................................................................................................108 Appendix. The inner completion of pseudo-normed spaces............................................................................................114 References......................................................................................................................................................................117 List of symbols.................................................................................................................................................................122 Index................................................................................................................................................................................124
Institute of Mathematics, The Catholic University of Lublin, P.O. Box 129, Al. Racławickie 14, 20-950 Lublin, Poland
Bibliografia
[AG] S. B. Agard and F. W. Gehring, Angles and quasiconformal mappings, Proc. London Math. Soc. (3) 14A (1965), 1-21.
[A1] L. V. Ahlfors, Zur Theorie der Überlagerungsflächen, Acta Math. 65 (1935), 157-194.
[A2] L. V. Ahlfors, Remarks on the Neumann-Poincaré integral equation, Pacific J. Math. 2 (1952), 271-280.
[A3] L. V. Ahlfors, Lectures on Quasiconformal Mappings, D. Van Nostrand, Princeton, 1966.
[AVV] G. D. Anderson, M. K. Vamanamurthy and M. Vuorinen, Distortion function for plane quasiconformal mappings, Israel J. Math. 62 (1988), 1-16.
[ABR] S. Axler, P. Bourdon and W. Ramey, Harmonic Function Theory, Grad. Texts in Math. 137, Springer, New York, 1992.
[Ba] S. Banach, Théorie des Opérations Linéaires, Warszawa, 1932.
[BeS] H. Behnke and F. Sommer, Theorie der analytischen Funktionen, Springer, Berlin, 1976.
[BS] S. Bergman and M. Schiffer, Kernel functions and conformal mapping, Compositio Math. 8 (1951), 205-249.
[BA] A. Beurling and L. V. Ahlfors, The boundary correspondence under quasiconformal mappings, Acta Math. 96 (1956), 125-142.
[B1] B. Bojarski, Homeomorphic solution of Beltrami systems, Dokl. Akad. Nauk SSSR 102 (1955), 661-664 (in Russian).
[B2] B. Bojarski, Generalized solutions of a system of differential equations of the first order and elliptic type with discontinuous coefficients, Mat. Sb. 43 (1957), 451-503 (in Russian).
[BH] D. Bshouty and W. Hengartner, Univalent harmonic mappings in the plane, Ann. Univ. Mariae Curie-Skłodowska Sect. A 48 (1994), 12-42.
[C] G. Choquet, Sur un type de transformation analytique généralisant la représentation conforme et définie au moyen de fonctions harmoniques, Bull. Sci. Math. (2) 69 (1945), 156-165.
[D] G. David, Opérateurs intégraux singuliers sur certaines courbes du plan complexe, Ann. Sci. École Norm. Sup. 17 (1984), 157-189.
[DS] N. Dunford and J. T. Schwartz, Linear Operators, Vols. I, II, Interscience, New York, 1958, 1963.
[DE] A. Douady and C. J. Earle, Conformally natural extension of homeomorphisms of the circle, Acta Math. 157 (1986), 23-48.
[Du] P. Duren, Theory of $H^p$-spaces, Academic Press, New York and London, 1970.
[F] O. J. Farrell, On approximation to an analytic function by polynomials, Bull. Amer. Math. Soc. 40 (1934), 908-914.
[Fe2] R. Fehlmann, Quasiconformal mappings with free boundary components, Ann. Acad. Sci. Fenn. Ser. A I Math. 7 (1982), 337-347.
[FS] R. Fehlmann and K. Sakan, On the set of substantial boundary points for extremal quasiconformal mappings, Complex Variables Theory Appl. 6 (1986), 323-335.
[Fr] K. Friedrichs, Spektraltheorie halbbeschränkter Operatoren I-III, Math. Ann. 109 (1934), 465-487; 685-713; 110 (1935), 777-779.
[G1] D. Gaier, Konstruktive Methoden der konformen Abbildung, Springer, Berlin, 1964.
[G2] D. Gaier, Lectures on Complex Approximation, Birkhäuser, Boston, 1987.
[Ga] J. B. Garnett, Bounded Analytic Functions, Academic Press, New York, 1981.
[Ha] P. R. Halmos, Introduction to Hilbert Space and the Theory of Spectral Multiplicity, Chelsea, New York, 1957.
[H] J. Hersch, Longueurs extrémales, mesure harmonique et distance hyperbolique, C. R. Acad. Sci. Paris 235 (1952), 569-571.
[HP] J. Hersch et A. Pfluger, Généralisation du lemme de Schwarz et du principe de la mesure harmonique pour les fonctions pseudo-analytiques, C. R. Acad. Sci. Paris 234 (1952), 43-45.
[Hü] O. Hübner, Remarks on a paper by Ławrynowicz on quasiconformal mappings, Bull. Acad. Polon. Sci. 18 (1970), 183-186.
[KK] L. V. Kantorovich and V. I. Krylov, Approximate Methods of Higher Analysis, Interscience-Noordhoff, New York and Groningen, 1958; translated from the 3rd Russian edition by C. D. Benster.
[Ke] O. D. Kellogg, Foundations of Potential Theory, Dover, New York, 1953.
[Kn] H. Kneser, Lösung der Aufgabe 41, Jahresber. Deutsch. Math.-Verein. 35 (1926), 123-124.
[Kr] N. Y. Kruglyak, A family of metric spaces, in: Qualitative and Approximate Methods for the Investigation of Operator Equations, No. 3, Yaroslav. Gos. Univ., Yaroslavl' 1978, 112-121 (in Russian).
[KZ] W. Kawa and J. Zając (Jr.), Dynamical approximation of special functions in quasiconformal theory, Folia Sci. Univ. Tech. Resoviensis Math. 17 (1995), 35-42.
[K1] J. G. Krzyż, Generalized Fredholm eigenvalues of a Jordan curve, Ann. Polon. Math. 46 (1985), 157-163.
[K2] J. G. Krzyż, Conjugate holomorphic eigenfunctions and extremal quasiconformal reflection, Ann. Acad. Sci. Fenn. Ser. A I Math. 10 (1985), 305-311.
[K3] J. G. Krzyż, Quasicircles and harmonic measure, Ann. Acad. Sci. Fenn. Ser. A I Math. 12 (1987), 19-24.
[K4] J. G. Krzyż, Generalized Neumann-Poincaré operator and chord-arcs curves, Ann. Univ. Mariae Curie-Skłodowska Sect. A 43 (1989), 69-78.
[K5] J. G. Krzyż, Fredholm eigenvalues and complementary Hardy spaces, Ann. Univ. Mariae Curie-Skłodowska Sect. A 44 (1990), 23-36.
[K6] J. G. Krzyż, Quasisymmetric functions and quasihomographies, Ann. Univ. Mariae Curie-Skłodowska Sect. A 47 (1993), 90-95.
[KP] J. G. Krzyż and D. Partyka, Generalized Neumann-Poincaré operator, chord-arc curves and Fredholm eigenvalues, Complex Variables Theory Appl. 21 (1993), 253-263.
[Kü1] R. Kühnau, Verzerrungssätze und Koeffizientenbedingungen vom Grunskyschen Typ für quasikonforme Abbildungen, Math. Nachr. 48 (1971), 77-105.
[Kü2] R. Kühnau, Zu den Grunskyschen Koeffizientenbedingungen, Ann. Acad. Sci. Fenn. Ser. A I Math. 6 (1981), 125-130.
[Kü3] R. Kühnau, Quasikonforme Fortsetzbarkeit, Fredholmsche Eigenwerte und Grunskysche Koeffizientenbedingungen, Ann. Acad. Sci. Fenn. Ser. A I Math. 7 (1982), 383-391.
[Kü4] R. Kühnau, Wann sind die Grunskyschen Koeffizientenbedingungen hinreichend für Q-quasikonforme Fortsetzbarkeit?, Comment. Math. Helv. 61 (1986), 290-307.
[Kü5] R. Kühnau, Zur Berechnung der Fredholmschen Eigenwerte ebener Kurven, Z. Angew. Math. Mech. 66 (1986), 193-200.
[Kü6] R. Kühnau, Möglichst konforme Spiegelung an einer Jordankurve, Jahresber. Deutsch. Math.-Verein. 90 (1988), 90-109.
[LP] A. Lecko and D. Partyka, An alternative proof of a result due to Douady and Earle, Ann. Univ. Mariae Curie-Skłodowska Sect. A 42 (1988), 59-68.
[Le] M. Lehtinen, Remarks on the maximal dilatation of the Beurling-Ahlfors extension, Ann. Acad. Sci. Fenn. Ser. A I Math. 9 (1984), 133-139.
[L] O. Lehto, Univalent Functions and Teichmüller Spaces, Grad. Texts in Math. 109, Springer, New York, 1987.
[LV] O. Lehto and K. I. Virtanen, Quasiconformal Mappings in the Plane, 2nd ed., Grundlehren Math. Wiss. 126, Springer, Berlin, 1973.
[LVV] O. Lehto, K. I. Virtanen and J. Väisälä, Contributions to the distortion theory of quasiconformal mappings, Ann. Acad. Sci. Fenn. Ser. A I Math. 273 (1959), 1-14.
[ŁK] J. Ławrynowicz and J. G. Krzyż, Quasiconformal Mappings in the Plane: Parametrical Methods, Lecture Notes in Math. 978, Springer, Berlin, 1983.
[M] A. I. Markushevich [A. Markuschewitsch], Conformal Mapping of Regions with Variable Boundary and Application to the Approximation of Analytic Functions by Polynomials, Dissertation, Moscow, 1934 (in Russian).
[Me] S. N. Mergelyan, Uniform approximations to functions of a complex variable, Uspekhi Mat. Nauk (N. S.) 7 (2) (1952), 31-122 (in Russian); English transl. in Amer. Math. Soc. Transl. 101 (1954), 99 pp.
[Ml] W. Mlak, Hilbert Spaces and Operator Theory, Math. Appl., Kluwer, Dordrecht, and PWN-Polish Scientific Publishers, Warszawa, 1991.
[Mo] A. Mori, On an absolute constant in the theory of quasiconformal mappings, J. Math. Soc. Japan 8 (1956), 156-166.
[N] J. C. C. Nitsche, Lectures on Minimal Surfaces, Vol. 1, Cambridge Univ. Press, Cambridge, 1989.
[P0] D. Partyka, Generalized Fredholm Eigenvalues of a Jordan Curve in the Plane, Ph. D. thesis, Institute of Mathematics, Maria Curie-Skłodowska University, 1989 (in Polish).
[P1] D. Partyka, A sewing theorem for complementary Jordan domains, Ann. Univ. Mariae Curie-Skłodowska Sect. A 41 (1987), 99-103.
[P2] D. Partyka, The maximal dilatation of Douady and Earle extension of a quasisymmetric automorphism of the unit circle, Ann. Univ. Mariae Curie-Skłodowska Sect. A 44 (1990), 45-57.
[P3] D. Partyka, A distortion theorem for quasiconformal automorphisms of the unit disk, Ann. Polon. Math. 55 (1991), 277-281.
[P4] D. Partyka, Approximation of the Hersch-Pfluger distortion function. Applications, Ann. Univ. Mariae Curie-Skłodowska Sect. A 45 (1992), 99-111.
[P5] D. Partyka, The inner completion of normed spaces, Hokkaido Math. J. 21 (1992), 305-318.
[P6] D. Partyka, Generalized harmonic conjugation operator, Ber. Univ. Jyväskylä Math. Inst. 55 (1993), 143-155 (Proc. of the Fourth Finnish-Polish Summer School in Complex Analysis at Jyväskylä, August 1992).
[P7] D. Partyka, Approximation of the Hersch-Pfluger distortion function, Ann. Acad. Sci. Fenn. Ser. A I Math. 18 (1993), 343-354.
[P8] D. Partyka, Spectral values and eigenvalues of a quasicircle, Ann. Univ. Mariae Curie-Skłodowska Sect. A 46 (1993), 81-98.
[P9] D. Partyka, On the maximal dilatation of the Douady-Earle extension, Ann. Univ. Mariae Curie-Skłodowska Sect. A 48 (1994), 80-97.
[P10] D. Partyka, The smallest positive eigenvalue of a quasisymmetric automorphism of the unit circle, in: Topics in Complex Analysis, Banach Center Publ. 31, Inst. Math., Polish Acad. Sci., Warszawa, 1995, 303-310.
[P11] D. Partyka, The maximal value of the function $[0,1] ∋ r ↦ Φ²_K(√r) - r$, Bull. Soc. Sci. Lettres Łódź 45, Sér. Rech. Déformations 20 (1995), 49-55.
[P12] D. Partyka, On approximation of complex-valued harmonic functions in Jordan domains, Folia Sci. Univ. Tech. Resoviensis Math. 20 (154) (1996), 121-129.
[P13] D. Partyka, Spectral values of a quasicircle, Complex Variables Theory Appl., to appear.
[PZ] D. Partyka and J. Zając, On modification of the Beurling-Ahlfors extension of a quasisymmetric function, Bull. Soc. Sci. Lettres Łódź 15 (1990), 45-52.
[Po] Ch. Pommerenke, Univalent Functions, Vandenhoeck and Ruprecht, Göttingen, 1975.
[Pr] I. I. Priwalow, Randeigenschaften analytischer Funktionen, Deutscher Verlag Wiss., Berlin, 1956.
[R] E. Reich, On the relation between local and global properties of boundary values for extremal quasiconformal mappings, in: Discontinuous Groups and Riemann Surfaces, Ann. of Math. Stud. 79, Princeton Univ. Press, 1974, 391-407.
[Re] H. Renelt, Behandlung von Abbildungsproblemen der quasikonformen Abbildung mittels direkter Variationsmethoden, Dissertation, Martin-Luther-Univ., Halle, 1971.
[R-F1] F. Riesz, Über die Randwerte einer analytischen Funktion, Math. Z. 18 (1923), 87-95.
[R-F2] F. Riesz, Zur Theorie des Hilbertschen Raumes, Acta Sci. Math. (Szeged) 7 (1934), 34-38.
[R-M] M. Riesz, Sur les fonctions conjuguées, Math. Z. 27 (1927), 218-244.
[Ru] W. Rudin, Real and Complex Analysis, McGraw-Hill, New York, 1966.
[SZ] K. Sakan and J. Zając, The Douady-Earle extension of quasihomographies, in: Generalizations of Complex Analysis and Their Applications in Physics, Banach Center Publ. 37, Inst. Math., Polish Acad. Sci., Warszawa, 1996, 35-44.
[Sa] S. Saks, Théorie de l'intégrale, Monografje Matematyczne 2, z subwencji Funduszu Kultury Narodowej, Warszawa-Lwów, 1933.
[S1] M. Schiffer, The Fredholm eigenvalues of plane domains, Pacific J. Math. 7 (1957), 1187-1225.
[S2] M. Schiffer, Fredholm eigenvalues and conformal mapping, Rend. Mat. 22 (1963), 447-468.
[S3] M. Schiffer, Fredholm eigenvalues and Grunsky matrices, Ann. Polon. Math. 39 (1981), 149-164.
[SS] M. Schiffer and G. Schober, An extremal problem for the Fredholm eigenvalues, Arch. Rational Mech. Anal. 44 (1971//72), 83-92.
[Sc1] G. Schober, On the Fredholm eigenvalues of plane domains, J. Math. Mech. 16 (1966), 535-541.
[Sc2] G. Schober, Neumann's lemma, Proc. Amer. Math. Soc. 19 (1968), 306-311.
[Sc3] G. Schober, Continuity of curve functionals and a technique involving quasiconformal mapping, Arch. Rational Mech. Anal. 29 (1968), 378-389.
[Sc4] G. Schober, Semicontinuity of curve functionals, Arch. Rational Mech. Anal. 33 (1969), 374-376.
[Sc5] G. Schober, Estimates for Fredholm eigenvalues based on quasiconformal mapping, in: Numerische, insbesondere approximationstheoretische Behandlung von Funktionalgleichungen, Lecture Notes in Math. 333, Springer, Berlin, 1973, 211-217.
[Sh] H. S. Shapiro, Some observations concerning weighted polynomial approximation of holomorphic functions, Mat. Sb. 73 (1967), 320-330 (in Russian); English transl.: Math. USSR-Sb. 2 (1967), 285-294.
[Sp] G. Springer, Fredholm eigenvalues and quasiconformal mapping, Acta Math. 111 (1964), 121-142.
[St1] K. Strebel, Zur Frage der Eindeutigkeit extremaler quasikonformer Abbildungen des Einheitskreises I, Comment. Math. Helv. 36 (1962), 306-323.
[St2] K. Strebel, Zur Frage der Eindeutigkeit extremaler quasikonformer Abbildungen des Einheitskreises II, Comment. Math. Helv. 39 (1964), 77-89.
[T] O. Taari, Charakterisierung der Quasikonformität mit Hilfe der Winkelverzerrung, Ann. Acad. Sci. Fenn. Ser. A I Math. 390 (1966), 43 pp.
[Va] J. V. Vainio, Conditions for the possibility of conformal sewing, Ann. Acad. Sci. Fenn. Ser. A I Math. Dissertationes 53 (1985), 43 pp.
[V] M. Vuorinen, Conformal Geometry and Quasiregular Mappings, Lecture Notes in Math., 1319, Springer, Berlin, 1988.
[VV] M. K. Vamanamurthy and M. Vuorinen, Functional inequalities, Jacobi products, and quasiconformal maps, Illinois J. Math., to appear.
[Wa] C.-F. Wang, On the precision of Mori's theorem in Q-mappings, Science Record 4 (1960), 329-333.
[W1] S. E. Warschawski, On the solution of the Lichtenstein-Gershgorin integral equation in conformal mapping: I. Theory, Nat. Bur. Standards Appl. Math. Ser. 42 (1955), 7-29.
[W2] S. E. Warschawski, On the differentiability at the boundary in conformal mapping, Proc. Amer. Math. Soc. 12 (1961), 614-620.