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Near-Circularity in Capacity and Maximally Convergent Polynomials

  • Hans-Peter Blatt

摘要

If f is a power series with radius R of convergence, \(R > 1\) R > 1 , it is well-known that the method of Carathéodory–Fejér constructs polynomial approximations of f on the closed unit disk which show the typical phenomenon of near-circularity on the unit circle. Let E be compact and connected and let f be holomorphic on E. If \(\left\{ p_n\right\} _{n\in \mathbb {N}}\) p n n N is a sequence of polynomials converging maximally to f on E, it is shown that the modulus of the error functions \(f-p_n\) f - p n is asymptotically constant in capacity on level lines of the Green’s function \(g_\Omega (z,\infty )\) g Ω ( z , ) of the complement \(\Omega \) Ω of E in \(\overline{\mathbb {C}}\) C ¯ with pole at infinity, thereby reflecting a type of near-circularity, but without gaining knowledge of the winding numbers of the error curves with respect to the point 0.