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Transactions of the American Mathematical Society
Transactions of the American Mathematical Society
ISSN 1088-6850(online) ISSN 0002-9947(print)

 

Approximation on disks


Author: Kenneth John Preskenis
Journal: Trans. Amer. Math. Soc. 171 (1972), 445-467
MSC: Primary 41A20; Secondary 30A82
MathSciNet review: 0312123
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Abstract: Let $ D$ be a closed disk in the complex plane, $ f$ a complex valued continuous function on $ D$ and $ {R_f}(D) = $ the uniform closure on $ D$ of rational functions in $ z$ and $ f$ which are finite. Among other results we obtain the following. Theorem. If $ f$ is of class $ {C^1}$ in a neighborhood of $ D$ and $ \vert{f_{\bar z}}\vert > \vert{f_z}\vert$ everywhere (i.e., $ f$ is an orientation reversing immersion of $ D$ in the plane), then $ {R_f}(D) = C(D)$. Theorem. Let $ f$ be a polynomial in $ z$ and $ \bar z$. If for each a in $ D,f - \Sigma {(j!)^{ - 1}}{D^j}f(a){(z - a)^j} = {(\bar z - \bar a)^k}g$ with $ \vert{g_{\bar z}}\vert > \vert{g_z}\vert$ at he zeros of $ g$ in $ D$ where $ Df = {f_z}$, then $ {R_f}(D) = C(D)$. Corollary. Let $ f$ be a polynomial in $ z$ and $ \bar z$ and let $ \vert{f_{z\bar z}}(0)\vert < \vert{f_{\bar z\,\bar z}}(0)\vert/2$. Then there exists an $ r > 0$ such that, for $ D = (\vert z\vert \leqslant r),{R_f}(D) = C(D)$. The proofs of the theorems use measures and the conditions involved in the theorems are independent of each other. Concerning the corollary, results of E. Bishop and G. Stolzenberg show that $ {f_{\bar z}}(0) = 0$ and $ \vert{f_{\bar z\,\bar z}}(0)\vert < \vert{f_{z\bar z}}(0)\vert$, then there exists no $ r$ such that $ {R_f}(D) = C(D)$ where $ D = (\vert z\vert \leqslant r)$.

Let $ F = ({f_1}, \cdots ,{f_n})$ be a map on $ B$ = unit polydisk in $ {{\mathbf{C}}^n}$ with values in $ {{\mathbf{C}}^n},{P_F}$ = uniform closure on $ B$ of polynomials in $ {z_1}, \cdots {z_n},{f_1}, \cdots ,{f_n}$. Theorem. If $ F$ is of class $ {C^1}$ in a neighborhood of $ B,{F_{\bar z}}$ is invertible and if for each $ a = ({a_1}, \cdots ,{a_n})$ in $ B$, there exist complex constants $ \{ {c_j}\} ,\{ {d_{ij}}\} ,i,j = 1, \cdots ,n$, such that $ \Sigma {c_j}({z_j} - {a_j})({f_j}(z) - {f_j}(a)) + \Sigma {d_{ij}}({z_i} - {a_i})({z_j} - {a_j})$ has positive real part for all $ z \ne a$, then $ \{ (\zeta ,F(\zeta )):\zeta \in B\} $ is a polynomially convex set. Corollary. If $ F = (f,g)$ where $ f(z,w) = \bar z + cz\bar z + d{\bar z^2} + q\bar zw,g(z,w) = \bar w + sw\bar w + t{\bar w^2} + p\bar wz$ and the coefficients satisfy $ \vert\bar c + d\vert + \vert d\vert + \vert q\vert < 1$ and $ \vert\bar s + t\vert + \vert t\vert + \vert p\vert < 1$, then $ {P_F} = C(B)$. Corollary. If $ F(z) = \bar z + R(z)$ where $ R = ({R_1}, \cdots ,{R_n})$ is of class $ {C^2}$ and satisfies the Lipschitz condition $ \vert R(\zeta ) - R(\eta )\vert \leqslant k\vert\zeta - \eta\vert$ with $ k < 1$, then $ {P_F} = C(B)$. This last corollary is a result of Hörmander and Wermer. The proof of the theorem uses methods from several complex variables.


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DOI: http://dx.doi.org/10.1090/S0002-9947-1972-0312123-3
PII: S 0002-9947(1972)0312123-3
Article copyright: © Copyright 1972 American Mathematical Society