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In mathematics, the Hartogs–Rosenthal theorem is a classical result in
complex analysis Complex analysis, traditionally known as the theory of functions of a complex variable, is the branch of mathematical analysis that investigates functions of complex numbers. It is helpful in many branches of mathematics, including algebra ...
on the
uniform approximation In the mathematical field of analysis, uniform convergence is a mode of convergence of functions stronger than pointwise convergence. A sequence of functions (f_n) converges uniformly to a limiting function f on a set E if, given any arbitrarily ...
of continuous functions on compact subsets of the
complex plane In mathematics, the complex plane is the plane formed by the complex numbers, with a Cartesian coordinate system such that the -axis, called the real axis, is formed by the real numbers, and the -axis, called the imaginary axis, is formed by th ...
by
rational function In mathematics, a rational function is any function that can be defined by a rational fraction, which is an algebraic fraction such that both the numerator and the denominator are polynomials. The coefficients of the polynomials need not be ...
s. The theorem was proved in 1931 by the German mathematicians
Friedrich Hartogs Friedrich Moritz "Fritz" Hartogs (20 May 1874 – 18 August 1943) was a German-Jewish mathematician, known for his work on set theory and foundational results on several complex variables. Life Hartogs was the son of the merchant Gustav H ...
and Arthur Rosenthal and has been widely applied, particularly in
operator theory In mathematics, operator theory is the study of linear operators on function spaces, beginning with differential operators and integral operators. The operators may be presented abstractly by their characteristics, such as bounded linear oper ...
.


Statement

The Hartogs–Rosenthal theorem states that if ''K'' is a compact subset of the complex plane with
Lebesgue measure In measure theory, a branch of mathematics, the Lebesgue measure, named after French mathematician Henri Lebesgue, is the standard way of assigning a measure to subsets of ''n''-dimensional Euclidean space. For ''n'' = 1, 2, or 3, it coincides ...
zero, then any continuous complex-valued function on ''K'' can be uniformly approximated by rational functions.


Proof

By the
Stone–Weierstrass theorem In mathematical analysis, the Weierstrass approximation theorem states that every continuous function defined on a closed interval can be uniformly approximated as closely as desired by a polynomial function. Because polynomials are among the s ...
any complex-valued continuous function on ''K'' can be uniformly approximated by a polynomial in z and \overline. So it suffices to show that \overline can be uniformly approximated by a rational function on ''K''. Let ''g(z)'' be a
smooth function In mathematical analysis, the smoothness of a function is a property measured by the number of continuous derivatives it has over some domain, called ''differentiability class''. At the very minimum, a function could be considered smooth if ...
of compact support on C equal to 1 on ''K'' and set : f(z)=g(z)\cdot \overline. By the generalized Cauchy integral formula :f(z) = \frac\iint_ \frac\frac, since ''K'' has measure zero. Restricting ''z'' to ''K'' and taking Riemann approximating sums for the integral on the right hand side yields the required uniform approximation of \bar by a rational function.


See also

*
Runge's theorem In complex analysis, Runge's theorem (also known as Runge's approximation theorem) is named after the German mathematician Carl Runge who first proved it in the year 1885. It states the following: Denoting by C the set of complex numbers, let ''K ...
*
Mergelyan's theorem Mergelyan's theorem is a result from approximation by polynomials in complex analysis proved by the Armenian mathematician Sergei Mergelyan in 1951. Statement :Let ''K'' be a compact subset of the complex plane C such that C∖''K'' is con ...


Notes


References

* * * * {{DEFAULTSORT:Hartogs-Rosenthal theorem Rational functions Theorems in approximation theory Theorems in complex analysis