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In complex analysis a complex-valued function ''ƒ'' of a complex variable ''z'': *is said to be holomorphic at a point ''a'' if it is differentiable at every point within some open disk centered at ''a'', and * is said to be analytic at ''a'' if in some open disk centered at ''a'' it can be expanded as a convergent power series :: :(this implies that the radius of convergence is positive). One of the most important theorems of complex analysis is that holomorphic functions are analytic. Among the corollaries of this theorem are * the identity theorem that two holomorphic functions that agree at every point of an infinite set ''S'' with an accumulation point inside the intersection of their domains also agree everywhere in every connected open subset of their domains that contains the set ''S'', and * the fact that, since power series are infinitely differentiable, so are holomorphic functions (this is in contrast to the case of real differentiable functions), and * the fact that the radius of convergence is always the distance from the center ''a'' to the nearest singularity; if there are no singularities (i.e., if ''ƒ'' is an entire function), then the radius of convergence is infinite. Strictly speaking, this is not a corollary of the theorem but rather a by-product of the proof. * no bump function on the complex plane can be entire. In particular, on any connected open subset of the complex plane, there can be no bump function defined on that set which is holomorphic on the set. This has important ramifications for the study of complex manifolds, as it precludes the use of partitions of unity. In contrast the partition of unity is a tool which can be used on any real manifold. == Proof == The argument, first given by Cauchy, hinges on Cauchy's integral formula and the power series development of the expression : Let ''D'' be an open disk centered at ''a'' and suppose ''ƒ'' is differentiable everywhere within an open neighborhood containing the closure of ''D''. Let ''C'' be the positively oriented (i.e., counterclockwise) circle which is the boundary of ''D'' and ''z'' a point in ''D''. Starting with Cauchy's integral formula, we have : Interchange of the integral and infinite sum is justified by observing that is bounded on ''C'' by some positive number ''M'', while for all ''w'' in ''C'' : for some positive ''r'' as well. We therefore have : on ''C'', and as the Weierstrass M-test shows the series converges uniformly, the sum and the integral may be interchanged. As the factor (''z'' − ''a'')''n'' does not depend on the variable of integration ''w'', it may be factored out to yield : which has the desired form of a power series in ''z'': : with coefficients : 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Analyticity of holomorphic functions」の詳細全文を読む スポンサード リンク
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