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In complex analysis, Fatou's theorem, named after Pierre Fatou, is a statement concerning holomorphic functions on the unit disk and their pointwise extension to the boundary of the disk. ==Motivation and statement of theorem== If we have a holomorphic function defined on the open unit disk , it is reasonable to ask under what conditions we can extend this function to the boundary of the unit disk. To do this, we can look at what the function looks like on each circle inside the disk centered at 0, each with some radius . This defines a new function on the circle , defined by , where . Then it would be expected that the values of the extension of onto the circle should be the limit of these functions, and so the question reduces to determining when converges, and in what sense, as , and how well defined is this limit. In particular, if the norms of these are well behaved, we have an answer: :Theorem: Let be a holomorphic function such that :: Then converges to some function pointwise almost everywhere and in . That is, :: :and :: :for almost every . Now, notice that this pointwise limit is a radial limit. That is, the limit being taken is along a straight line from the center of the disk to the boundary of the circle, and the statement above hence says that : for almost every . The natural question is, now with this boundary function defined, will we converge pointwise to this function by taking a limit in any other way? That is, suppose instead of following a straight line to the boundary, we follow an arbitrary curve , : for every nontangential limit converging to , where is defined as above. 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「Fatou's theorem」の詳細全文を読む スポンサード リンク
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