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In the mathematical theory of conformal and quasiconformal mappings, the extremal length of a collection of curves is a measure of the size of that is invariant under conformal mappings. More specifically, suppose that is an open set in the complex plane and is a collection of paths in and is a conformal mapping. Then the extremal length of is equal to the extremal length of the image of under . One also works with the conformal modulus of , the reciprocal of the extremal length. The fact that extremal length and conformal modulus are conformal invariants of makes them useful tools in the study of conformal and quasi-conformal mappings. One also works with extremal length in dimensions greater than two and certain other metric spaces, but the following deals primarily with the two dimensional setting. ==Definition of extremal length== To define extremal length, we need to first introduce several related quantities. Let be an open set in the complex plane. Suppose that is a collection of rectifiable curves in . If is Borel-measurable, then for any rectifiable curve we let : denote the -length of , where denotes the Euclidean element of length. (It is possible that .) What does this really mean? If is parameterized in some interval , then is the integral of the Borel-measurable function with respect to the Borel measure on for which the measure of every subinterval is the length of the restriction of to . In other words, it is the Lebesgue-Stieltjes integral is the length of the restriction of to . Also set : The area of is defined as : and the extremal length of is : where the supremum is over all Borel-measureable with 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「Extremal length」の詳細全文を読む スポンサード リンク
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