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・ Quasi-fibration
・ Quasi-finite field
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Quasi-isometry
・ Quasi-isomorphism
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・ Quasi-Lie algebra
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・ Quasi-Newton Inverse Least Squares Method
・ Quasi-Newton Least Squares Method


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Quasi-isometry : ウィキペディア英語版
Quasi-isometry
In mathematics, quasi-isometry is an equivalence relation on metric spaces that ignores their small-scale details in favor of their coarse structure. The concept is especially important in Gromov's geometric group theory.〔

==Definition==
Suppose that f is a (not necessarily continuous) function from one metric space (M_1,d_1) to a second metric space (M_2,d_2). Then f is called a ''quasi-isometry'' from (M_1,d_1) to (M_2,d_2) if there exist constants A\ge 1, B\ge 0, and C\ge 0 such that the following two properties both hold:〔P. de la Harpe, ''Topics in geometric group theory''. Chicago Lectures in Mathematics. University of Chicago Press, Chicago, IL, 2000. ISBN 0-226-31719-6〕
#For every two points x and y in M_1, the distance between their images is (up to the additive constant B) within a factor of A of their original distance. More formally:
#:\forall x,y\in M_1: \frac\; d_1(x,y)-B\leq d_2(f(x),f(y))\leq A\; d_1(x,y)+B.
#Every point of M_2 is within the constant distance C of an image point. More formally:
#:\forall z\in M_2:\exists x\in M_1: d_2(z,f(x))\le C.
The two metric spaces (M_1,d_1) and (M_2,d_2) are called quasi-isometric if there exists a quasi-isometry f from (M_1,d_1) to (M_2,d_2).

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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