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Łukaszyk–Karmowski metric : ウィキペディア英語版
Łukaszyk–Karmowski metric
In mathematics, the Łukaszyk–Karmowski metric is a function defining a distance between two random variables or two random vectors.〔(Metryka Pomiarowa, przykłady zastosowań aproksymacyjnych w mechanice doświadczalnej (Measurement metric, examples of approximation applications in experimental mechanics) ), (PhD thesis ), Szymon Łukaszyk (author), Wojciech Karmowski (supervisor), Tadeusz Kościuszko Cracow University of Technology, submitted December 31, 2001, completed March 31, 2004〕〔(A new concept of probability metric and its applications in approximation of scattered data sets ), Łukaszyk Szymon, Computational Mechanics
Volume 33, Number 4, 299–304, Springer-Verlag 2003 〕 This function is not a metric as it does not satisfy the identity of indiscernibles condition of the metric, that is for two identical arguments its value is greater than zero. The concept is named after Szymon Łukaszyk and Wojciech Karmowski.
==Continuous random variables==
The Łukaszyk–Karmowski metric ''D'' between two continuous independent random variables ''X'' and ''Y'' is defined as:
:D(X, Y) = \int_^\infty \int_^\infty |x-y|f(x)g(y) \, dx\, dy
where ''f''(''x'') and ''g''(''y'') are the probability density functions of ''X'' and ''Y'' respectively.
One may easily show that such ''metrics'' above do not satisfy the identity of indiscernibles condition required to be satisfied by the metric of the metric space. In fact they satisfy this condition if and only if both arguments ''X'', ''Y'' are certain events described by Dirac delta density probability distribution functions. In such a case:
:D_(X, Y) = \int_^\infty \int_^\infty |x-y|\delta(x-\mu_x)\delta(y-\mu_y) \, dx\, dy = |\mu_x-\mu_y|
the Łukaszyk–Karmowski metric simply transforms into the metric between expected values \mu_x, \mu_y of the variables ''X'' and ''Y'' and obviously:
:D_(X, X) = |\mu_x-\mu_x| = 0.
For all the other cases however:
:D\left(X, X\right) > 0. \,
The Łukaszyk–Karmowski metric satisfies the remaining non-negativity and symmetry conditions of metric directly from its definition (symmetry of modulus), as well as subadditivity/triangle inequality condition:
:\begin
&^\infty |x-z|f(x)h(z) \, dx\, dz\ = \int_^\infty \int_^\infty |x-z|f(x)h(z) \, dx\, dz \int_^\infty g(y) dy\ \\
&^\infty \int_^\infty |(x-y)+(y-z)|f(x)g(y)h(z) \, dx\, dy\, dz\ \\
&^\infty \int_^\infty (|x-y|+|y-z|)f(x)g(y)h(z) \, dx\, dy\, dz\ \\
&^\infty \int_^\infty |x-y|f(x)g(y)h(z) \, dx\, dy\, dz\ + \int_^\infty \int_^\infty \int_^\infty |y-z|f(x)g(y)h(z) \, dx\, dy\, dz\ \\
&^\infty |x-y|f(x)g(y) \, dx\, dy\ + \int_^\infty \int_^\infty |y-z|g(y)h(z) \, dy\, dz\ \\
&^\infty \int_^\infty |x-y| f(x, y) \, dx\, dy.

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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