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Elasticity of a function : ウィキペディア英語版
Elasticity of a function
In mathematics, the elasticity or point elasticity of a positive differentiable function ''f'' of a positive variable (positive input, positive output)〔The elasticity can also be defined if the input and/or output is consistently negative, or simply away from any points where the input or output is zero, but in practice the elasticity is used for positive quantities.〕 at point ''a'' is defined as
:Ef(a) = \fracf'(a)
:=\lim_\frac\frac=\lim_\frac\frac=\lim_\frac}}\approx \frac
or equivalently
:Ef(x) = \frac.
It is thus the ratio of the relative (percentage) change in the function's output f(x) with respect to the relative change in its input x, for infinitesimal changes from a point (a, f(a)). Equivalently, it is the ratio of the infinitesimal change of the logarithm of a function with respect to the infinitesimal change of the logarithm of the argument.
The elasticity of a function is a constant \alpha if and only if the function has the form f(x) = C x ^ \alpha for a constant C>0.
The elasticity at a point is the limit of the arc elasticity between two points as the separation between those two points approaches zero.
The concept of elasticity is widely used in economics; see elasticity (economics) for details. 〔• Hanoch, G. (1975) “The elasticity of scale and the shape of average costs,” American Economic Review 65, pp. 492-497.〕 〔• Panzar, J.C. and R.D. Willig (1977) “Economies of scale in multi-output production, Quarterly Journal of Economics 91, 481-493.〕〔• (Zelenyuk, V. (2013) “A scale elasticity measure for directional distance function and its dual: Theory and DEA estimation.” European Journal of Operational Research 228:3, pp 592–600 )〕
==Rules==
Rules for finding the elasticity of products and quotients are simpler than those for derivatives. Let ''f, g'' be differentiable. Then〔
:E ( f(x) \cdot g(x) ) = E f(x) + E g(x)
:E \frac = E f(x) - E g(x)
:E ( f(x) + g(x) ) = \frac
:E ( f(x) - g(x) ) = \frac
The derivative can be expressed in terms of elasticity as
:D f(x) = \frac
Let ''a'' and ''b'' be constants. Then
:E ( a ) = 0 \
: E ( a \cdot f(x) ) = E f(x) ,
: E (b x^a) = a \ .

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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