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Half-exponential function : ウィキペディア英語版
Half-exponential function
In mathematics, a half-exponential function is a function ''ƒ'' so that if ''ƒ'' is composed with itself the result is exponential:
: f(f(x)) = ab^x. \,
Another definition is that ''ƒ'' is half-exponential if it is non-decreasing and ''ƒ''−1(''x''''C'') ≤ o(log ''x'').
for every ''C'' > 0.
It has been proven that every function ''ƒ'' composed of basic arithmetic operations, exponentials, and logarithms, then ''ƒ''(''ƒ''(''x'')) is either subexponential or superexponential: half-exponential functions are not expressible in terms of elementary functions.
There are infinitely many functions whose self-composition is the same exponential function as each other. In particular, for every A in the open interval (0,1) and for every continuous strictly increasing function ''g'' from () onto (), there is an extension of this function to a continuous monotonic function f on the real numbers such that f(f(x))=\exp x. In particular,
: f (x) =
\begin
g (x) & \mbox x \in (), \\
\exp (g^ (x)) & \mbox x \in (A,1], \\
\exp (f ( \ln (x))) & \mbox x \in (1,\infty), \\
\ln (f ( \exp (x))) & \mbox x \in (-\infty,0). \\
\end

Half-exponential functions are used in computational complexity theory for growth rates "intermediate" between polynomial and exponential.〔
==See also==

* Functional square root

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「Half-exponential function」の詳細全文を読む



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