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A differentiability criterion for continuous functions
Monatshefte für Mathematik ( IF 0.8 ) Pub Date : 2021-05-24 , DOI: 10.1007/s00605-021-01574-0
Stefan Catoiu

We show that, with the exception of the symmetric derivative, each limit of the form

$$\begin{aligned} \lim _{h\rightarrow 0}\frac{Af(x+ah)+Bf(x+bh)}{h},\qquad (A+B=0,Aa+Bb=1), \end{aligned}$$

is equivalent to the ordinary derivative, for all continuous functions at x. And, up to a non-zero scalar multiple, these are the only criteria for differentiating all continuous functions at x, by taking limits of first order difference quotients with two function evaluations.



中文翻译:

连续函数的微分准则

我们证明,除了对称导数之外,形式的每个极限

$$ \ begin {aligned} \ lim _ {h \ rightarrow 0} \ frac {Af(x + ah)+ Bf(x + bh)} {h},\ qquad(A + B = 0,Aa + Bb = 1),\ end {aligned} $$

对于x处的所有连续函数,等价于普通导数。而且,对于非零标量倍数,这是通过对带有两个函数求值的一阶差分商的限制来区分x处所有连续函数的唯一标准。

更新日期:2021-05-25
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