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On the equality problem of generalized Bajraktarević means
Aequationes Mathematicae ( IF 0.9 ) Pub Date : 2019-08-07 , DOI: 10.1007/s00010-019-00670-9
Richárd Grünwald , Zsolt Páles

The purpose of this paper is to investigate the equality problem of generalized Bajraktarević means, i.e., to solve the functional equation

$$\begin{aligned} f^{(-1)}\bigg (\frac{p_1(x_1)f(x_1)+\dots +p_n(x_n)f(x_n)}{p_1(x_1)+\dots +p_n(x_n)}\bigg )=g^{(-1)}\bigg (\frac{q_1(x_1)g(x_1)+\dots +q_n(x_n)g(x_n)}{q_1(x_1)+\dots +q_n(x_n)}\bigg ), \end{aligned}$$(*)

which holds for all \((x_1,\dots ,x_n)\in I^n\), where \(n\ge 2\), I is a nonempty open real interval, the unknown functions \(f,g:I\rightarrow {\mathbb {R}}\) are strictly monotone, \(f^{(-1)}\) and \(g^{(-1)}\) denote their generalized left inverses, respectively, and \(p=(p_1,\dots ,p_n):I\rightarrow {\mathbb {R}}_{+}^n\) and \(q=(q_1,\dots ,q_n):I\rightarrow {\mathbb {R}}_{+}^n\) are also unknown functions. This equality problem in the symmetric two-variable (i.e., when \(n=2\)) case was already investigated and solved under sixth-order regularity assumptions by Losonczi (Aequationes Math 58(3):223–241, 1999). In the nonsymmetric two-variable case, assuming the three times differentiability of f, g and the existence of \(i\in \{1,2\}\) such that either \(p_i\) is twice continuously differentiable and \(p_{3-i}\) is continuous on I, or \(p_i\) is twice differentiable and \(p_{3-i}\) is once differentiable on I, we prove that (*) holds if and only if there exist four constants \(a,b,c,d\in {\mathbb {R}}\) with \(ad\ne bc\) such that

$$\begin{aligned} cf+d>0,\qquad g=\frac{af+b}{cf+d},\qquad \text{ and }\qquad q_\ell =(cf+d)p_\ell \qquad (\ell \in \{1,\dots ,n\}). \end{aligned}$$

In the case \(n\ge 3\), we obtain the same conclusion with weaker regularity assumptions. Namely, we suppose that f and g are three times differentiable, p is continuous and there exist \(i,j,k\in \{1,\dots ,n\}\) with \(i\ne j\ne k\ne i\) such that \(p_i,p_j,p_k\) are differentiable.



中文翻译:

关于广义Bajraktarević的均等问题

本文的目的是研究广义Bajraktarević均值问题,即求解泛函

$$ \ begin {aligned} f ^ {(-1)} \ bigg(\ frac {p_1(x_1)f(x_1)+ \ dots + p_n(x_n)f(x_n)} {p_1(x_1)+ \ dots + p_n(x_n)} \ bigg)= g ^ {(-1)} \ bigg(\ frac {q_1(x_1)g(x_1)+ \ dots + q_n(x_n)g(x_n)} {q_1(x_1) + \点+ q_n(x_n)} \ bigg),\ end {aligned} $$(*)

它适用于所有\((x_1,\ dots,x_n)\ in I ^ n \),其中\(n \ ge 2 \)I是一个非空的开放实数区间,未知函数\ {f,g:I \ rightarrow {\ mathbb {R}} \)严格是单调的,\(f ^ {(-1)} \)\(g ^ {(-1)} \)分别表示它们的广义左逆,而\ (p = {p_1,\ dots,p_n):I \ rightarrow {\ mathbb {R}} _ {+} ^ n \)\(q = {q_1,\ dots,q_n):I \ rightarrow {\ mathbb {R}} _ {+} ^ n \)也是未知函数。对称二元变量中的等式问题(即\(n = 2 \))案例已经由Losonczi在六阶正则性假设下进行了调查和解决(Aequationes Math 58(3):223-241,1999)。在非对称二变量情况下,假设fg的三倍可微性和\(i \ in \ {1,2 \} \)的存在,使得\(p_i \)两次连续可微,而\(p p_ {3-i} \)I上是连续的,或者\(p_i \)I上是可微的两次,而\(p_ {3-i} \)I上是可微的,我们证明(*)成立且仅当在{\ mathbb {R}} \中存在四个常量\(a,b,c,d \)\(ad \ ne bc \) 这样

$$ \ begin {aligned} cf + d> 0,\ qquad g = \ frac {af + b} {cf + d},\ qquad \ text {和} \ qquad q_ \ ell =(cf + d)p_ \ ell \ qquad(\ ell \ in \ {1,\ dots,n \})。\ end {aligned} $$

\(n \ ge 3 \)的情况下,我们以较弱的规律性假设获得了相同的结论。即,我们假设fg是可微的三倍,p是连续的,并且存在\(i,j,k \ in \ {1,\ dots,n \} \)\(i \ ne j \ ne k \ ne i \)使得\(p_i,p_j,p_k \)是可微的。

更新日期:2019-08-07
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