Substabilizability and superstabilizability for bivariate means
 Mustapha Raïssouli^{1, 2}Email author and
 József Sándor^{3}
https://doi.org/10.1186/1029242X201428
© Raïssouli and Sándor; licensee Springer. 2014
Received: 27 July 2013
Accepted: 18 December 2013
Published: 24 January 2014
Abstract
The stability and stabilizability concepts for means in two variables have been introduced in (Raïssouli in Appl. Math. ENotes 11:159174, 2011). It has been proved that the arithmetic, geometric, and harmonic means are stable, while the logarithmic and identric means are stabilizable. In the present paper, we introduce new concepts, the socalled substabilizability and superstabilizability, and we apply them to some standard means.
MSC:26E60.
Keywords
1 Introduction
and are known as the arithmetic, geometric, harmonic, logarithmic, and identric means, respectively.
are known as the first Seiffert mean [2], the second Seiffert mean [3] and the NeumanSándor mean [4], respectively.
A mean m is symmetric if $m(a,b)=m(b,a)$ for all $a,b>0$, and monotone if $(a,b)\mapsto m(a,b)$ is increasing in a and in b, that is, if ${a}_{1}\le {a}_{2}$ (resp. ${b}_{1}\le {b}_{2}$) then $m({a}_{1},b)\le m({a}_{2},b)$ (resp. $m(a,{b}_{1})\le m(a,{b}_{2})$). For more details as regards monotone means, see [5].
For a given mean m, we set ${m}^{\ast}(a,b)={(m({a}^{1},{b}^{1}))}^{1}$, and it is easy to see that ${m}^{\ast}$ is also a mean, called the dual mean of m. Every mean m satisfies ${m}^{\ast \ast}:={({m}^{\ast})}^{\ast}=m$, and if ${m}_{1}$ and ${m}_{2}$ are two means such that ${m}_{1}<{m}_{2}$ then ${m}_{1}^{\ast}>{m}_{2}^{\ast}$. Further, the arithmetic and harmonic means are mutually dual (i.e. ${A}^{\ast}=H$, ${H}^{\ast}=A$) and the geometric mean is selfdual (i.e. ${G}^{\ast}=G$).
Let p be a real number. The next means are of interest.

The power (binomial) mean:$\{\begin{array}{l}{B}_{p}:={B}_{p}(a,b):={G}_{p,0}(a,b)={(\frac{{a}^{p}+{b}^{p}}{2})}^{1/p},\\ {B}_{1}=H,\phantom{\rule{2em}{0ex}}{B}_{0}=G,\phantom{\rule{2em}{0ex}}{B}_{1}=A,\phantom{\rule{2em}{0ex}}{B}_{2}:=Q.\end{array}$

The power logarithmic mean:$\{\begin{array}{l}{L}_{p}:={L}_{p}(a,b)={(\frac{{a}^{p}{b}^{p}}{p(lnalnb)})}^{1/p},\phantom{\rule{2em}{0ex}}{L}_{p}(a,a)=a,\\ {L}_{1}={L}^{\ast},\phantom{\rule{2em}{0ex}}{L}_{0}=G,\phantom{\rule{2em}{0ex}}{L}_{1}=L,\phantom{\rule{2em}{0ex}}{L}_{2}={(AL)}^{1/2}.\end{array}$
We end this section by recalling the next result which will be needed in the sequel.
Further these inequalities are the best possible i.e. ${L}_{2}$, ${L}_{4}$, ${L}_{5}$ are the best power logarithmic means lower bounds of P, M, T, while ${B}_{2/3}$, ${B}_{4/3}$, ${B}_{5/3}$ are the best power (binomial) means upper bounds of P, M, T, respectively. Otherwise, there is no $p>0$ such that P, M or T is strictly less that ${L}_{p}$.
For some details as regards the above theorem, we refer the reader to [6–10].
2 Needed tools
For the sake of simplicity for the reader, we recall here more basic notions and results that will be needed in the sequel, see [11] for more details. We begin by the next definition.
called the resultant meanmap of ${m}_{1}$, ${m}_{2}$, and ${m}_{3}$.
For the computation of $\mathcal{R}({m}_{1},{m}_{2},{m}_{3})$ when ${m}_{1}$, ${m}_{2}$, ${m}_{3}$ belong to the set of the above standard means, some examples can be found in [11–14]. Here we state another example which will be of interest.
We also recall the next result, see [13].
As already proved [11–13], the resultant meanmap’s importance stems from the fact that it is a tool for introducing the stability and stabilizability concepts, which we recall in the following.
 (a)
Stable if $\mathcal{R}(m,m,m)=m$.
 (b)
Stabilizable if there exist two nontrivial stable means ${m}_{1}$ and ${m}_{2}$ satisfying the relation $\mathcal{R}({m}_{1},m,{m}_{2})=m$. We then say that m is $({m}_{1},{m}_{2})$stabilizable.
A developed study about the stability and stabilizability of the standard means was presented in [11]. In particular the next result has been proved there.
 (1)
The power binomial mean ${B}_{p}$ is stable for all real number p. In particular, the arithmetic, geometric, and harmonic means A, G, and H are stable.
 (2)
The power logarithmic mean ${L}_{p}$ is $({B}_{p},G)$stabilizable for all real number p.
 (3)
The logarithmic mean L is $(H,A)$stabilizable and $(A,G)$stabilizable while the identric mean I is $(G,A)$stabilizable.
Remark 2.1 The symmetry character of the above involved mean is, by definition, taken as essential hypothesis. In fact, if we attempt to extend the above concepts to nonsymmetric means by keeping the same definitions (Definition 2.1 and Definition 2.2), the simple means $m={A}_{1/3},{G}_{1/3}$, with ${A}_{1/3}(a,b)=(1/3)a+(2/3)b$, ${G}_{1/3}(a,b)={a}^{1/3}{b}^{2/3}$, do not satisfy $\mathcal{R}(m,m,m)=m$. In another way, the definition of ℛ, together with that related to the stability and stabilizability concepts, is not exactly the same as above, but must be investigated for nonsymmetric means. We leave the details as regards the latter point to a later time.
The next definition is also needed here [13].
A symmetric mean m will be called cross mean if the map ${m}^{\otimes 2}:=m\otimes m$ is symmetric in its four variables.
It is proved in [11] that every cross mean is stable. The reverse of the latter assertion is still an open problem. Otherwise, it is conjectured [13] that the first Seiffert mean P is not stabilizable and such a problem is also still open. We also conjecture here that the second Seiffert mean and the NeumanSándor mean are not stabilizable either.
The next result needed here has also been proved in [14].
Theorem 2.3 Let ${m}_{1}$ and ${m}_{2}$ be two nontrivial stable symmetric monotone means such that ${m}_{1}\le {m}_{2}$ (resp. ${m}_{2}\le {m}_{1}$). Assume that ${m}_{1}$ is moreover a cross mean. Then there exists one and only one $({m}_{1},{m}_{2})$stabilizable mean m such that ${m}_{1}\le m\le {m}_{2}$ (resp. ${m}_{2}\le m\le {m}_{1}$).
3 Two special subsets of means
is obvious. By virtue of this equivalence, it will be sufficient to study the properties of one the sets ${\mathcal{E}}^{}({m}_{1},{m}_{2})$ and ${\mathcal{E}}^{+}({m}_{1},{m}_{2})$ and to deduce that of the other by duality.
Example 3.1 With the help of Theorem 2.1, it is simple to see that $G<\mathcal{R}(G,G,A)$ and $A>\mathcal{R}(G,A,A)$. So $G\in {\mathcal{E}}^{+}(G,A)$ and $A\in {\mathcal{E}}^{}(G,A)$. We can also verify that $T\in {\mathcal{E}}^{}(A,G)$ and $M\in {\mathcal{E}}^{}(A,G)$. Other more interesting examples will be seen later.
The next result is of interest.
Proposition 3.1 Let ${m}_{1}$, ${m}_{2}$ be two nontrivial monotone (symmetric) stable means where ${m}_{1}$ is a cross mean. Then the intersection between ${\mathcal{E}}^{}({m}_{1},{m}_{2})$ and ${\mathcal{E}}^{+}({m}_{1},{m}_{2})$ is reduced to the unique mean m which is the $({m}_{1},{m}_{2})$stabilizable mean.
Proof Following Theorem 2.3, let m be the unique $({m}_{1},{m}_{2})$stabilizable mean. Then $\mathcal{R}({m}_{1},m,{m}_{2})=m$ and so $m\in {\mathcal{E}}^{}({m}_{1},{m}_{2})$ and $m\in {\mathcal{E}}^{+}({m}_{1},{m}_{2})$. Inversely, let $m\in {\mathcal{E}}^{}({m}_{1},{m}_{2})\cap {\mathcal{E}}^{+}({m}_{1},{m}_{2})$; then $\mathcal{R}({m}_{1},m,{m}_{2})=m$ and so m is the unique $({m}_{1},{m}_{2})$stabilizable mean. □
Now, we are in a position to state the next result ensuring the existence of a maximal superstabilizable (resp. minimal substabilizable) mean.
Theorem 3.2 Let ${m}_{1}$, ${m}_{2}$ be two symmetric monotone means. Then the set ${\mathcal{E}}^{+}({m}_{1},{m}_{2})$ has at least a maximal element.
Before giving the proof of the last theorem we state the next corollary, which is immediate from the above.
Corollary 3.3 Let ${m}_{1}$, ${m}_{2}$ be as in the above theorem. Then the set ${\mathcal{E}}^{}({m}_{1},{m}_{2})$ has at least a minimal element.
Since ${m}_{1}$ and ${m}_{1}$ are monotone, we deduce by Theorem 2.1, ${m}_{i}\le \mathcal{R}({m}_{1},{sup}_{i\in J}{m}_{i},{m}_{2})$ for all $i\in J$ and so ${sup}_{i\in J}{m}_{i}\le \mathcal{R}({m}_{1},{sup}_{i\in J}{m}_{i},{m}_{2})$, that is, ${sup}_{i\in J}{m}_{i}\in {\mathcal{E}}^{+}({m}_{1},{m}_{2})$. It follows that every nonempty totally ordered subset of ${\mathcal{E}}^{+}({m}_{1},{m}_{2})$ has an upper bound in ${\mathcal{E}}^{+}({m}_{1},{m}_{2})$, that is, ${\mathcal{E}}^{+}({m}_{1},{m}_{2})$ is inductive. We can then apply the classical Zorn lemma to conclude and the proof of the theorem is complete. □
 (1)
The sets ${\mathcal{E}}^{}(A,m)$ and ${\mathcal{E}}^{+}(A,m)$ are (linearly) convex.
 (2)
The sets ${\mathcal{E}}^{}(G,m)$ and ${\mathcal{E}}^{+}(G,m)$ are geometrically convex.
Proof (1) follows from the linearaffine character of A with the definition of ℛ, while (2) comes from the geometric character of G. The details are simple and omitted here. □
4 Substabilizability and superstabilizability
The next definition may be stated.
 (a)
$({m}_{1},{m}_{2})$substabilizable if $\mathcal{R}({m}_{1},m,{m}_{2})\le m$ and m is between ${m}_{1}$ and ${m}_{2}$,
 (b)
$({m}_{1},{m}_{2})$superstabilizable if $m\le \mathcal{R}({m}_{1},m,{m}_{2})$ and m is between ${m}_{1}$ and ${m}_{2}$.
Following Theorem 2.3, the above definition extends that of stabilizability in the sense that a mean m is $({m}_{1},{m}_{2})$stabilizable if and only if (a) and (b) hold. It follows that the above concepts bring something new for nonstable and nonstabilizable means. For this, we say that m is strictly $({m}_{1},{m}_{2})$substabilizable if $\mathcal{R}({m}_{1},m,{m}_{2})<m$ and m is strictly $({m}_{1},{m}_{2})$superstabilizable if $m<\mathcal{R}({m}_{1},m,{m}_{2})$, with in both cases m being strictly between ${m}_{1}$ and ${m}_{2}$.
Example 4.1 We can easily see that G is $(G,A)$superstabilizable (but not strictly) while A is $(G,A)$substabilizable. However, T and M are not $(G,A)$substabilizable, since they are not between G and A. More interesting examples, presented as main results, will be stated in the section below.
 (1)
If there exists a symmetric mean ${m}_{1}$ such that m is $({m}_{1},G)$substabilizable then $m\ge {m}_{1}^{\pi}$.
 (2)
If there exists a symmetric mean ${m}_{1}$ such that m is $({m}_{1},G)$superstabilizable then $m\le {m}_{1}^{\pi}$.
 (2)
It is similar to that the above. The details are omitted here. □
The above theorem has various consequences, which we will state in what follows.
 (i)
If m is $({B}_{p},G)$substabilizable for some $p\ge 0$ then ${L}_{p}\le m\le {B}_{p}$. In particular, if m is $(A,G)$substabilizable then $L\le m\le A$.
 (ii)
If m is $({B}_{p},G)$superstabilizable for some $p\le 0$ then ${B}_{p}\le m\le {L}_{p}$. In particular, if m is $(A,G)$superstabilizable then $G\le m\le L$.
Proof It is immediate by combining the above theorem with the fact that ${B}_{p}^{\pi}={L}_{p}$ for each real number p, and ${B}_{1}=A$, ${L}_{1}=L$. □
 (ii)
The above corollary implies that I is not $(A,G)$superstabilizable, but it is perhaps $(A,G)$substabilizable. See more details as regards the latter point in the section below.
Corollary 4.3 Let $m>G$ be a strictly $({B}_{p},G)$substabilizable mean. Then $0<q<p<r$, where q is the greatest number such that $m>{L}_{q}$ and r is the smallest number such that $m<{B}_{r}$.
Proof If $m>G$ is strictly $({B}_{p},G)$substabilizable then, by definition, $m<{B}_{p}$ and, by the above corollary, $m\ge {L}_{p}$. Combining these latter meaninequalities we deduce the desired result. □
 (ii)
If M is strictly $({B}_{p},G)$substabilizable for some p then $4/3<p\le 4$.
 (iii)
If T is strictly $({B}_{p},G)$substabilizable then $5/3<p\le 5$.
 (iv)
There is no $p\in \mathbb{R}$ such that P, M or T is $({B}_{p},G)$superstabilizable.
Proof Combining the above corollary with Theorem 1.1, we immediately deduce the assertions (i), (ii), and (iii).
Assertion (iv) follows from Corollary 4.2(ii) with Theorem 1.1 again. Details are omitted here. □
5 Application to some standard means
This section will be devoted to an application of the above concepts to some known means. We begin with the next result.
Theorem 5.1 The logarithmic mean L is strictly $(G,A)$superstabilizable.
for all $a,b>0$ with $a\ne b$. We will present two different proofs for equation (5.1). By the symmetric character of the involved means, we can assume, without loss the generality, that $a<b$.

The first method is much more natural: Since $Aa=bA=(ba)/2$, we have$L(a,A)L(A,b)=\frac{{(ba)}^{2}}{4ln(A/a)\cdot ln(b/A)}.$
This gives equation (5.1), so it completes the proof of the first method.

The second method is based on the fact that we can always set $a={e}^{x}G$ and $b={e}^{x}G$ with $x>0$. A simple computation leads to$L(a,b)=\frac{shx}{x}G,\phantom{\rule{2em}{0ex}}L(a,\frac{a+b}{2})=\frac{shx}{xln(chx)}G,\phantom{\rule{2em}{0ex}}L(a,\frac{a+b}{2})=\frac{shx}{x+ln(chx)}G.$
for all $x>0$, which clearly holds and inequality (5.1) is again proved.
In summary, we have shown that L is strictly $(G,A)$superstabilizable. □
which, with $H<L<A$, means that L is strictly $(A,H)$substabilizable.
Theorem 5.2 The identric mean I is strictly $(A,G)$substabilizable.
Proof We will present here two different methods for proving our claim: The first is direct and based on some meaninequalities already stated in the literature, while the second one is similar to above.

First method: We have to show$I(a,G)+I(b,G)<2I(a,b)$(5.2)
which, when combined with equation (5.3), gives equation (5.2), so it completes the proof of the first method.

Second method: To show equation (5.2) is equivalent to proving that$A(\sqrt{a},\sqrt{b})I(\sqrt{a},\sqrt{b})<I(a,b).$(5.4)
It follows that Φ is strictly decreasing for $x>0$ and so $\mathrm{\Phi}(x)<\mathrm{\Phi}(0):={lim}_{t\to 0}\mathrm{\Phi}(t)=0$. The second method is complete. □
Remark 5.2 Another method for proving equation (5.4) can be stated as follows: It is well known (and easy to verify) that $I({a}^{2},{b}^{2})=I(a,b)S(a,b)$ for all $a,b>0$, where $S:=S(a,b)={({a}^{a}{b}^{b})}^{1/(a+b)}$ is the socalled weighted geometric mean. With this, equation (5.4) is equivalent to $A(\sqrt{a},\sqrt{b})<S(\sqrt{a},\sqrt{b})$ i.e. $A<S$, which is a wellknown meaninequality.
As a consequence of the above, the next result gives a double inequality refining $L<I$ and involving the four standard means G, L, I, and A.
This, with Example 2.1 and a simple manipulation, gives the desired result. □
Of course, the above theorems when combined with the properties of subsuperstabilizability imply that ${L}^{\ast}$ is, simultaneously, strictly $(G,H)$substabilizable and strictly $(H,A)$superstabilizable, while ${I}^{\ast}$ is strictly $(H,G)$superstabilizable.
As already pointed out before, whether the first Seiffert mean P is stabilizable still is an open problem. However, the next result may be stated.
Theorem 5.4 The first Seiffert mean P is strictly $(A,G)$substabilizable.
holds for all $a,b>0$ with $a\ne b$. We also present here two different methods.

First method: this method is analogous to the above. Simple computation leads to$P(a,b)=G\frac{shx}{arcsin(thx)}$
for all $x>0$. The desired inequality follows in the same way as previously.

Second method: this method is based on an integral form of $P(a,b)$. It is easy to see that, for all $a,b>0$ (with $a<b$ without loss the generality), we have$P(a,b)={\left(\frac{4}{ba}{\int}_{1}^{\sqrt{b/a}}\frac{dx}{1+{x}^{2}}\right)}^{1}.$(5.7)
from which equation (5.10) follows. The proof is complete. □
which is exactly equation (5.6).
6 Some open problems
for all $0<t<1$. We then present the following.
Problem 1: Prove or disprove that the first Seiffert mean P is strictly $(G,A)$superstabilizable.
Problem 2: Find the best real numbers $p>0$ and $q>0$ for which P is strictly $({B}_{p},{B}_{q})$substabilizable.
Problem 3: Are the means T and M strictly $({B}_{p},{B}_{q})$substabilizable for some real numbers $p>0$, $q>0$?
Declarations
Acknowledgements
The present work was supported by the Deanship of Scientific Research of Taibah University.
Authors’ Affiliations
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