Journal of Applied Mathematics and Physics
Vol.1 No.7(2013), Article ID:40715,4 pages DOI:10.4236/jamp.2013.17001
An Optimal Double Inequality among the One-Parameter, Arithmetic and Geometric Means
College of Mathematics and Computer Science, Hebei University, Baoding, China
Email: ghy@hbu.cn, 563211828@qq.com, 347764565@qq.com, 602580999@qq.com
Copyright © 2013 Hongya Gao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Received October 12, 2013; revised November 12, 2013; accepted November 17, 2013
Keywords: Optimal Double Inequality; One-Parameter Mean; Arithmetic Mean; Geometric Mean
ABSTRACT
In the present paper, we answer the question: for fixed, what are the greatest value
and the least value
such that the double inequality
holds for all
with
? where for
, the one-parameter mean
, arithmetic mean
and geometric mean
of two positive real numbers
and
are defined by
and
, respectively.
1. Introduction
For, the one-parameter mean
, arithmetic mean
and geometric mean
of two positive real numbers
and
are defined by
(1)
and
, respectively.
It is well-known that the one-parameter mean is continuous and strictly increasing with respect to for fixed
with
. Many means are special cases of the one-parameter mean, for example:
is the arithmetic mean,
is the Heronian mean,
is the geometric mean, and
is the harmonic mean.
The one-parameter mean and its inequalities have been studied intensively, see [1-6].
The purpose of this paper is to answer the question: for, what are the greatest value
and the least value
such that the double inequality
holds for all
with
?
2. Main Result
The main result of this paper is the following theorem.
Theorem 2.1. Let. Then for any
with
, we have 1)
for
2)
for
3)
for
.
The numbers and
in 2) and 3) are optimal.
In order to prove Theorem 2.1, we need a preliminary lemma.
Lemma 2.1. For, one has
(2)
Proof. Simple calculations lead to
(3)
(4)
(5)
(6)
(2) follows from (3)-(6).
Proof of Theorem 2.1. Without loss of generality we assume and take
We first consider the case
. 1) follows from
From now on we assume Let
then (1) leads to
(7)
where
Simple calculations lead to
(8)
where
(9)
(10)
(11)
where
(12)
(13)
where
(14)
(15)
(16)
We shall distinguish between two cases.
Case 1.. The left-hand side inequality of 2)
for follows from Lemma 2.1 because in this case
for all. In the sequel we assume
.
We clearly see from (16) that
Thus is strictly decreasing for
and strictly increasing for
. (2.14) yields
then
for
and
for
. The same reasoning applies to
and
as well, and noticing (13) and (12), one has
This result together with (11) implies
Thus is strictly decreasing for
and strictly increasing for
The same reasoning applies to
and as well, and applying (8)-(10), we derive
Since for
and
for
, then we know from (7) that
This implies the left-hand side of 2) and the right-hand side of 3).
Case 2.. From (14) we know that
From (13) we know that for
and
for
. This implies
is strictly decreasing for
and strictly increasing for
. From (12) we know
Therefore
(11) implies has the same property as
thus
is strictly decreasing for
and strictly increasing for
. The same reasoning applies to
,
and
as well, and noticing (9) and (8), one has
which together with (7) implies
This implies the right-hand side of 2) and the left-hand side of 3).
We are now in the position to prove the constants
and
are optimal.
For any (positive or negative, with
sufficiently small) we consider the case
. (12)
implies
By the continuity of, there exists
such that
By (11), as the same property as
. The same reasoning applies to
,
,
and
as well, and noticing (10)-(8), we know
has the same property as
. By (7) one has
This proves the optimality for.
To prove the optimality for in the right-hand side of 2) and the left-hand side of 3), we notice from
that there exists such that
for and
and
for This ends the proof of Theorem 2.1.
3. Acknowledgements
This paper is supported by NSF of Hebei Province (A2011201011).
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