In this paper, we provide a new simple proof of a double inequality of Masjed-Jamei type proved by L. Zhu [
Citation: Fen Wang. A new proof of a double inequality of Masjed-Jamei type[J]. AIMS Mathematics, 2024, 9(4): 8768-8775. doi: 10.3934/math.2024425
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In this paper, we provide a new simple proof of a double inequality of Masjed-Jamei type proved by L. Zhu [
In 2010, Masjed-Jamei [2] obtained some interesting inequalities for several special functions, one of which is about the relation of the inverse tangent function arctanx and inverse hyperbolic sine function sinh−1(x) as follows:
(arctanx)2⩽xsinh−1(x)√1+x2, x∈(−1,1). | (1.1) |
The study related to (1.1) attracted much attention in last decade. At first, Zhu and Male˘sević [3] proved that (1.1) holds for any x∈(−∞,+∞). They also obtained some refinements of (1.1) as follows:
Proposition 1.1. [3, Theorem 1.3] For any x∈(−∞,+∞), we have
−145x6⩽(arctanx)2−xsinh−1x√1+x2⩽−145x6+4105x8, | (1.2) |
−145x6+4105x8−11225x10⩽(arctanx)2−xsinh−1x√1+x2⩽−145x6+4105x8−11225x10+58610395x12. | (1.3) |
Define
vn=1n[n!2n−1(2n−1)!!−(1+13+⋯+12n−1)],n⩾3. | (1.4) |
By using flexible analysis tools, Zhu and Male˘sević [4] extended (1.2) and (1.3) to a general form as follows:
Proposition 1.2. [4, Theorem 1.1] For any x∈(−∞,+∞), we have
2m+1∑n=3(−1)nvnx2n⩽(arctanx)2−xsinh−1x√1+x2⩽2m+2∑n=3(−1)nvnx2n. | (1.5) |
Proposition 1.3. [5, Theorem 2.1] The double inequality
xsinh−1(x)√1+x2+145x2<(arctanx)2<xsinh−1(x)√1+x2 | (1.6) |
holds for any x∈(0,+∞) with best constants 0 and 1/45.
Please see [6,7] for more generalizations.
Motivated by (1.1)–(1.6), Zhu and Male˘sević [3] also studied the relation of the inverse hyperbolic tangent function tanh−1(x) and inverse sine function arcsinx as follows:
Proposition 1.4. [3, Theorem 1.4] The inequality
[tanh−1(x)]2<xarcsinx√1−x2 | (1.7) |
holds for any x∈(0,1) with the the best power number 2.
Proposition 1.5. [3, Theorem 1.6] The inequality
xarcsinx√1−x2−[tanh−1(x)]2<N∑n=3vnx2n | (1.8) |
holds for any x∈(0,1).
Moreover, by investigating the power series of the following function:
[tanh−1(x)]2arcsinx√1−x2=x−145x5−22945x7−612835x9+O(x10), | (1.9) |
Zhu [1] obtained the following interesting double inequality of Masjed-Jamei type.
Theorem 1.1. [1, Theorem 1] The double inequality
(x−x5)arcsinx√1−x2<[tanh−1(x)]2<(x−145x5)arcsinx√1−x2 | (1.10) |
holds for any x∈(0,1) with best constants −1 and −145.
The goal of this paper is to give a new and elementary proof of Theorem 1.1, which is much simpler than the proof of Zhu [1]. Zhu's proof [1] used the power series of the functions 1/cosnx and sinx/cosnx and properties of the Bernoulli numbers and Euler numbers. Our proof only relies on the power series of hyperbolic sine and cosine functions and some elementary computations.
We first establish two lemmas about the monotonicity of two functions.
Lemma 2.1. Let
f(x)=[tanh−1(x)]2√1−x2x−x5−arcsinx, |
then f(x) is strictly increasing on (0,1).
Proof. Let t=tanh−1(x)∈(0,+∞), then x=tanh(t). Define
F(t):=f(tanh(t))=t2cosh(t)tanh(t)−tanh5(t)−arcsin(tanh(t))=t2cosh4t(cosh2t+sinh2t)sinht−arcsin(tanh(t)). |
In order to prove that f(x) is strictly increasing on (0,1), we only need to prove F(t) is strictly increasing on (0,+∞). In fact,
φ(t):=[(cosh2t+sinh2t)2coshtsinh2t]F′(t)=t2(2cosh4t−7cosh2t+4)cosh4t+2t(cosh2t+sinh2t)sinhtcosh5t−(cosh2t+sinh2t)2sinh2t. | (2.1) |
Since
A1:=(2cosh4t−7cosh2t+4)cosh4t=126[cosh(8t)−6cosh(6t)−24cosh(4t)−26cosh(2t)−9]=126[∞∑n=0(8t)2n(2n)!−6⋅∞∑n=0(6t)2n(2n)!−24⋅∞∑n=0(4t)2n(2n)!−26⋅∞∑n=0(2t)2n(2n)!−9], |
B1:=2(cosh2t+sinh2t)sinhtcosh5t=125[sinh(8t)+4sinh(6t)+6sinh(4t)+4sinh(2t)]t=125[∞∑n=0(8t)2n+1(2n+1)!+4⋅∞∑n=0(6t)2n+1(2n+1)!+6⋅∞∑n=0(4t)2n+1(2n+1)!+4⋅∞∑n=0(2t)2n+1(2n+1)!] |
and
C1:=−(cosh2t+sinh2t)2sinh2t=123[cosh(6t)−2cosh(4t)+3cosh(2t)−2]=123[∞∑n=0(6t)2n(2n)!−2⋅∞∑n=0(4t)2n(2n)!+3⋅∞∑n=0(2t)2n(2n)!−2], |
we have
φ(t)=A1t2+B1t+C1=126∞∑n=1a2n+2t2n+2, | (2.2) |
where
a2n+2=82n(2n)!−6⋅62n(2n)!−24⋅42n(2n)!−26⋅22n(2n)!+2⋅82n+1(2n+1)!+8⋅62n+1(2n+1)!+12⋅42n+1(2n+1)!+8⋅22n+1(2n+1)!−8⋅62n+2(2n+2)!+16⋅42n+2(2n+2)!−24⋅22n+2(2n+2)!. |
It is easy to check that
a4=0,a6=28169,a8=42245. | (2.3) |
When n⩾4,
a2n+2>82n(2n)![1−6⋅(34)2n−24⋅(12)2n−26⋅(14)2n]+62n+1(2n+1)!(1−3n+1)+8⋅22n+1(2n+1)!(1−3n+1)>82n(2n)![1−6⋅(34)8−24⋅(12)8−26⋅(14)8]=82n(2n)!⋅625211>0. | (2.4) |
Combining (2.2)–(2.4), we obtain that φ(t)>0 for any t∈(0,+∞), which implies
F′(t)>0 for any t∈(0,+∞). |
So, F(t) is strictly increasing on (0,+∞). The proof of Lemma 2.1 is completed.
Lemma 2.2. Let
g(x)=arcsinx−[tanh−1(x)]2√1−x2x−145x5, |
then g(x) is strictly increasing on (0,1).
Proof. Let t=tanh−1(x)∈(0,+∞), then x=tanh(t). Define
G(t):=g(tanh(t))=arcsin(tanh(t))−t2cosh(t)tanh(t)−145tanh5(t)=arcsin(tanh(t))−45t2cosh4t(45cosh4t−sinh4t)sinht. |
In order to prove that g(x) is strictly increasing on (0,1), we only need to prove G(t) is strictly increasing on (0,+∞). Define
ψ(t):=[(45cosh4t−sinh4t)2coshtsinh2t]G′(t)=t2(1980cosh6t−90cosh4t+315cosh2t−180)cosh4t−90t(45cosh4t−sinh4t)sinhtcosh5t+(45cosh4t−sinh4t)2sinh2t. | (2.5) |
Since
A2:=(1980cosh6t−90cosh4t+315cosh2t−180)cosh4t=127[495cosh(10t)+4860cosh(8t)+22815cosh(6t)+61560cosh(4t)+106290cosh(2t)+63180]=127[495⋅∞∑n=0(10t)2n(2n)!+4860⋅∞∑n=0(8t)2n(2n)!+22815⋅∞∑n=0(6t)2n(2n)!+61560⋅∞∑n=0(4t)2n(2n)!+106290⋅∞∑n=0(2t)2n(2n)!+63180], |
B2:=−90(45cosh4t−sinh4t)sinhtcosh5t=127[−990sinh(10t)−8100sinh(8t)−27450sinh(6t)−48600sinh(4t)−42300sinh(2t)]=127[−990⋅∞∑n=0(10t)2n+1(2n+1)!−8100⋅∞∑n=0(8t)2n+1(2n+1)!−27450⋅∞∑n=0(6t)2n+1(2n+1)!−48600⋅∞∑n=0(4t)2n+1(2n+1)!−42300⋅∞∑n=0(2t)2n+1(2n+1)!], |
and
C2:=(45cosh4t−sinh4t)2sinh2t=127[484cosh(10t)+3080cosh(8t)+6660cosh(6t)+3840cosh(4t)−7080cosh(2t)−6984]=127[484⋅∞∑n=0(10t)2n(2n)!+3080⋅∞∑n=0(8t)2n(2n)!+6660⋅∞∑n=0(6t)2n(2n)!+3840⋅∞∑n=0(4t)2n(2n)!−7080⋅∞∑n=0(2t)2n(2n)!−6984], |
we have
ψ(t)=A2t2+B2t+C2=127∞∑n=1b2n+2t2n+2, | (2.6) |
where
b2n+2=495⋅102n(2n)!+4860⋅82n(2n)!+22815⋅62n(2n)!+61560⋅42n(2n)!+106290⋅22n(2n)!−990⋅102n+1(2n+1)!−8100⋅82n+1(2n+1)!−27450⋅62n+1(2n+1)!−48600⋅42n+1(2n+1)!−42300⋅22n+1(2n+1)!+484⋅102n+2(2n+2)!+3080⋅82n+2(2n+2)!+6660⋅62n+2(2n+2)!+3840⋅42n+2(2n+2)!−7080⋅22n+2(2n+2)!. |
It is easy to check that
b2n+2>0, 1⩽n⩽9. | (2.7) |
When n⩾10,
b2n+2>102n(2n)!⋅(495−990×102n+1)+82n(2n)!⋅(4860−8100×82n+1)+62n(2n)!⋅(22815−27450×62n+1)+42n(2n)!⋅(61560−48600×42n+1)+22n(2n)!⋅(106290−42300×22n+1)+22n+2(2n+2)!⋅(3840⋅22n+2−7840)>0. | (2.8) |
Combining (2.5)–(2.8), we obtain that ψ(t)>0 for any t∈(0,+∞), which implies
G′(t)>0 for any t∈(0,+∞). |
So, G(t) is strictly increasing on (0,+∞). The proof of Lemma 2.2 is completed.
Proof of Theorem 1.1. By Lemma 2.1 and
limx→0+f(x)=0, |
we get f(x)>0 for any x∈(0,1), which implies
[tanh−1(x)]2>(x−x5)arcsinx√1−x2, x∈(0,1). | (2.9) |
By Lemma 2.2 and
limx→0+g(x)=0, |
we get g(x)>0 for any x∈(0,1), which implies
[tanh−1(x)]2<(x−145x5)arcsinx√1−x2, x∈(0,1). | (2.10) |
Since
limx→0+[tanh−1(x)]2⋅√1−x2−xarcsinxx5arcsinx=−1, | (2.11) |
limx→1−[tanh−1(x)]2⋅√1−x2−xarcsinxx5arcsinx=−145, | (2.12) |
Theorem 1.1 follows from (2.9)–(2.12).
In this paper, we give a new simple proof of a double inequality of Masjed-Jamei type proved by Zhu [1]. We believe that the technique used in this paper can be used to obtain other interesting analytic inequalities.
Based on numerical experiments and (1.9), we propose the following conjectures:
Conjecture 1.
ϕ(x)=√1−x2[tanh−1(x)]2−xarcsinxx5arcsinx |
is strictly increasing on (0,1).
Conjecture 2.
h(x)=x−√1−x2[tanh−1(x)]2arcsinx |
is absolutely monotonic on (0,1).
The authors declare they have not used Artificial Intelligence (AI) tools in the creation of this article.
The author was supported by the Foundation of Hubei Provincial Department of Eduction (No. Q20233003), the Scientific Research Fund of Hubei Provincial Department of Eduction (No. B2022207) and Hubei University of Education, Bigdata Modeling and Intelligent Computing Research Institute.
The author declares no conflict of interest in this paper.
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L. Zhu, B. Male˘sević, Natural approximation of Masjed-Jamei's inequality, Rev. R. Acad. Cienc. Exactas Fís. Nat. Ser. A Mat., 114 (2020), 25. https://doi.org/10.1007/s13398-019-00735-z doi: 10.1007/s13398-019-00735-z
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X. D. Chen, L. Nie, W. K. Huang, New inequalities between the inverse hyperbolic tangent and the analogue for corresponding functions, J. Inequal. Appl., 2020 (2020), 1–8. https://doi.org/10.1186/s13660-020-02396-8 doi: 10.1186/s13660-020-02396-8
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