Research article

A new proof of a double inequality of Masjed-Jamei type

  • Received: 23 November 2023 Revised: 30 January 2024 Accepted: 26 February 2024 Published: 29 February 2024
  • MSC : 26D05, 26D15

  • In this paper, we provide a new simple proof of a double inequality of Masjed-Jamei type proved by L. Zhu [1].

    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 [1].



    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 sinh1(x) as follows:

    (arctanx)2xsinh1(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)2xsinh1x1+x2145x6+4105x8, (1.2)
    145x6+4105x811225x10(arctanx)2xsinh1x1+x2145x6+4105x811225x10+58610395x12. (1.3)

    Define

    vn=1n[n!2n1(2n1)!!(1+13++12n1)],n3. (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+1n=3(1)nvnx2n(arctanx)2xsinh1x1+x22m+2n=3(1)nvnx2n. (1.5)

    Proposition 1.3. [5, Theorem 2.1] The double inequality

    xsinh1(x)1+x2+145x2<(arctanx)2<xsinh1(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 tanh1(x) and inverse sine function arcsinx as follows:

    Proposition 1.4. [3, Theorem 1.4] The inequality

    [tanh1(x)]2<xarcsinx1x2 (1.7)

    holds for any x(0,1) with the the best power number 2.

    Proposition 1.5. [3, Theorem 1.6] The inequality

    xarcsinx1x2[tanh1(x)]2<Nn=3vnx2n (1.8)

    holds for any x(0,1).

    Moreover, by investigating the power series of the following function:

    [tanh1(x)]2arcsinx1x2=x145x522945x7612835x9+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

    (xx5)arcsinx1x2<[tanh1(x)]2<(x145x5)arcsinx1x2 (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)=[tanh1(x)]21x2xx5arcsinx,

    then f(x) is strictly increasing on (0,1).

    Proof. Let t=tanh1(x)(0,+), then x=tanh(t). Define

    F(t):=f(tanh(t))=t2cosh(t)tanh(t)tanh5(t)arcsin(tanh(t))=t2cosh4t(cosh2t+sinh2t)sinhtarcsin(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(2cosh4t7cosh2t+4)cosh4t+2t(cosh2t+sinh2t)sinhtcosh5t(cosh2t+sinh2t)2sinh2t. (2.1)

    Since

    A1:=(2cosh4t7cosh2t+4)cosh4t=126[cosh(8t)6cosh(6t)24cosh(4t)26cosh(2t)9]=126[n=0(8t)2n(2n)!6n=0(6t)2n(2n)!24n=0(4t)2n(2n)!26n=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)!+4n=0(6t)2n+1(2n+1)!+6n=0(4t)2n+1(2n+1)!+4n=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)!2n=0(4t)2n(2n)!+3n=0(2t)2n(2n)!2],

    we have

    φ(t)=A1t2+B1t+C1=126n=1a2n+2t2n+2, (2.2)

    where

    a2n+2=82n(2n)!662n(2n)!2442n(2n)!2622n(2n)!+282n+1(2n+1)!+862n+1(2n+1)!+1242n+1(2n+1)!+822n+1(2n+1)!862n+2(2n+2)!+1642n+2(2n+2)!2422n+2(2n+2)!.

    It is easy to check that

    a4=0,a6=28169,a8=42245. (2.3)

    When n4,

    a2n+2>82n(2n)![16(34)2n24(12)2n26(14)2n]+62n+1(2n+1)!(13n+1)+822n+1(2n+1)!(13n+1)>82n(2n)![16(34)824(12)826(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[tanh1(x)]21x2x145x5,

    then g(x) is strictly increasing on (0,1).

    Proof. Let t=tanh1(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(45cosh4tsinh4t)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):=[(45cosh4tsinh4t)2coshtsinh2t]G(t)=t2(1980cosh6t90cosh4t+315cosh2t180)cosh4t90t(45cosh4tsinh4t)sinhtcosh5t+(45cosh4tsinh4t)2sinh2t. (2.5)

    Since

    A2:=(1980cosh6t90cosh4t+315cosh2t180)cosh4t=127[495cosh(10t)+4860cosh(8t)+22815cosh(6t)+61560cosh(4t)+106290cosh(2t)+63180]=127[495n=0(10t)2n(2n)!+4860n=0(8t)2n(2n)!+22815n=0(6t)2n(2n)!+61560n=0(4t)2n(2n)!+106290n=0(2t)2n(2n)!+63180],
    B2:=90(45cosh4tsinh4t)sinhtcosh5t=127[990sinh(10t)8100sinh(8t)27450sinh(6t)48600sinh(4t)42300sinh(2t)]=127[990n=0(10t)2n+1(2n+1)!8100n=0(8t)2n+1(2n+1)!27450n=0(6t)2n+1(2n+1)!48600n=0(4t)2n+1(2n+1)!42300n=0(2t)2n+1(2n+1)!],

    and

    C2:=(45cosh4tsinh4t)2sinh2t=127[484cosh(10t)+3080cosh(8t)+6660cosh(6t)+3840cosh(4t)7080cosh(2t)6984]=127[484n=0(10t)2n(2n)!+3080n=0(8t)2n(2n)!+6660n=0(6t)2n(2n)!+3840n=0(4t)2n(2n)!7080n=0(2t)2n(2n)!6984],

    we have

    ψ(t)=A2t2+B2t+C2=127n=1b2n+2t2n+2, (2.6)

    where

    b2n+2=495102n(2n)!+486082n(2n)!+2281562n(2n)!+6156042n(2n)!+10629022n(2n)!990102n+1(2n+1)!810082n+1(2n+1)!2745062n+1(2n+1)!4860042n+1(2n+1)!4230022n+1(2n+1)!+484102n+2(2n+2)!+308082n+2(2n+2)!+666062n+2(2n+2)!+384042n+2(2n+2)!708022n+2(2n+2)!.

    It is easy to check that

    b2n+2>0,  1n9. (2.7)

    When n10,

    b2n+2>102n(2n)!(495990×102n+1)+82n(2n)!(48608100×82n+1)+62n(2n)!(2281527450×62n+1)+42n(2n)!(6156048600×42n+1)+22n(2n)!(10629042300×22n+1)+22n+2(2n+2)!(384022n+27840)>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

    limx0+f(x)=0,

    we get f(x)>0 for any x(0,1), which implies

    [tanh1(x)]2>(xx5)arcsinx1x2,   x(0,1). (2.9)

    By Lemma 2.2 and

    limx0+g(x)=0,

    we get g(x)>0 for any x(0,1), which implies

    [tanh1(x)]2<(x145x5)arcsinx1x2,   x(0,1). (2.10)

    Since

    limx0+[tanh1(x)]21x2xarcsinxx5arcsinx=1, (2.11)
    limx1[tanh1(x)]21x2xarcsinxx5arcsinx=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)=1x2[tanh1(x)]2xarcsinxx5arcsinx

    is strictly increasing on (0,1).

    Conjecture 2.

    h(x)=x1x2[tanh1(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.



    [1] L. Zhu, New double inequality of Masjed-Jamei-type, Rev. R. Acad. Cienc. Exactas Fís. Nat. Ser. A Mat., 117 (2023), 41. https://doi.org/10.1007/s13398-022-01375-6 doi: 10.1007/s13398-022-01375-6
    [2] M. Masjed-Jamei, A main inequality for several special functions, Comput. Math. Appl., 60 (2010), 1280–1289. https://doi.org/10.1016/j.camwa.2010.06.007 doi: 10.1016/j.camwa.2010.06.007
    [3] L. Zhu, B. Male˘sević, Inequalities between the inverse hyperbolic tangent and the inverse sine and the analogue for corresponding functions, J. Inequal. Appl., 2019 (2019), 1–10. https://doi.org/10.1186/s13660-019-2046-2 doi: 10.1186/s13660-019-2046-2
    [4] 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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    [6] C. Chesneau, , Y. J. Bagul, On a reverse trigonometric Masjed-Jamei inequality, Asia Pac. J. Math., 8 (2021), 1–5. https://doi.org/10.28924/APJM/8-13 doi: 10.28924/APJM/8-13
    [7] 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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