<p>For <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(a,b&gt;0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>&gt;</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation> with <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(a\ne b\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>a</mi> <mo>≠</mo> <mi>b</mi> </mrow> </math></EquationSource> </InlineEquation>, let <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\( L=L(a,b),I=I(a,b),A=A(a,b),G=G(a,b),A_{p}=A_{p}(a,b)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>L</mi> <mo>=</mo> <mi>L</mi> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mi>I</mi> <mo>=</mo> <mi>I</mi> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mi>A</mi> <mo>=</mo> <mi>A</mi> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <mi>G</mi> <mo>=</mo> <mi>G</mi> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo stretchy="false">)</mo> </mrow> <mo>,</mo> <msub> <mi>A</mi> <mi>p</mi> </msub> <mo>=</mo> <msub> <mi>A</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> denote the logarithmic mean, identric mean, arithmetic mean, geometric mean and <i>p</i>-order power mean, respectively. In 2003, Alzer and Qiu asserted “there does not exist a real number p such that <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\sqrt{LI}&lt;A_{p}(A,G)&lt;(L+I)/2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msqrt> <mrow> <mi mathvariant="italic">LI</mi> </mrow> </msqrt> <mo>&lt;</mo> <msub> <mi>A</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>A</mi> <mo>,</mo> <mi>G</mi> <mo stretchy="false">)</mo> </mrow> <mo>&lt;</mo> <mrow> <mo stretchy="false">(</mo> <mi>L</mi> <mo>+</mo> <mi>I</mi> <mo stretchy="false">)</mo> </mrow> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation> is valid for all <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(a,b&gt;0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>&gt;</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation>” , which inspired us to prove the assertion and to find new sharp bounds for the product and sum of logarithmic and identric means. Precisely, we proved that the inequalities <Equation ID="Equ37"> <EquationSource Format="TEX">\(\begin{aligned} \sqrt{LI}&lt; &amp; \sqrt{A_{1/3}A_{2/3}}&lt;A_{8/9}\left( A,G\right)&lt;A_{1/2}, \\ A_{q_{0}}&lt; &amp; A_{\ln 2}\left( A,G\right)&lt;\frac{L+I}{2}, \\ \frac{L+I}{2}&lt; &amp; \frac{A_{1/3}+A_{2/3}}{2}&lt;A_{1/2} \end{aligned}\)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <mtable> <mtr> <mtd columnalign="right"> <mrow> <msqrt> <mrow> <mi mathvariant="italic">LI</mi> </mrow> </msqrt> <mo>&lt;</mo> </mrow> </mtd> <mtd columnalign="left"> <mrow> <msqrt> <mrow> <msub> <mi>A</mi> <mrow> <mn>1</mn> <mo stretchy="false">/</mo> <mn>3</mn> </mrow> </msub> <msub> <mi>A</mi> <mrow> <mn>2</mn> <mo stretchy="false">/</mo> <mn>3</mn> </mrow> </msub> </mrow> </msqrt> <mo>&lt;</mo> <msub> <mi>A</mi> <mrow> <mn>8</mn> <mo stretchy="false">/</mo> <mn>9</mn> </mrow> </msub> <mfenced close=")" open="("> <mi>A</mi> <mo>,</mo> <mi>G</mi> </mfenced> <mo>&lt;</mo> <msub> <mi>A</mi> <mrow> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </msub> <mo>,</mo> </mrow> </mtd> </mtr> <mtr> <mtd columnalign="right"> <mrow> <mrow /> <msub> <mi>A</mi> <msub> <mi>q</mi> <mn>0</mn> </msub> </msub> <mo>&lt;</mo> </mrow> </mtd> <mtd columnalign="left"> <mrow> <msub> <mi>A</mi> <mrow> <mo>ln</mo> <mn>2</mn> </mrow> </msub> <mfenced close=")" open="("> <mi>A</mi> <mo>,</mo> <mi>G</mi> </mfenced> <mo>&lt;</mo> <mfrac> <mrow> <mi>L</mi> <mo>+</mo> <mi>I</mi> </mrow> <mn>2</mn> </mfrac> <mo>,</mo> </mrow> </mtd> </mtr> <mtr> <mtd columnalign="right"> <mrow> <mrow /> <mfrac> <mrow> <mi>L</mi> <mo>+</mo> <mi>I</mi> </mrow> <mn>2</mn> </mfrac> <mo>&lt;</mo> </mrow> </mtd> <mtd columnalign="left"> <mrow> <mfrac> <mrow> <msub> <mi>A</mi> <mrow> <mn>1</mn> <mo stretchy="false">/</mo> <mn>3</mn> </mrow> </msub> <mo>+</mo> <msub> <mi>A</mi> <mrow> <mn>2</mn> <mo stretchy="false">/</mo> <mn>3</mn> </mrow> </msub> </mrow> <mn>2</mn> </mfrac> <mo>&lt;</mo> <msub> <mi>A</mi> <mrow> <mn>1</mn> <mo stretchy="false">/</mo> <mn>2</mn> </mrow> </msub> </mrow> </mtd> </mtr> </mtable> </mrow> </math></EquationSource> </Equation>hold for <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(a,b&gt;0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>&gt;</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation> with <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(a\ne b\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>a</mi> <mo>≠</mo> <mi>b</mi> </mrow> </math></EquationSource> </InlineEquation>, where <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(q_{0}=\left( \ln 2\right) /\left( 1+\ln 2\right) \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>q</mi> <mn>0</mn> </msub> <mo>=</mo> <mfenced close=")" open="("> <mo>ln</mo> <mn>2</mn> </mfenced> <mo stretchy="false">/</mo> <mfenced close=")" open="("> <mn>1</mn> <mo>+</mo> <mo>ln</mo> <mn>2</mn> </mfenced> </mrow> </math></EquationSource> </InlineEquation>. These yield several nice chains of inequalities for means. Finally, a conjecture is proposed.</p>

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Sharp bounds for the product and sum of logarithmic and identric means

  • Hui-Zuo Xu

摘要

For \(a,b>0\) a , b > 0 with \(a\ne b\) a b , let \( L=L(a,b),I=I(a,b),A=A(a,b),G=G(a,b),A_{p}=A_{p}(a,b)\) L = L ( a , b ) , I = I ( a , b ) , A = A ( a , b ) , G = G ( a , b ) , A p = A p ( a , b ) denote the logarithmic mean, identric mean, arithmetic mean, geometric mean and p-order power mean, respectively. In 2003, Alzer and Qiu asserted “there does not exist a real number p such that \(\sqrt{LI}<A_{p}(A,G)<(L+I)/2\) LI < A p ( A , G ) < ( L + I ) / 2 is valid for all \(a,b>0\) a , b > 0 ” , which inspired us to prove the assertion and to find new sharp bounds for the product and sum of logarithmic and identric means. Precisely, we proved that the inequalities \(\begin{aligned} \sqrt{LI}< & \sqrt{A_{1/3}A_{2/3}}<A_{8/9}\left( A,G\right)<A_{1/2}, \\ A_{q_{0}}< & A_{\ln 2}\left( A,G\right)<\frac{L+I}{2}, \\ \frac{L+I}{2}< & \frac{A_{1/3}+A_{2/3}}{2}<A_{1/2} \end{aligned}\) LI < A 1 / 3 A 2 / 3 < A 8 / 9 A , G < A 1 / 2 , A q 0 < A ln 2 A , G < L + I 2 , L + I 2 < A 1 / 3 + A 2 / 3 2 < A 1 / 2 hold for \(a,b>0\) a , b > 0 with \(a\ne b\) a b , where \(q_{0}=\left( \ln 2\right) /\left( 1+\ln 2\right) \) q 0 = ln 2 / 1 + ln 2 . These yield several nice chains of inequalities for means. Finally, a conjecture is proposed.