<p>This paper investigates the absolute monotonicity of two function families associated with the Gaussian hypergeometric function <i>F</i>(<i>a</i>,&#xa0;<i>b</i>;&#xa0;<i>c</i>;&#xa0;<i>x</i>) (where <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(a,b,c\in \mathbb {R}_+\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>,</mo> <mi>c</mi> <mo>∈</mo> <msub> <mi mathvariant="double-struck">R</mi> <mo>+</mo> </msub> </mrow> </math></EquationSource> </InlineEquation>): <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\mathcal {F}_p(x)=(1-x)^pF(a,b;c;x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="script">F</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msup> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>-</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mi>p</mi> </msup> <mi>F</mi> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>;</mo> <mi>c</mi> <mo>;</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\mathcal {G}_p(x)=(1-x)^p \exp (F(a,b;c;x))\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi mathvariant="script">G</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msup> <mrow> <mo stretchy="false">(</mo> <mn>1</mn> <mo>-</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mi>p</mi> </msup> <mo>exp</mo> <mrow> <mo stretchy="false">(</mo> <mi>F</mi> <mrow> <mo stretchy="false">(</mo> <mi>a</mi> <mo>,</mo> <mi>b</mi> <mo>;</mo> <mi>c</mi> <mo>;</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>, as well as the logarithmic transform <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\ln \mathcal {F}_p(x)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>ln</mo> <msub> <mi mathvariant="script">F</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>. Our main goal is to establish necessary and sufficient conditions for the parameter <i>p</i>, such that <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(-\mathcal {F}'_p\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>-</mo> <msubsup> <mrow> <mi mathvariant="script">F</mi> </mrow> <mi>p</mi> <mo>′</mo> </msubsup> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\pm \mathcal {G}'_p\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>±</mo> <msubsup> <mrow> <mi mathvariant="script">G</mi> </mrow> <mi>p</mi> <mo>′</mo> </msubsup> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\pm (\ln \mathcal {F}_p)'\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo>±</mo> <msup> <mrow> <mo stretchy="false">(</mo> <mo>ln</mo> <msub> <mi mathvariant="script">F</mi> <mi>p</mi> </msub> <mo stretchy="false">)</mo> </mrow> <mo>′</mo> </msup> </mrow> </math></EquationSource> </InlineEquation> are absolutely monotonic on (0,&#xa0;1). Moreover, we derive several results regarding the absolute monotonicity of their higher-order derivatives. As applications, we derive several new inequalities for the Gaussian hypergeometric function <i>F</i>(<i>a</i>,&#xa0;<i>b</i>;&#xa0;<i>c</i>;&#xa0;<i>x</i>). We develop a novel constructive approach based on Jurkat’s criterion for power series ratios, which avoids limitations of cumbersome recursive/inductive methods in the existing literature.</p>

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Necessary and Sufficient Conditions for Absolute Monotonicity of Functions Related to Gaussian Hypergeometric Functions

  • Tiehong Zhao

摘要

This paper investigates the absolute monotonicity of two function families associated with the Gaussian hypergeometric function F(abcx) (where \(a,b,c\in \mathbb {R}_+\) a , b , c R + ): \(\mathcal {F}_p(x)=(1-x)^pF(a,b;c;x)\) F p ( x ) = ( 1 - x ) p F ( a , b ; c ; x ) and \(\mathcal {G}_p(x)=(1-x)^p \exp (F(a,b;c;x))\) G p ( x ) = ( 1 - x ) p exp ( F ( a , b ; c ; x ) ) , as well as the logarithmic transform \(\ln \mathcal {F}_p(x)\) ln F p ( x ) . Our main goal is to establish necessary and sufficient conditions for the parameter p, such that \(-\mathcal {F}'_p\) - F p , \(\pm \mathcal {G}'_p\) ± G p and \(\pm (\ln \mathcal {F}_p)'\) ± ( ln F p ) are absolutely monotonic on (0, 1). Moreover, we derive several results regarding the absolute monotonicity of their higher-order derivatives. As applications, we derive several new inequalities for the Gaussian hypergeometric function F(abcx). We develop a novel constructive approach based on Jurkat’s criterion for power series ratios, which avoids limitations of cumbersome recursive/inductive methods in the existing literature.