<p>This paper investigates the boundedness properties of a convolution operator associated with the fractional Fourier transform (often abbreviated as FrFT) of order <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\alpha \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>α</mi> </math></EquationSource> </InlineEquation>. The operator under study incorporates chirp weight functions <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\gamma _{1,2}(x)=e^{\pm i(x-a(\alpha )x^{2})}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>γ</mi> <mrow> <mn>1</mn> <mo>,</mo> <mn>2</mn> </mrow> </msub> <mrow> <mo stretchy="false">(</mo> <mi>x</mi> <mo stretchy="false">)</mo> </mrow> <mo>=</mo> <msup> <mi>e</mi> <mrow> <mo>±</mo> <mi>i</mi> <mo stretchy="false">(</mo> <mi>x</mi> <mo>-</mo> <mi>a</mi> <mrow> <mo stretchy="false">(</mo> <mi>α</mi> <mo stretchy="false">)</mo> </mrow> <msup> <mi>x</mi> <mn>2</mn> </msup> <mo stretchy="false">)</mo> </mrow> </msup> </mrow> </math></EquationSource> </InlineEquation> and serves as a natural generalization of classical convolution structures within the fractional domain, which was first introduced in <i>Wirel. Pers. Commun.</i> <b>92</b>, 623–637, (2017). First, we establish a Young-type inequality for this operator, proving that it maps <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(L_{p}(\mathbb {R})\times L_{q}(\mathbb {R})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mi>p</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">R</mi> <mo stretchy="false">)</mo> </mrow> <mo>×</mo> <msub> <mi>L</mi> <mi>q</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">R</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> into <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(L_{r}(\mathbb {R})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <mi>r</mi> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">R</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> for <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(p,q,r&gt;1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>p</mi> <mo>,</mo> <mi>q</mi> <mo>,</mo> <mi>r</mi> <mo>&gt;</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> satisfying <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(1/p+1/q=1+1/r\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>1</mn> <mo stretchy="false">/</mo> <mi>p</mi> <mo>+</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mi>q</mi> <mo>=</mo> <mn>1</mn> <mo>+</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mi>r</mi> </mrow> </math></EquationSource> </InlineEquation>, with an explicit constant depending solely on <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\alpha \)</EquationSource> <EquationSource Format="MATHML"><math> <mi>α</mi> </math></EquationSource> </InlineEquation>. Second, we prove a Hausdorff–Young type inequality which guarantees boundedness into the conjugate space <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(L_{s_{1}}(\mathbb {R})\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>L</mi> <msub> <mi>s</mi> <mn>1</mn> </msub> </msub> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="double-struck">R</mi> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation> whenever <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(1\le p,q,s\le 2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>1</mn> <mo>≤</mo> <mi>p</mi> <mo>,</mo> <mi>q</mi> <mo>,</mo> <mi>s</mi> <mo>≤</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(1/p_1 +1/q_1=1/s\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mn>1</mn> <mo stretchy="false">/</mo> <msub> <mi>p</mi> <mn>1</mn> </msub> <mo>+</mo> <mn>1</mn> <mo stretchy="false">/</mo> <msub> <mi>q</mi> <mn>1</mn> </msub> <mo>=</mo> <mn>1</mn> <mo stretchy="false">/</mo> <mi>s</mi> </mrow> </math></EquationSource> </InlineEquation>, thereby extending the range of exponents to the dual setting. Third, we derive a Saitoh-type weighted inequality valid for all <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(p&gt;1\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>p</mi> <mo>&gt;</mo> <mn>1</mn> </mrow> </math></EquationSource> </InlineEquation> (including <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(p=2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>p</mi> <mo>=</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation>), providing <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(L_{p}\)</EquationSource> <EquationSource Format="MATHML"><math> <msub> <mi>L</mi> <mi>p</mi> </msub> </math></EquationSource> </InlineEquation>-boundedness with respect to suitable weight functions. The proofs rely on the Riesz-Thorin interpolation theorem, Hölder’s inequality, and Plancherel-type identities for the FrFT. These results unify and substantially extend classical convolution estimates to the fractional Fourier framework.</p>

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Young type inequality and Hausdorff–Young type inequality for convolution operator associated with fractional Fourier transform of \(\alpha \)-order

  • Trinh Tuan,
  • Lai Tien Minh

摘要

This paper investigates the boundedness properties of a convolution operator associated with the fractional Fourier transform (often abbreviated as FrFT) of order \(\alpha \) α . The operator under study incorporates chirp weight functions \(\gamma _{1,2}(x)=e^{\pm i(x-a(\alpha )x^{2})}\) γ 1 , 2 ( x ) = e ± i ( x - a ( α ) x 2 ) and serves as a natural generalization of classical convolution structures within the fractional domain, which was first introduced in Wirel. Pers. Commun. 92, 623–637, (2017). First, we establish a Young-type inequality for this operator, proving that it maps \(L_{p}(\mathbb {R})\times L_{q}(\mathbb {R})\) L p ( R ) × L q ( R ) into \(L_{r}(\mathbb {R})\) L r ( R ) for \(p,q,r>1\) p , q , r > 1 satisfying \(1/p+1/q=1+1/r\) 1 / p + 1 / q = 1 + 1 / r , with an explicit constant depending solely on \(\alpha \) α . Second, we prove a Hausdorff–Young type inequality which guarantees boundedness into the conjugate space \(L_{s_{1}}(\mathbb {R})\) L s 1 ( R ) whenever \(1\le p,q,s\le 2\) 1 p , q , s 2 and \(1/p_1 +1/q_1=1/s\) 1 / p 1 + 1 / q 1 = 1 / s , thereby extending the range of exponents to the dual setting. Third, we derive a Saitoh-type weighted inequality valid for all \(p>1\) p > 1 (including \(p=2\) p = 2 ), providing \(L_{p}\) L p -boundedness with respect to suitable weight functions. The proofs rely on the Riesz-Thorin interpolation theorem, Hölder’s inequality, and Plancherel-type identities for the FrFT. These results unify and substantially extend classical convolution estimates to the fractional Fourier framework.