<p>We investigate <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(L^1\rightarrow L^\infty \)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mi>L</mi> <mn>1</mn> </msup> <mo stretchy="false">→</mo> <msup> <mi>L</mi> <mi>∞</mi> </msup> </mrow> </math></EquationSource> </InlineEquation> dispersive estimates for the Dirac equation with a potential in four spatial dimensions. We classify the structure of the obstructions at the thresholds as being composed of an at most two dimensional space of resonances per threshold, and finitely many eigenfunctions. Similar to the Schrödinger evolution, we prove the natural <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(t^{-2}\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mi>t</mi> <mrow> <mo>-</mo> <mn>2</mn> </mrow> </msup> </math></EquationSource> </InlineEquation> decay rate when the thresholds are regular. When there is a threshold resonance or eigenvalue, we show that there is a time dependent, finite rank operator satisfying <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\Vert F_t\Vert _{L^1\rightarrow L^\infty }\lesssim (\log t)^{-1}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mrow> <mo stretchy="false">‖</mo> </mrow> <msub> <mi>F</mi> <mi>t</mi> </msub> <msub> <mrow> <mo stretchy="false">‖</mo> </mrow> <mrow> <msup> <mi>L</mi> <mn>1</mn> </msup> <mo stretchy="false">→</mo> <msup> <mi>L</mi> <mi>∞</mi> </msup> </mrow> </msub> <mo>≲</mo> <msup> <mrow> <mo stretchy="false">(</mo> <mo>log</mo> <mi>t</mi> <mo stretchy="false">)</mo> </mrow> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </mrow> </math></EquationSource> </InlineEquation> for <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(t&gt;2\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>t</mi> <mo>&gt;</mo> <mn>2</mn> </mrow> </math></EquationSource> </InlineEquation> such that <Equation ID="Equ81"> <EquationSource Format="TEX">\( \Vert e^{it\mathcal H}P(\mathcal H)-F_t\Vert _{L^1\rightarrow L^\infty }\lesssim t^{-1} \quad \text {for } t&gt;2, \)</EquationSource> <EquationSource Format="MATHML"><math display="block"> <mrow> <mrow> <mo stretchy="false">‖</mo> </mrow> <msup> <mi>e</mi> <mrow> <mi>i</mi> <mi>t</mi> <mi mathvariant="script">H</mi> </mrow> </msup> <mi>P</mi> <mrow> <mo stretchy="false">(</mo> <mi mathvariant="script">H</mi> <mo stretchy="false">)</mo> </mrow> <mo>-</mo> <msub> <mi>F</mi> <mi>t</mi> </msub> <msub> <mrow> <mo stretchy="false">‖</mo> </mrow> <mrow> <msup> <mi>L</mi> <mn>1</mn> </msup> <mo stretchy="false">→</mo> <msup> <mi>L</mi> <mi>∞</mi> </msup> </mrow> </msub> <mo>≲</mo> <msup> <mi>t</mi> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> <mspace width="1em" /> <mtext>for</mtext> <mspace width="0.333333em" /> <mi>t</mi> <mo>&gt;</mo> <mn>2</mn> <mo>,</mo> </mrow> </math></EquationSource> </Equation>with <i>P</i> a projection onto a subspace of the absolutely continuous spectrum in a small neighborhood of the thresholds. We further show that the operator <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(F_t=0\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msub> <mi>F</mi> <mi>t</mi> </msub> <mo>=</mo> <mn>0</mn> </mrow> </math></EquationSource> </InlineEquation> if there is a threshold eigenvalue but no threshold resonance. We pair this with high energy bounds for the evolution and provide a complete description of the dispersive bounds.</p>

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Dispersive estimates for Dirac Operators in dimension four with obstructions at threshold energies

  • William R. Green,
  • Connor Lane,
  • Benjamin Lyons

摘要

We investigate \(L^1\rightarrow L^\infty \) L 1 L dispersive estimates for the Dirac equation with a potential in four spatial dimensions. We classify the structure of the obstructions at the thresholds as being composed of an at most two dimensional space of resonances per threshold, and finitely many eigenfunctions. Similar to the Schrödinger evolution, we prove the natural \(t^{-2}\) t - 2 decay rate when the thresholds are regular. When there is a threshold resonance or eigenvalue, we show that there is a time dependent, finite rank operator satisfying \(\Vert F_t\Vert _{L^1\rightarrow L^\infty }\lesssim (\log t)^{-1}\) F t L 1 L ( log t ) - 1 for \(t>2\) t > 2 such that \( \Vert e^{it\mathcal H}P(\mathcal H)-F_t\Vert _{L^1\rightarrow L^\infty }\lesssim t^{-1} \quad \text {for } t>2, \) e i t H P ( H ) - F t L 1 L t - 1 for t > 2 , with P a projection onto a subspace of the absolutely continuous spectrum in a small neighborhood of the thresholds. We further show that the operator \(F_t=0\) F t = 0 if there is a threshold eigenvalue but no threshold resonance. We pair this with high energy bounds for the evolution and provide a complete description of the dispersive bounds.