<p>The notion of mild solutions for autonomous linear retarded functional differential equations (RFDEs) has been introduced in [J. Nishiguchi, Electron. J. Qual. Theory Differ. Equ. <b>2023</b>, No.&#xa0;32, 1–77] for the purpose of defining fundamental matrix solutions and obtaining a variation of constants formula for the RFDEs. This notion gives a straightforward definition of solutions to the RFDEs under discontinuous history functions compared with previous studies in the literature. For a given autonomous linear RFDE, it holds that the fundamental matrix solutions are locally Lipschitz continuous on the interval <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\([0, \infty )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">[</mo> <mn>0</mn> <mo>,</mo> <mi>∞</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation>. However, it is not apparent whether a similar property is true for the mild solutions. Here we obtain a result which shows the regularity of mild solutions on <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\([0, \infty )\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">[</mo> <mn>0</mn> <mo>,</mo> <mi>∞</mi> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> for autonomous linear RFDEs. The result makes clear a connection between the mild solutions and solution concepts in previous studies.</p>

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On regularity of mild solutions for autonomous linear retarded functional differential equations

  • Junya Nishiguchi

摘要

The notion of mild solutions for autonomous linear retarded functional differential equations (RFDEs) has been introduced in [J. Nishiguchi, Electron. J. Qual. Theory Differ. Equ. 2023, No. 32, 1–77] for the purpose of defining fundamental matrix solutions and obtaining a variation of constants formula for the RFDEs. This notion gives a straightforward definition of solutions to the RFDEs under discontinuous history functions compared with previous studies in the literature. For a given autonomous linear RFDE, it holds that the fundamental matrix solutions are locally Lipschitz continuous on the interval \([0, \infty )\) [ 0 , ) . However, it is not apparent whether a similar property is true for the mild solutions. Here we obtain a result which shows the regularity of mild solutions on \([0, \infty )\) [ 0 , ) for autonomous linear RFDEs. The result makes clear a connection between the mild solutions and solution concepts in previous studies.