<p>Investigating the level sets <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\varvec{L^t}\)</EquationSource> </InlineEquation> of Archimax copulas <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\varvec{C \in \mathcal {C}_{am}}\)</EquationSource> </InlineEquation>, we establish that these sets can be characterized in terms of certain convex functions <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\varvec{f^s}\)</EquationSource> </InlineEquation> and non-decreasing functions <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\varvec{g^t}\)</EquationSource> </InlineEquation>. Motivated by the results in Mai and Scherer (Extremes <i>14</i>, 311-324 <CitationRef CitationID="CR32">2011</CitationRef>) and Trutschnig et al. (Extremes <i>19</i>, 405-427 <CitationRef CitationID="CR43">2016</CitationRef>), which examine the way bivariate Extreme Value copulas distribute their mass, we extend these findings to the larger family of bivariate Archimax copulas <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\varvec{{C}_{am}}\)</EquationSource> </InlineEquation>. Working with Markov kernels (conditional distributions), we analyze the mass distributions of Archimax copulas <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(\varvec{C \in \mathcal {C}_{am}}\)</EquationSource> </InlineEquation> and show that the support of <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(\varvec{C}\)</EquationSource> </InlineEquation> is determined by some functions <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(\varvec{f^0}\)</EquationSource> </InlineEquation>, <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(\varvec{g^L}\)</EquationSource> </InlineEquation>, and <InlineEquation ID="IEq10"> <EquationSource Format="TEX">\(\varvec{g^R}\)</EquationSource> </InlineEquation>. Additionally, we prove that the discrete component (if any) of <InlineEquation ID="IEq11"> <EquationSource Format="TEX">\(\varvec{C}\)</EquationSource> </InlineEquation> concentrates its mass on the graphs of the afore-mentioned functions <InlineEquation ID="IEq12"> <EquationSource Format="TEX">\(\varvec{f^s}\)</EquationSource> </InlineEquation> or <InlineEquation ID="IEq13"> <EquationSource Format="TEX">\(\varvec{g^t}\)</EquationSource> </InlineEquation>. Recognizing the close relationship between the level sets <InlineEquation ID="IEq14"> <EquationSource Format="TEX">\(\varvec{L^t}\)</EquationSource> </InlineEquation> of a copula <InlineEquation ID="IEq15"> <EquationSource Format="TEX">\(\varvec{C}\)</EquationSource> </InlineEquation> and its Kendall distribution function <InlineEquation ID="IEq16"> <EquationSource Format="TEX">\(\varvec{F_C^K}\)</EquationSource> </InlineEquation>, we provide an alternative proof for the representation of <InlineEquation ID="IEq17"> <EquationSource Format="TEX">\(\varvec{F_C^K}\)</EquationSource> </InlineEquation> for arbitrary Archimax copulas <InlineEquation ID="IEq18"> <EquationSource Format="TEX">\(\varvec{C \in \mathcal {C}_{am}}\)</EquationSource> </InlineEquation> and derive simple expressions for the level set masses <InlineEquation ID="IEq19"> <EquationSource Format="TEX">\(\varvec{\mu _C(L^t)}\)</EquationSource> </InlineEquation>. Building upon the fact that Archimax copulas <InlineEquation ID="IEq20"> <EquationSource Format="TEX">\(\varvec{C \in \mathcal {C}_{am}}\)</EquationSource> </InlineEquation> can be represented via two univariate probability measures <InlineEquation ID="IEq21"> <EquationSource Format="TEX">\(\boldsymbol\gamma\)</EquationSource> </InlineEquation> and <InlineEquation ID="IEq22"> <EquationSource Format="TEX">\(\varvec{\vartheta }\)</EquationSource> </InlineEquation> — so-called Williamson and Pickands dependence measures — we show that absolute continuity, discreteness, and singularity properties of these measures <InlineEquation ID="IEq23"> <EquationSource Format="TEX">\(\varvec{\gamma }\)</EquationSource> </InlineEquation> and <InlineEquation ID="IEq24"> <EquationSource Format="TEX">\(\varvec{\vartheta }\)</EquationSource> </InlineEquation> carry over to the corresponding Archimax copula <InlineEquation ID="IEq25"> <EquationSource Format="TEX">\(\varvec{C_{\gamma , \vartheta }}\)</EquationSource> </InlineEquation>. Finally, we derive conditions on <InlineEquation ID="IEq26"> <EquationSource Format="TEX">\(\varvec{\gamma }\)</EquationSource> </InlineEquation> and <InlineEquation ID="IEq27"> <EquationSource Format="TEX">\(\varvec{\vartheta }\)</EquationSource> </InlineEquation> such that the support of the absolutely continuous, discrete, or singular component of <InlineEquation ID="IEq28"> <EquationSource Format="TEX">\(\varvec{C_{\gamma , \vartheta }}\)</EquationSource> </InlineEquation> coincides with the support of <InlineEquation ID="IEq29"> <EquationSource Format="TEX">\(\varvec{C_{\gamma , \vartheta }}\)</EquationSource> </InlineEquation>.</p>

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On bivariate Archimax copulas: Level sets, mass distributions and related results

  • Nicolas Dietrich

摘要

Investigating the level sets \(\varvec{L^t}\) of Archimax copulas \(\varvec{C \in \mathcal {C}_{am}}\) , we establish that these sets can be characterized in terms of certain convex functions \(\varvec{f^s}\) and non-decreasing functions \(\varvec{g^t}\) . Motivated by the results in Mai and Scherer (Extremes 14, 311-324 2011) and Trutschnig et al. (Extremes 19, 405-427 2016), which examine the way bivariate Extreme Value copulas distribute their mass, we extend these findings to the larger family of bivariate Archimax copulas \(\varvec{{C}_{am}}\) . Working with Markov kernels (conditional distributions), we analyze the mass distributions of Archimax copulas \(\varvec{C \in \mathcal {C}_{am}}\) and show that the support of \(\varvec{C}\) is determined by some functions \(\varvec{f^0}\) , \(\varvec{g^L}\) , and \(\varvec{g^R}\) . Additionally, we prove that the discrete component (if any) of \(\varvec{C}\) concentrates its mass on the graphs of the afore-mentioned functions \(\varvec{f^s}\) or \(\varvec{g^t}\) . Recognizing the close relationship between the level sets \(\varvec{L^t}\) of a copula \(\varvec{C}\) and its Kendall distribution function \(\varvec{F_C^K}\) , we provide an alternative proof for the representation of \(\varvec{F_C^K}\) for arbitrary Archimax copulas \(\varvec{C \in \mathcal {C}_{am}}\) and derive simple expressions for the level set masses \(\varvec{\mu _C(L^t)}\) . Building upon the fact that Archimax copulas \(\varvec{C \in \mathcal {C}_{am}}\) can be represented via two univariate probability measures \(\boldsymbol\gamma\) and \(\varvec{\vartheta }\) — so-called Williamson and Pickands dependence measures — we show that absolute continuity, discreteness, and singularity properties of these measures \(\varvec{\gamma }\) and \(\varvec{\vartheta }\) carry over to the corresponding Archimax copula \(\varvec{C_{\gamma , \vartheta }}\) . Finally, we derive conditions on \(\varvec{\gamma }\) and \(\varvec{\vartheta }\) such that the support of the absolutely continuous, discrete, or singular component of \(\varvec{C_{\gamma , \vartheta }}\) coincides with the support of \(\varvec{C_{\gamma , \vartheta }}\) .