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PROCEEDINGS May 15, 16, 17, 18, 2005 - Casualty Actuarial Society

PROCEEDINGS May 15, 16, 17, 18, 2005 - Casualty Actuarial Society

PROCEEDINGS May 15, 16, 17, 18, 2005 - Casualty Actuarial Society

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102 WHY LARGER RISKS HAVE SMALLER INSURANCE CHARGESFirst, we observe:3.2. Decomposability Equivalence to Closure in Unique SizeModelM is decomposable , M is closed: (3.2)We prove one direction and leave the other as an ex-Proofercise.“ )” Omitted.“(” SinceM is complete, there exist T ¹ 2 M, T ¹12 M, andT ¹22 M. By assumption, M is closed under independent summation.So the independent sum, T ¹1+ T ¹2, is in M. Takingexpectations one has E[T ¹1+ T ¹2]=¹ 1 + ¹ 2 . In a unique sizemodel, we know T ¹1 +¹ 2has the unique distribution in M withE[T ¹1 +¹ 2]=¹ 1 + ¹ 2 .If we assume size differentiability in a decomposable model,we can obtain some results constraining the behavior of the cumulativedistribution and the limited expected value functionwhen these are viewed as functions of risk size.3.3. Inequalities for Risk Size Partials in Decomposable ModelsIf M is a continuously differentiable decomposable risk-sizemodel, then1 ¸ @E[T ¹ ;t]@¹@F T¹@¹ · 0¸ 0(3.3a)(3.3b)@ 2 E[T ¹ ;t]@¹ 2 · 0: (3.3c)

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