Monday, 1 January 2018

integration - The partial expectation mathbbE(X;X>K) for an alpha-stable distributed random variable

The partial expectation E(X;X>K) for an alpha-stable distributed random variable:




By playing with convolutions of Characteristic Functions of alpha-Stable distributions S(α,β,μ,σ) and a payoff K, assuming μ=0 and deriving under the integral sign, found the partial expectation E(X;X>K) , i.e., where F(x) is the distribution function of X, KxdF(x) (which is not to be confused with the conditional expectation).



I am ending up with a difficult integral (easy to evaluate numerically but hard to get explicitly). With 1<α2:



ψ(α,β,σ,K)=12πασα|u|α2(1+iβtan(πα2)sgn(u))exp(|uσ|α(1iβtan(πα2)sgn(u))+iKu)du



The solutions is easy for K=0, so
ψ(α,β,σ,0)=σΓ(1α)((1+iβtan(πα2))1/α+(1iβtan(πα2))1/α)πα.
Also, there is a well known solution for symmetric cases in Zolotarev's book. But it is the K0 that is critical.

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