# is Pareto distribution exponential dispersion family and the form is unique?

I am trying to show that Pareto distribution with $$f(y;\alpha)=\alpha y^{-\alpha-1}$$ is exponential dispersion (ED) family which means that it can be rewritten as: $$f(y;\theta,\phi)={\rm exp}\{\frac {y\theta-b(\theta)} {a(\phi)} + c(y;\phi)\}$$ with $E[Y]=b^{\prime}(\theta)$ and $Var[Y]=b^{\prime\prime}(\theta)a(\phi)$.

What I found is there are multiple ways to rewrite the PDF in the exponential dispersion family form, but since I know that $$E[Y]=\frac {\alpha} {\alpha-1}$$ and $$Var[Y]=\frac {\alpha} {(\alpha-1)^2(\alpha-2)},$$ I can solve the differential equations to get $$b(\theta)=\alpha + {\rm log}(\alpha-1)$$ and $$a(\phi)=\frac {-\alpha} {\alpha-2}.$$ This suggests that the form of $f(y;\theta,\phi)$ is unique.

So, is the ED family form of the PDF for Pareto distribution unique? If so, what is the correct form?

• Your question is confusing as (a) you use three different notations for the parameter, ie $\alpha,\theta,\phi$ and (b) you use the same notation for $y$ in the Pareto and $y$ in the exponential family. Since $f(y;\alpha)=\exp\{\log(y)(-\alpha-1)+\log(\alpha)\}$, you get the natural representation. Note also that you are missing the indicator function in the Pareto density. – Xi'an Nov 10 '16 at 9:05
• Thanks! (a) I forgot to mention that this is a reparameterization, we can find $\theta$ and $\phi$ in terms of $\alpha$ (b) yes, this is the natural form for exponential family, but not in the form of ED as I illustrated above. Yes, I skip the indicator function $I(Y>1)$. – Sheldon Nov 10 '16 at 15:15
• You still didn't fix the missing indicator in the density. We need to know the range! – kjetil b halvorsen Oct 18 '17 at 16:14
• Have you found the solution? Because, me either, I'm interested in showing that the GPD comes from EDM and I can't find a solution – Maria Cristina Apr 1 '19 at 21:05