josemiotto / pylevy

Levy distributions for Python
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Parameterizations 0 and 1 use notation from unknown source #13

Closed ragibson closed 4 years ago

ragibson commented 4 years ago

Where does the notation for your first two parameterizations come from?

https://github.com/josemiotto/pylevy/blob/64c525f273d00d89cbbe531a6557b17b74d18f88/levy/__init__.py#L61-L63

The documentation says this is "in the notation of Nolan", but this appears to be incorrect. He seems to use "alpha, beta, gamma, delta" notation: S(α,β,γ,δ;0) and S(α,β,γ,δ;1).

I've also seen some parameterizations c and mu in place of gamma and delta, respectively. Notably, these notation conventions seem to have the distribution scale come before the location, whereas pylevy does the opposite.

josemiotto commented 4 years ago

Nolan calls it gamma and delta and I call them my and sigma, without loss. Where have you seen the other parameterizations?

ragibson commented 4 years ago

Voit's "The Statistical Mechanics of Financial Markets" uses

levy_notation

and e.g. Wikipedia introduces stable distributions with

stable_dist

but I'm not actually sure where this comes from originally (the sources here don't appear to use this notation).

josemiotto commented 4 years ago

I see, thanks for the Volt reference. I follow Nolan parametrization since this is the most quoted thing on Google Scholar. Personally, I'm not happy with that, because it's from a chapter of an unpublished book. I'd rather have Zolotarev as a reference, but I would agree in that it's a bit obscure to read.

I'm not sure about the following statement, but I think that because this distribution is stable, the order of location and scale may be irrelevant, like in the Normal distribution.