## the Grumble distribution and an ODE

As ‘Og’s readers may have noticed, I paid some recent visits to Cross Validated (although I find this too addictive to be sustainable on a long term basis!, and as already reported a few years ago frustrating at several levels from questions asked without any preliminary personal effort, to a lack of background material to understand hints towards the answer, to not even considering answers [once the homework due date was past?], &tc.). Anyway, some questions are nonetheless great puzzles, to with this one about the possible transformation of a random variable R with density

into a Gumble distribution. While the better answer is that it translates into a power law,

,

I thought using the S=R² transform could work but obtained a wrong sign in the pseudo-Gumble density

and then went into seeking another transform into a Gumbel rv T, which amounted to solve the differential equation

As I could not solve analytically the ODE, I programmed a simple Runge-Kutta numerical resolution as follows:

solvR=function(prec=10^3,maxz=1){ z=seq(1,maxz,le=prec) t=rep(1,prec) #t(1)=1 for (i in 2:prec) t[i]=t[i-1]+(z[i]-z[i-1])*exp(-z[i-1]+ exp(-z[i-1])+t[i-1]+exp(-t[i-1])) zold=z z=seq(.1/maxz,1,le=prec) t=c(t[-prec],t) for (i in (prec-1):1) t[i]=t[i+1]+(z[i]-z[i+1])*exp(-z[i+1]+ exp(-z[i+1])+t[i+1]+exp(-t[i+1])) return(cbind(c(z[-prec],zold),t)) }

Which shows that [the increasing] t(w) quickly gets too large for the function to be depicted. But this is a fairly useless result in that a transform of the original variable and of its parameter into an arbitrary distribution is always possible, given that W above has a fixed distribution… Hence the pun on Gumble in the title.

December 3, 2014 at 2:49 am

Unless I’m missing something here, the solution to your ODE is

isn’t it? You could solve for t or w by taking two logarithms.

December 3, 2014 at 8:00 am

Thank you, this is indeed a solution to the ODE.