## ratio-of-uniforms [#3]

**B**eing still puzzled (!) by the ratio-of-uniform approach, mostly failing to catch its relevance for either standard distributions in a era when computing a cosine or an exponential is negligible, or non-standard distributions for which computing bounds and boundaries is out-of-reach, I kept searching for solutions that would include unbounded densities and still produce compact boxes, as this seems essential for accept-reject simulation if not for slice sampling. And after exploring some dead-ends (in tune with running in Venezia!), I came upon the case of the generalised logistic transform

which ensures that the [ratio-of-almost-uniform] set I defined in my slides last week

is bounded in u. Since the transform g is the derivative of the inverse of h (!),

the parametrisation of the boundary of * H* is

which means it remains bounded if (a) a≤1 [to ensure boundedness at infinity] and (b) the limit of v(x) at zero [where I assume the asymptote stands] is bounded. Meaning

Working a wee bit more on the problem led me to realise that resorting to an arbitrary cdf Φ instead of the logistic cdf could solve the problem for most distributions, including all Gammas! Indeed, the boundary of * H* is now

which means it remains bounded if φ has very heavy tails, like 1/x². To handle the explosion when x=0. And an asymptote itself at zero, to handle the limit at infinity when f(x) goes to zero.

November 4, 2016 at 11:31 am

Unreadable formulas (maybe related to the age of my eyes)

November 4, 2016 at 10:00 pm

Sorry, they look fine on my computer, but I presume mileage may vary depending on the device!