Motivation: I’m trying to verify that some disagreement between an rstan and brms analysis is the result the use of a flat prior for a variable in brms
and a flat prior for a transformed version of that variable in my rstan
code.
Issue: for a parameter x_0 I am estimating, I’d like to be able to specify a prior \pi where \pi(x0) \propto 1/(x_0  x_{\max})^2. where x_0 is constrained as x_{\min} < x_0 < x_{\max} and x_{\min} > 0.

Is there an easy way to do this in
brms
? I can’t find any documentation about such improper priors. 
Alternatively, is there a distribution that follows this formula?