Hi, i’m fairly new to stan and need some help constructing my prior.

I’ve created a model for an enzymatic reaction E+S <-> ES -> P+ E which has reaction rates kf,kr,kcat ( My parameters which i have declared in the parameter block).

Most of the stan code is similar to the lotka volterra example (http://mc-stan.org/users/documentation/case-studies/lotka-volterra-predator-prey.html).

For the prior i want to code the joint distribution p_{kf,kr,kcat} (x,y,z) = p_kf (x)*p_km ((y+z)/x)*p_kcat (z) .

(N.B. Km = (kr+kcat)/kf is the Michaelis constant ). x,y,z are the random variables

Each of p_kf , p_km , p_kcat follow a known distribution (in stan) e.g. lognormal so p_kf (x) ~ lognormal(mu,sigma)

How do i code this? Is this possibly? I assume you need to use a user defined function but i couldn’t find any mention for multiple variables.

I would appreciate any help and let me know if it isn’t clear enough. I haven’t included any code because i don’t see how it would help but let me know if it would.