# Approximating a probability distribution by a mixture

Let us consider a probability distribution $(g_n)_{n \in \mathbb{N}}$ which we want to approximate by a mixture of $(f_n(\lambda))_{n \in \mathbb{N}}$ where $\lambda \in \mathbb{R}$ is a parameter.

Are there known techniques that allow one to find the mixture minimizing the $L^1$ norm: $$\min_{p} \sum_{n=0}^{\infty} \left|g_n - \int \rm{d} \lambda \; p(\lambda) f_n(\lambda) \right|$$ where $p(\lambda)$ is a normalized probability distribution?

The motivation of this problem is linked to experimental physics: ideally one would like to generate an experimental process characterized by the probability distribution $g$ but this is really not practical. What is really easy, however, is to generate an experimental process with the distribution $f(\lambda)$ where $\lambda$ is a tunable parameter. Therefore, the goal is to approximate $g$ as closely as possible with such a mixture of $f(\lambda)$, where the distance between the two distribution is computed with the $L^1$ norm, that is, I want to minimize the variation distance between the two distributions.

In the specific problem I consider, $f(\lambda)$ is a Poisson distribution with parameter $\lambda \geq 0$, but I really am interested in a general method to approach this problem-

Any pointer to the relevant literature would be greatly appreciated. Thanks a lot!

-
Can you motivate a bit more your problem (why do you need that in two lines, is it some sort of least favorable prior for simultaneous testing)? it is probability over $\mathbb{R}$ ? The norm you use in your sum is the $L^1$ norm between distribution right ? note that $p$ should have integral = 1. – robin girard Sep 1 '10 at 11:02
I edited my question according to your remarks. – Anthony Leverrier Sep 1 '10 at 12:08
Maybe it's better to bound $L^1$ distance by $L^2$, then Fourier analytic techniques can be used. That is also the strategy used in length minimization via energy minimization, well known to differential geometers. – John Jiang Sep 1 '10 at 20:58
Unfortunately, it is really the $L^1$ distance which is relevant in my problem so I cannot switch from the $L^1$ to the $L^2$ distance. Furthermore, as the distributions are defined over $\mathbb{N}$, I cannot see how a bound on the $L^2$ distance could give any information concerning the $L^1$ distance? – Anthony Leverrier Sep 2 '10 at 16:05
The iteration algorithm called Expectation-maximization is often suitable for approximations with mixitures, though it might not actually converge to the minimum you asked for. – Zsbán Ambrus Sep 16 '10 at 14:39

perhaps for starters you could take $p$ to be supported on a finite number of points. then the constraints on $p$ become simple linear inequalities and you have a [convex - perhaps even linear] programming problem.
$$\sum_{i=1}^n |x_i - a|$$
is the sample median - rather than the sample mean one gets for $a$ using least squares.]
you could see if references in the monograph by yadolah dodge [L$_1$ statistical procedures and related topics, ims lecture notes - monograph series vol 31 1997] give anything useful.
Thanks for the reference! Yes, I indeed started by taking $p$ supported on a finite number of points, and it works pretty well in practice. Still, it's a bit frustrating as a continuous density would be much more natural. – Anthony Leverrier Sep 16 '10 at 16:12