# Products of random stochastic matrices

As I mentioned, Behrouz Touri gave a really great presentation at ITA on some of his work with Angelia Nedić on products of stochastic matrices, some of which was in this paper on ArXiV. The setup of the paper is relatively straightforward — we have a sequence of independent random matrices $\{W(k)\}$, each of which is row-stochastic almost surely, and we want to know when the product $\lim_{k \to \infty} W(k) W(k-1) \cdots W(t_0)$ converges almost surely. The main result is that if the chain is balanced and strongly aperiodic, then the limit is a random stochastic matrix such that the rows in the same connected component of the infinite flow graph are equal.

Recall that for a chain $W$ a lazy version of the chain is $\alpha W + (1 - \alpha) I$. Laziness helps avoid periodicity by letting the chain be “stuck” with probability $(1 - \alpha)$. Strongly aperiodic means that there is a $\gamma \in (0,1]$ such that $\mathbb{E}[ W_{ii}(k) W_{ij}(k) ] \ge \gamma \mathbb{E}[ W_{ij}(k) ]$. Basically this is a sort of “expected laziness condition” which says that there is enough self-transition probability to avoid some sort of weak periodicity in the chain.

Consider a cut of the chain into a set of states $S$ and a set $\bar{S}$. A chain is balanced if there is an $\alpha > 0$ such that for all cuts $(S,\bar{S})$, we have $\mathbb{E}[ W_{S \bar{S}}(k) ] \ge \alpha \mathbb{E}[ W_{\bar{S} S}(k) ]$. So this is saying that at each time, the flow out of $S$ is commensurate with the flow into $S$.

The infinite flow graph has the same vertex set as the original chain, but with edge $(i,j)$ existing only if $\sum_{k} W_{ij}(k) + W_{ji}(k) = \infty$. That is, the edge has to exist infinitely often in the graph.

So to add it all up, the well-behaved independent products of stochastic matrices that converge are those which don’t behave badly over time — for each time $k$, they don’t shift around the mass too much and they are not too periodic. Basically, they don’t become volatile and cyclical, which sounds perfectly reasonable. So how does he prove it?

The first part is to connect the problem with a dynamic system whose state $x(k+1) = W(k+1) x(k)$ and look to see if the state converges in the limit. The next part uses a connection to absolute probability processes, which were used by Kolmogorov in 1936. A random vector process $\pi(k)$ is an absolute probability process for $\{W(k)\}$ if it is adapted to the same filtration as the chain, it’s stochastic almost surely, and

$\mathbb{E}[ \pi(k+1) W(k+1) | \mathcal{F}_k ] = \pi(k)$

Kolmogorov showed that for a deterministic sequence of stochastic matrices there is always an deterministic absolute probability process, so for independent chains we can always find a random absolute probability process. Using this, Touri and Nedić define a class of comparison functions which are super-martingales for each $\latex x(0)$ and have a limit. By choosing a particular comparison function they can get a version of the main result. It’s a nicely written paper and worth a skim if you’re interested in these things (as I am).