## Approximating Large Powers of Stochastic Matrices in Small Space

by Gil Cohen, Dean Doron, and Ori Sberlo

This work reports another step in the research journey towards better derandomization of space-bounded randomized computation, where prior recent steps were recommended by me in postings number 283 and 299. Again, favorite themes such as first obtaining a mild approximation and then improving it (via the Richardson iteration) play a pivotal role.* In addition, the paper provides an exceptionally nice exposition, which seems accessible also to non-experts.

*) I view this as a special case of the ZigZag Method.

Update (by Ted Pyne, 17-Nov-22): This work was recently superseded by a pair of papers, ECCC TR22-149 and ECCC TR22-150, which obtain near-optimal complexity for iterated matrix multiplication (whereas the prior result held for powering a single matrix): Interestingly, although the final result is the same, the ideas used to go beyond the single-matrix case are totally different.

#### The original abstract

We give a deterministic space-efficient algorithm for approximating powers of stochastic matrices. On input a $w \times w$ stochastic matrix $A$, our algorithm approximates $A^{n}$ in space $\widetilde{O}(\log n + \sqrt{\log n}\cdot \log w)$ to within high accuracy. This improves upon the seminal work by Saks and Zhou (FOCS95), that requires $O(\log^{3/2}n + \sqrt{\log n} \cdot \log w)$ space, in the regime $n \gg w$.

Available from ECCC TR22-008.

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