# Finite Calls to Stability | 31 Oct. 2019 |

Consider an unknown dynamical system parametrized by . In this short note we look at a relatively simple, yet neat method to assess stability of via finite calls to a particular oracle. Here, we will follow the notes by Roman Vershynin (RV2011).

Assume we have no knowledge of , but we are able to obtain for any desired . It turns out that we can use this oracle to bound from above.

First, recall that since is symmetric, . Now, the common definition of is . The constraint set is not per se convenient from a computational point of view. Instead, we can look at a compact constraint set: , where is the sphere in . The latter formulation has an advantage, we can discretize these compact constraints sets and tame functions over the individual chunks.

To do this formally, we can use -nets, which are convenient in quantitative analysis on compact metric spaces . Let be a -net for if . Hence, we measure the amount of balls to cover .

For example, take as our metric space where we want to bound (the cardinality, i.e., the amount of balls). Then, to construct a minimal set of balls, consider an -net of balls , with . Now the insight is simply that . Luckily, we understand the volume of -dimensional Euclidean balls well such that we can establish Which is indeed sharper than the bound from Lemma 5.2 in (RV2011).

Next, we recite Lemma 5.4 from (RV2011). Given an -net for with we have This can be shown using the Cauchy-Schwartz inequality. Now, it would be interesting to see how sharp this bound is. To that end, we do a -dimensional example ( ). Note, here we can easily find an optimal grid using polar coordinates. Let the unknown system be parametrized by such that . In the figures below we compute the upper-bound to for several nets . Indeed, for finer nets, we see the upper-bound approaching . However, we see that the convergence is slow while being inversely related to the exponential growth in . Overall, we see that after about a 100 calls to our oracle we can be sure about being exponentially stable. Here we just highlighted an example from (RV2011), at a later moment we will discuss its use in probability theory.