Let (T, ρ) be a metric† space, e > 0. A subset S ⊂ T is called the e-net for T if, for any t ∈ T, there exists s ⊂ S such that ρ (s, t) ≤ e. In other words, T may be covered by the balls of radius e centered at points of S. Denote by N (T, e) the least possible number of points in an e-net for the set T. Those e-nets which contain exactly N (T, e) points will be called minimal. The quantity H (T, e) ≡ log N (T, e) is called the metric entropy of the space T.