摘要：Example 1. Let $E=\{a_n\}_{n\ge 1}\subset R$ satisfying the conditon : there exist $C>0$ and $0<\alpha<1$ such that $$C^{-1}\alpha^n\le a_n\le C\alpha 阅读全文

摘要：Let $E$ be a subset of a metric sapce $(X, d)$ and $r>0.$ The set $E$ is called $r$-discrete if $d(x,y)\ge r$ whenever $x,y\in E, x\not=y.$ Let $N_r(B 阅读全文