Exercise 8, § 3

Give an example of a family which is summable but not absolutely summable in the field $\mathbf{Q}_p$ of $p$-adic numbers. [Show that a family $(x_i)_{i \in I}$ of points of $\mathbf{Q}_p$ is summable if and only if $\lim x_i = 0$ with respect to the filter of complements of finite subsets of $I$.]