TITLE: Name for Theorem 3.27 from baby Rudin?
QUESTION [8 upvotes]: Rudin rarely gives names to the theorems in this book.  Theorem 3.27 states if
$\{a_n\}$ is a monotonically decreasing sequence of  positive reals, then
$$\sum_{n=1}^\infty a_n\,\text{ converges}\iff\sum_{k=0}^\infty 2^ka_{2^k} \text{ converges}.$$

REPLY [8 votes]: This is called the Cauchy condensation test. See here.