## Thinking Mathematically (6th Edition)

false $\emptyset \subseteq \{\emptyset, \{\emptyset\}\}$
$\emptyset \not\subseteq \{\emptyset, \{\emptyset\}\}$ This is incorrect, for it should say: $\emptyset \subseteq \{\emptyset, \{\emptyset\}\}$ By definition, an empty set is a subset of every set.