Chapter R - Review of Basic Concepts - R.1 Sets - R.1 Exercises - Page 8: 95

$N \cup \emptyset = N$

RECALL: (1) $\emptyset$ is the empty set (2) The symbol $\cup$ represents union of sets which lists down in a set the combined elements of the given sets. Since the empty set has no elements., then its union with any other set will be the other set itself. Thus, $N \cup \emptyset = N$