Empty set

From CS2800 wiki

The empty set (written [math]\href{/cs2800/wiki/index.php/%E2%88%85}{∅} [/math] or [math]\href{/cs2800/wiki/index.php/Enumerated_set}{\{\}} [/math]) is a set; [math]x \href{/cs2800/wiki/index.php/%5Cnotin}{\notin} \href{/cs2800/wiki/index.php/%E2%88%85}{∅} [/math] for all [math]x [/math].