Definition:Empty Set/Notation

Notation for the Empty Set
The symbols $\O$ and $\emptyset$ used for the empty set are properly considered as stylings of $0$ (zero), and not variants of the Greek Phi: $\Phi, \phi, \varphi$.

Some sources maintain that it is a variant on the Norwegian / Danish / Faeroese letter Ø.

Some sources use $\Box$ as the symbol for the empty set, but this is rare.

Other sources use $0$, but this is not recommended for readily apparent reasons.

The preferred symbol on is $\varnothing$ for its completely unambiguous interpretation and aesthetically pleasing, clean presentation.