Null sign

Another suggestion is you can also type: Alt + 0216.

Best regards,

Dan

This is sufficiently rare, and handled in sufficiently many different ways, that you should always state explicitly how you're treating it. In my experience, the most common symbols are:

• $\mathsf{null}$, $\mathsf{nil}$, $\mathsf{NaN}$ or similar on the more computer-sciencey side, and

• $\perp$ or $\uparrow$ on the more logicy side.

  • Note that "$\perp$" is also used to denote contradiction, and "$\uparrow$" is also used as a predicate to denote "is undefined" or "doesn't halt" with "$\downarrow$" denoting "is defined"/"does halt."

But again, I'd explicitly state which you're using - although admittedly multiple of these would almost certainly make it obvious from context.

According to Wikipedia André Weil (and maybe others) introduced the symbol $\emptyset$ for the empty set. They were inspired by the letter Ø in the Norwegian and Danish alphabet. So, I guess you could argue that the symbol should be pronounced as it is pronounced in these alphabets.

In my experience most mathematicians just call the symbol "empty set".

Note also, that the Wikipedia article says that the symbol is not to be confused with the Greek letter $\phi$ or $\Phi$.