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How do you define a null set?
What is a null set example?
What is a null set called?
According to Wikipedia André Weil (and maybe others) introduced the symbol 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 or
.
Although not mathematics, I thought I would add that in the APL programming language the empty vector is represented by the symbol ⍬, which is pronounced "zilde".
My professor's slides use both, and I'm confused if they mean different things. In high school, we used the second representation of phi. Assuming the first one is also phi. I think.