“André Weil, in fact, invented for this book the universal notation we use today for the empty set: ∅.

This symbol comes from the Norwegian alphabet, which Weil had encountered in his travels. The Bourbaki text on set theory thus introduced this new mathematical symbol.”

(Amir Aczel, The Artist and the Mathematician: the story of Nicolas Bourbaki, the genius mathematician who never existed, p. 93.)