Hmm, never thought of ∅ as a φ. (According to Wikipedia, the symbol was introduced in the 1930s by Weil, inspired by the letter Ø in the Danish and Norwegian al

Hmm, never thought of ∅ as a φ. (According to Wikipedia, the symbol was introduced in the 1930s by Weil, inspired by the letter Ø in the Danish and Norwegian alphabets.)