The four words in the picture above are: φ, φ,? ,? . Among them, the third is used to represent the diameter. Their Unicode codes are 0X03A6, 0X03C6, 0X03D5 and 0X2205 respectively.

Strictly speaking, the font style of this lowercase letter "φ" (0x03c6) in the Song Dynasty Library of the system is wrong, and it is written as 0X03D5, so the writing in the above picture is correct. "?" For diameter (0X03D5) is an old lowercase writing. Now the lowercase of φ is generally written as "φ" (0x03c6). As for the empty set symbol, it has nothing to do with the other three symbols and cannot be confused.