Also of use is the Null set o available from the o key and option or ShiftOption O. Its a process but heres what youll need to do. Take a look at your keyboards numeric keypad right now just to reassure yourself.
Click on the symbol and then on the infinity symbol.
Its a third-party keyboard app that has every common and obscure mathematical symbol you might ever need to enter including Pi. Depending on your keyboard you may need to be sure you are in the Num Lock mode Codes for legal sections and commonly used monetary characters and symbols. You can use this online keyboard in alternation with your physical keyboard for example you can type regular numbers and letters on your keyboard and use the virtual math keyboard to type the mathematical characters. In this list below on the right side of colon is the key you need to press on your iPhones keyboard to see the symbols that are on the left of colon.