Human: I have a question about the history of mathematical notation. How did the <unk> symbol come to be used for the square root of -1? Assistant: 
The symbol <unk> was originally used for the square root of -1. It was later used for the square root of 2 as well. The <unk> symbol was originally used to represent the square root of -1 because it was the only symbol that could be used to represent the square root of -1.