Could someone explain the meaning of the "subway man" hallucination in the movie "Pi" (1998)?

In the movie Pi (1998), Max has several hallucinations. One hallucination I did not understand was the singing man in the subway. We see the man twice in the movie. We first see the man singing, then disappearing in the subway car. The second time, we see the man reading a newspaper in the subway car. Max asks for the newspaper and soon after the man asks for the paper back. The man also follows Max out of the subway. What does this man symbolize and what significance does he have?

I thought that the man could be real, but when Max comes out of the subway station, Marcey doesn't seem to notice him, so I can only assume that the man is a hallucination. Another part I don't understand is how the stock numbers in the imaginary man's newspaper are accurate. Max goes into a store and he picks up a newspaper that had matching stock numbers.