Maybe he "just" got it wrong. Maybe they're typos, and the manuscript was correct. Or...
Maybe Pemulis gave Hal an obviously wrong derivative, and when uncorrected, drove Pemulis to abruptly end the tutoring. Maybe Pemulis said it right but Hal heard it wrong. Or...
Maybe it's "just" another sign they're in an alternate universe where even the math is different. That's pretty much how I feel about it
Maybe Pemulis gave Hal an obviously wrong derivative, and when uncorrected, drove Pemulis to abruptly end the tutoring. Maybe Pemulis said it right but Hal heard it wrong. Or...
Maybe it's "just" another sign they're in an alternate universe where even the math is different. That's pretty much how I feel about it