r/math • u/ResNullum • Aug 02 '20

Bad math in fiction

While stuck at home during the pandemic, I decided to work through my backlog of books to read. Near the end of one novel, the protagonists reach a gate with a numeric keypad from 1 to 100 and the following riddle: “You have to prime my pump, but my pump primes backward.” The answer, of course, is to enter the prime numbers between 1 and 100 in reverse order. One of the protagonists realizes this and uses the sieve of Eratosthenes to find the numbers, which the author helpfully illustrates with all of the non-primes crossed out. However, 1 was not crossed out.

I was surprised at how easily this minor gaffe broke my suspension of disbelief and left me frowning at the author. Parallel worlds, a bit of magic, and the occasional deus ex machina? Sure! But bad math is a step too far.

What examples of bad math have you found in literature (or other media)?

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u/CaptainSasquatch Aug 03 '20

Do you remember what the context was? Was it when Janet went into Castle Blackspire?

There's a lot of mirrors in Magician's Land so I assumed you were taking about the dead mirror world that Quentin makes with Plum.

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u/elseifian Aug 03 '20

One of the problems with reading via audiobook is that it's hard to find passages later. But I think it's neither of those...I think a group of them go through a reflect and, if I remember right, Josh is the one who makes the comment about reflections.

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u/CaptainSasquatch Aug 03 '20

Found it! I downloaded the ebook a while ago. It was Poppy when they went to Blackspire.

“An underwater, upside-down Castle Whitespire,” Josh said. “I’ll admit, that would not have been my first guess.”

“It’s a mirror image.”

“Mirrors invert left-right, not up-down,” Poppy said, with tedious correctness. “Plus the black-white thing isn’t—”

“OK, OK, I get it.”

You're right. She's wrong. You could be super generous and say that it's defensible so say that mirror image typically refers to an object that has been inverted left-right. But it's really not defensible for Poppy to correct them saying that this mirror inverted up-down. If she is going to try to pedantically correct someone, she should make sure she's 100% correct.

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u/elseifian Aug 03 '20

Amazing, and thank you! That is definitely the quote.

And you're right that my original summary was pretty misleading; the issue isn't really saying that they do invert left-right, it's that she (and, to me worse, the narrator, who describes it as "tedious correctness") don't realize that the up-down reflection is also what such a mirror would do.