Ch 12 typo - modulo 21 (should be 31)

[gregbyrd]

In the first paragraph of the "Computing the Modulus" section of Chapter 12, we get the modulus for free when we are factoring 15, because it's 2^4-1. In the second paragraph, "this trick also works when we are factoring 21..." This should be 31 (2^5-1).

[gregbyrd]

Just realized that 31 is prime, so that's not what you meant. But 64 = 1 mod 21. I get it. Never mind.