# All Odd Numbers Are Prime — A Corollary

[Rescued from my old blog.]

Once again, Rudbeckia Hirta brings us some funny-but-sad mathematics. The test question was:

Without factoring it, explain how the number
N = (1 x 2 x 3 x 4 x 5 x 6 x 7 x 8 x 9 x 10 x 11) + 1
can be used to argue that there is a prime number larger than 11.

Take a few minutes to think about how you would answer that. Okay, if you haven’t seen a similar proof before, you are allowed to take as many minutes as you need! 😉 Euclid’s famous proof of a closely-related theorem (The Infinitude of Primes) is featured in one of my favorite math-for-the-layman books, Journey through Genius: The Great Theorems of Mathematics.

Then click over to Ms. Hirta’s blog (Learning Curves) to read the answer given by a college honors student. In the first comment post, oxeador explains the student’s reasoning.

And for those of you who haven’t heard the joke before, here is an extended list of reasons why all odd numbers are prime.

## 7 thoughts on “All Odd Numbers Are Prime — A Corollary”

1. I never commented on this one?

Comment: “But 2 is prime, and 2 is even”
Retort: “That’s odd!”

2. I saw that joke when you posted it on your blog, just in time to use it when my eighth-graders reviewed prime numbers last semester. Worked perfectly!

3. I did, didn’t I? It’s hard to pass up an opportunity for a good bad joke!

4. siwogedi johannes says:

2 is a even number simply because it is divisible by 2 and for a number to be odd, it must not be divided by 2 and is a prime nunber at the same. so not all prime numbers are odd and not all odd numbers are prime.

5. ROTFLOL Excellent!

I’d reconsider the “Linguist” one though and make it consistent with the “Computational Linguist” one:

“3 is an odd prime, 5 is an odd prime, 7 is an odd prime, 9 is an irregular odd prime, …”

This site uses Akismet to reduce spam. Learn how your comment data is processed.