So, where are we? We assumed that $s_{n-2}$ is divisible by $M_n$, but $M_n$ is not prime. We picked a divisor $q$ of $M_n$ and used it to define a group $X^*$, and yesterday we showed that $\omega$ has order $2^n$ in $X^*$. Today we’ll use this to derive a contradiction.

Recall that we picked $q$ so that $q^2 \leq M_n$—we can always pick a divisor of $M_n$ that is less than (or equal to) the square root of $M_n$. We then defined $X$ in terms of $q$ as $X = \{ a + b\sqrt 3 \mid a, b \in \mathbb{Z}; 0 \leq a, b < q \}$.

So how many elements are in the set $X$? That’s not too hard: there are $q$ choices for the coefficient $a$, and $q$ choices for the coefficient $b$; each pair of choices gives a different element of $X$, which therefore contains $q^2$ elements.

So what about the order of $X^*$? We got $X^*$ by throwing away elements from $X$ without an inverse. At least we know that $0$ doesn’t have an inverse. There might be more, depending on $q$, but at least we can say that $|X^*| \leq q^2 - 1$. $\omega$ is in the group $X^*$, and we showed that the order of an element cannot exceed the order of the group. But, check this out: $|X^*| \leq q^2 - 1 \leq M_n - 1 < M_n + 1 = 2^n = |\omega|$

The order of the group $|X^*|$ is less than the order of $|\omega|$! This is a contradiction. So, our assumption that $M_n$ has a nontrivial divisor $q$ must be wrong— $M_n$ is prime!

It took 23 posts, but we have finally proved one direction of the Lucas-Lehmer test: if computing $s_{n-2}$ yields something divisible by $M_n$, then $M_n$ is definitely prime.

And now I have to decide what to do with the remaining week. Of course, there is another direction to prove: we have only shown that the Lucas-Lehmer test correctly identifies primes (if $s_{n-2}$ is divisible by $M_n$, then $M_n$ is prime), but it is possible to also prove the converse, that the Lucas-Lehmer test identifies all the Mersenne primes (if $M_n$ is prime, then $s_{n-2}$ will be divisible by $M_n$). I am still trying to figure out how difficult this proof is. I don’t think we’ll be able to fit it in the last 7 days of the month, but it still might be worth starting it and finishing it on a more relaxed schedule.

Of course, I’m also open to questions, suggestions, etc.! Associate Professor of Computer Science at Hendrix College. Functional programmer, mathematician, teacher, pianist, follower of Jesus.
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### 5 Responses to MaBloWriMo 23: contradiction!

1. chushogilionhawk says:

I’d like to see some posts about ordinal numbers. I find them difficult.

• Brent says:

Oh, that’s a nice idea!

2. janhrcek says:

I’ve been watching your MaBloWriMo posts since the beginning. Good job explaining the proof! I have a suggestion for the following week: what about proving Lagrange’s theorem?

• Brent says:

Hmm, good idea!