Theorem 39: Euler’s criterion

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Theorem of the week

I’ve previously written about Fermat’s Little Theorem, which tells us that if $latex p$ is a prime and $latex a$ is not divisible by $latex p$, then $latex a^{p-1} \equiv 1 \pmod{p}$.  That’s not terribly exciting when $latex p = 2$, so let’s concentrate on odd primes $latex p$ in this post.  Then $latex \frac{p-1}{2}$ is a whole number, so it makes sense (in the modular arithmetic world) to let $latex x = a^{\frac{p-1}{2}}$.  Then Fermat‘s Little Theorem tells us that $latex x^2 \equiv 1 \pmod{p}$.

How many possibilities for $latex x$ are there (modulo $latex p$, of course), if we know that $latex x^2 \equiv 1 \pmod{p}$?

Well, that congruence tells us that $latex (x – 1)(x + 1) \equiv 0 \pmod{p}$.  But, in the jargon, “there are no zero-divisors modulo a prime”.  That is, if $latex mn \equiv 0 \pmod{p}$, then $latex m \equiv 0 \pmod{p}$…

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Every odd integer larger than 1 is the sum of at most five primes

Terence Tao has has just posted about his latest paper on

Every odd integer larger than 1 is the sum of at most five primes! Please check it out!

Fermat Numbers

Proofs from THE BOOK by Martin Aigner and Günter Ziegler begins by giving six proofs of the infinity of primes. We will go over the third proof. Before we go over this proof, lets cover some background.

— 1. Fermat Numbers —

Fermat numbers are defined by

so that and . They are of great interest in many ways: for example, it was proved by Gauss that, if , is a prime , then a regular polygon of sides can be inscribed in a circle by Euclidean methods. The property of the Fermat numbers which is relevant here is

Theorem 1 No two Fermat numbers have a common divisor greater than 1.

We will prove this theorem later.

The first four Fermat numbers are prime, and Fermat conjectured that all were prime. Euler, however, found in 1732 that

is composite.

In 1880 Landry proved that

It is currently known that , is composite for . Factoring Fermat numbers is extremely difficult as a result of their large size. has known factors with remaining (where denotes a composite number with digits). has known factors with remaining. has no known factors but is composite. There are currently four Fermat numbers that are known to be composite, but for which no single factor is known: and . In all the other cases proved to be composite a factor is known. No prime has been found beyond , and it seems unlikely that any more will be found using current computational methods and hardware.

— 2. Infinitude of Primes Theorem —

We are now ready to prove Euclid’s Second Theorem, also called the Infinitude of Primes Theorem using the third proof in Proofs from THE BOOK.

Theorem 2 (Euclid’s Second Theorem) The number of primes is infinite.

Proof: Next let us look at the Fermat numbers for . We will show that any two Fermat numbers are relatively prime (Theorem 1); hence there must be infinitely many primes. To this end, we verify the recursion

from which our assertion follows immediately. Indeed, if is a divisor of, say, and , then divides , and hence or . But is impossible since all Fermat numbers are odd.

To prove the recursion we use induction on . For we have and . With induction we now conclude

Finite Groups

Proofs from THE BOOK by Martin Aigner and Günter Ziegler begins by giving six proofs of the infinity of primes. We will go over the second proof. Before we go over this proof, lets cover some background.

— 1. Algebraic Structures —

One of the themes of modern algebra is to compare algebraic structures. An algebraic structure refers to a nonempty set equipped with a binary operation or several binary operations (usually two). Also, we shall refer to a nonempty set equipped with two binary operations satisfying certain properties as a number system. First, we shall define algebraic structures called group, ring and field. A group consists of nonempty set with one binary operation satisfying several conditions. It is the simplest of the three.

Definition 1 Let be a nonempty set and be a binary operation on Then the set together with the binary operation , denoted, is a group, if the following three conditions are satisfied:

i. The binary operation is associative, i.e., for all .

ii. There is a -identity , i.e., for all .

iii. For each , there is a -inverse , i.e., , where is the -identity.

Further more, a group is said to be commutative (or abelian), if the binary operation is commutative.

Infinity of primes (re-posted to test my changes to latex2wp.py to support LyX 1.6.9)

I have re-posted this to test my changes to the latex2wp.py and terrystyle.py programs, compiled with Python Software Foundation’s Python 2.7.2 64 bit version, to add support for LyX 1.6.9. The code change incorporates some additional theorem-like environments, macros, font styles, and the numbering has been change so that the different theorem-like types each have a separate counter (e.g., theorem 1, theorem 2, lemma 1, proposition 1, theorem 3, lemma 2, …, as opposed to theorem 1, theorem 2, lemma 3, proposition 4, …). Furthermore, I have provided more background information which will benefit those without a background in abstract algebra.

Proofs from THE BOOK by Martin Aigner and Günter Ziegler begins by giving six proofs of the infinity of primes. The first proof is what they call "the oldest Book Proof" attributed to Euclid. Before we go over this proof, lets cover some background.

Infinity of primes

Proofs from THE BOOK by Martin Aigner and Günter Ziegler begins by giving six proofs of the infinity of primes. The first proof is what they call “the oldest Book Proof” attributed to Euclid. Before we go over this proof, lets cover some background.

Tao’s AMS lecture: Structure and randomness in the prime numbers

This is a link to Terence Tao’s “retired” lecture Structure and randomness in the prime numbers. Mathematicians with Terence’s talent appear only once in a generation and he is arguably the best living mathematician in the world.

View a public lecture by Terence Tao on the topic here.