Prime number/Citable Version

A prime number is a whole number (i.e., one having no fractional or decimal part) that cannot be evenly divided by any numbers but 1 and itself. The first few prime numbers are 2, 3, 5, 7, 11, 13, and 17. With the exception of $2$, all prime numbers are odd numbers, but not every odd number is prime. For example, $9=3\cdot 3$ and $15=3\cdot 5$, so neither 9 nor 15 is prime. The study of prime numbers has a long history, going back to ancient times, and it remains an active part of number theory (a branch of mathematics) today. It is commonly believed that the study of prime numbers is an interesting, but not terribly useful, area of mathematical research. While this used to be the case, the theory of prime numbers has important applications now. Understanding properties of prime numbers and their generalizations is essential to modern cryptography, and to public key ciphers that are crucial to Internet commerce, wireless networks and, of course, military applications. Less well known is that other computer algorithms also depend on properties of prime numbers.

Definition

Prime numbers are usually defined to be positive integers (other than 1) with the property that they are only (evenly) divisible by 1 and themselves. In other words, a number $n\in \mathbb {N}$ is said to be prime if there are exactly two $m\in \mathbb {N}$ such that $m|n$, namely $m=1$ and $m=n$.

There is another way of defining prime numbers, and that is that a number is prime if whenever it divides the product of two numbers, it must divide one of those numbers. A nonexample (if you will) is that 4 divides 12, but 4 does not divide 2 and 4 does not divide 6 even though 12 is 2 times 6. This means that 4 is not a prime number. We may express this second possible definition in symbols (a phrase commonly used to mean "in mathematical notation") as follows: A number $p\in \mathbb {N}$ is prime if for any $a,b\in \mathbb {N}$ such that $p|ab$ (read p divides ab), either $p|a$ or $p|b$. If the first characterization of prime numbers is taken as the definition, the second is derived from it as a theorem, and vice versa.

Aside on mathematical notation: The second sentence above is a translation of the first into mathematical notation. It may seem difficult at first (perhaps even a form of obfuscation!), but mathematics relies on precise reasoning, and mathematical notation has proved to be a valuable, if not indispensible, aid to the study of mathematics. It is commonly noted while ancient Greek mathematicians hd a good understanding of prime numbers, and indeed Euclid was able to show that there are infinitely many prime numbers, the study of prime numbers (and algebra in general) was hampered by the lack of a good notation, and this is one reason ancient Greek mathematics (or mathematicians) excelled in geometry, making comparatively less progress in algebra and number theory.

Unique Factorization

Every integer N > 1 can be written in a unique way as a product of prime factors, up to reordering. to see why this is true, assume that N can be written as a product of prime factors in two ways

$N=p_{1}p_{2}\cdots p_{m}=q_{1}q_{2}\cdots q_{n}$

We may now use a technique known as mathematical induction to show that the two prime decompositions are really the same.

Given any prime divisor of N (call it p), we know that

$p|p_{1}p_{2}\cdots p_{m}$

and

$p|q_{1}q_{2}\cdots q_{n}$

Because p is prime, we know there are integers i and j such that $p|p_{i}$ and $p|q_{j}$. On the other hand, since $p_{i}$ and $q_{j}$ are prime, it must be that p is equal to $p_{i}$ and to $q_{j}$. This means that if we set $N_{1}=N/p$, we may write

$N_{1}=p_{1}p_{2}\cdots {\hat {p_{i}}}\cdots p_{m}$

and

$N_{1}=q_{1}q_{2}\cdots {\hat {q_{j}}}\cdots q_{n}$

where the circumflex ("hat symbol") indicates that $p=p_{i}=q_{j}$ is not part of the prime factorization of $N_{1}$

Continuing this way, we obtain a sequence of numbers $N=N_{0}>N_{1}>N_{2}>\ldots >N_{n}=1$ where each $N_{\alpha }$ is obtained by dividing $N_{\alpha -1}$ by a prime factor. In particular, we see that $m=n$ and that there is some permutation ("rearrangement") of the indices $1,2,\ldots n$ such that $p_{i}=q_{\sigma (i)}$. Said differently, the two factorizations of N must be the same up to a possible rearrangment of terms.

There are infinitely many primes

One basic fact about the prime numbers is that there are infinitely man of them. In other words, the list of prime numbers 2, 3, 5, 7, 11, 13, 17, ... doesn't ever stop. There are a number of ways of showing that this is so, but one of the oldest and most familiar proofs goes back go Euclid.

Euclid's Proof

Suppose the set of prime numbers is finite, say $\{p_{1},p_{2},p_{3},\ldots ,p_{n}\}$, and let

$N=p_{1}p_{2}\cdots p_{n}+1$

then for each $i\in 1,\ldots ,n$ we know that $p_{i}\not |N$ (because the remainder is 1). This means that N is not divisible by an prime, which is impossible. This contradiction shows that our assumption that there must only be a finite number of primes must have been wrong and thus proves the theorem.