Fundamental Theorem of Arithmetic

Theorem
For every integer $$n$$ such that $$n > 1$$, $$n$$ can be expressed as the product of one or more primes, uniquely up to the order in which they appear.

Proof

 * First we prove existence.

If $$n$$ is prime, the result is immediate.

Let $$n$$ be composite.

Then by Composite Number has Two Divisors Less Than It, $$\exists r, s \in \mathbb{Z}: n = r s, 1 < r < n, 1 < s < n$$.

This being the case, the set $$S_1 = \{d: d \backslash n, 1 < d < n\}$$ is nonempty, and bounded below by $$1$$.

By Integers Bounded Below has Minimal Element, $$S_1$$ has a least element, which we will call $$p_1$$.

If $$p_1$$ is composite, then by Composite Number has Two Divisors Less Than It, it has at least two divisors $$a, b$$, such that $$a, b \backslash p_1$$ and $$1 < a < p_1, 1 < b < p_1$$.

But by Divides is Ordering on Positive Integers, it follows that $$a, b \backslash n$$ and hence $$a, b \in S$$, which contradicts the assertion that $$p_1$$ is the smallest element of $$S_1$$.

Thus, $$p_1$$ is necessarily prime.

We may now write $$n = p_1 n_1$$, where $$n > n_1 > 1$$.

If $$n_1$$ is prime, we are done.

Otherwise, the set $$S_2 = \{d: d \backslash n_1, 1 < d < n_1\}$$ is nonempty, and bounded below by $$1$$.

By the above argument, the least element $$p_2$$ of $$S_2$$ is prime.

Thus we may write $$n_1 = p_2 n_2$$, where $$1 < n_2 < n_1$$.

This gives us $$ n = p_1 p_2 n_2$$.

If $$n_2$$ is prime, we are done.

Otherwise, we continue this process.

Since $$ n > n_1 > n_2 > \ldots > 1$$ is a decreasing sequence of positive integers, there must be a finite number of $$n_i$$'s.

That is, we will arrive at some prime number $$n_{k-1}$$, which we will call $$p_k$$.

This results in the prime factorization $$n={p_1}{p_2} \ldots {p_k}$$.


 * Now we prove uniqueness.

Now, suppose $$n$$ has two prime factorizations, namely $$n=p_1 p_2 \dots p_r=q_1 q_2 \dots q_s$$, where $$r \le s$$ and each $$p_i$$ and $$q_j$$ is prime with $$p_1 \le p_2 \le \dots \le p_r$$ and $$q_1 \le q_2 \le \dots \le q_s$$.

Since $$p_1 \backslash q_1 q_2 \dots q_s$$, it follows from Prime Divides Factors that $$p_1=q_j,$$ for some $$1 \le j \le s$$.

This means $$p_1 \ge q_1$$.

Similarly, since $$q_1 \backslash p_1 p_2 \dots p_r$$, from Prime Divides Factors we have $$q_1 \ge p_1$$.

Thus, $$p_1=q_1$$, so we may cancel these common factors, which gives us $${p_2}{p_3} \dots {p_r}={q_2}{q_3} \dots {q_s}$$.

We may repeat this process to show that $$p_2=q_2$$, $$p_3=q_3$$, $$\dots$$, $$p_r=q_r$$.

If $$r < s$$, we arrive at $$1 =q_{r+1} q_{r+2} \ldots q_s$$ after canceling all common factors.

Of course, this is impossible, which means $$r=s$$.

Thus, $$p_1=q_1, p_2=q_2, \ldots, p_r=q_s$$, which means the two factorizations are identical.

Therefore, the prime factorization of $$n$$ is unique.

Alternative Proof

 * First we prove existence.

Suppose this supposition is false. Let $$m$$ be the smallest number which can not be expressed as the product of primes.

As a prime number is trivially a product of primes, $$m$$ can not itself be prime.

Hence $$\exists r, s \in \mathbb{Z}: 1 < r < m, 1 < s < m: m = r s$$.

As $$m$$ is our least counterexample, both $$r$$ and $$s$$ can be expressed as the product of primes.

Say $$r = p_1 p_2 \ldots p_k$$ and $$s = q_1 q_2 \ldots q_l$$, where all of $$p_1 \ldots p_k, q_1 \ldots q_l$$ are prime.

Hence $$m = r s = p_1 p_2 \ldots p_k q_1 q_2 \ldots q_l$$, which is a product of primes.

Hence there is no such counterexample.


 * Now we prove uniqueness.

Suppose the supposition false, that is, there is at least one number that can be expressed in more than one way as a product of primes. Let the least of these be $m$.

That is, $$m$$ is the smallest number which can be expressed in at least two ways, that is: $$m = p_1 p_2 \ldots p_r = q_1 q_2 \ldots q_s$$, where all of $$p_1 \ldots p_r, q_1 \ldots q_s$$ are prime.

Clearly $$m$$ can not be prime, therefore $$r,s \ge 2$$.

Let us arrange that the primes are in order of size, that is, $$p_1 \le p_2 \le \dots \le p_r$$ and $$q_1 \le q_2 \le \dots \le q_s$$, and also let us arrange that $$p_1 \le q_1$$.

Now suppose $$p_1 = q_1$$. Then:

$$\frac m {p_1} = p_2 p_3 \ldots p_r = q_2 q_3 \ldots q_s = \frac m {q_1}$$

But then we have the integer $$\frac m {p_1}$$ being expressible in two different ways, thus contradicting the fact that $$m$$ is the smallest number that can be so expressed.

Therefore, $$p_1 \ne q_1 \Longrightarrow p_1 < q_1 \Longrightarrow p_1 < q_2, q_3, \ldots, q_s$$ as we arranged them in order.

Now: $$1 < p_1 < q_j: 1 < j < s \Longrightarrow p_1 \nmid q_j$$ from Prime Not Divisor then Coprime.

But $$p_1 \backslash m \Longrightarrow p_1 \backslash q_1 q_2 \ldots q_s$$.

Thus $$\exists j: 1 \le j \le s: p_1 \backslash q_j$$ from Prime Divides Factors.

But $$q_j$$ was supposed to be a prime.

This gives us our contradiction, and there is therefore no such counterexample to our supposition.