# Integer is Expressible as Product of Primes/Proof 3

## Theorem

Let $n$ be an integer such that $n > 1$.

Then $n$ can be expressed as the product of one or more primes.

## Proof

The proof proceeds by induction.

For all $n \in \N_{> 1}$, let $\map P n$ be the proposition:

- $n$ can be expressed as a product of prime numbers.

First note that if $n$ is prime, the result is immediate.

### Basis for the Induction

$\map P 2$ is the case:

- $n$ can be expressed as a product of prime numbers.

As $2$ itself is a prime number, and the result is immediate.

This is the basis for the induction.

### Induction Hypothesis

Now it needs to be shown that, if $\map P j$ is true, for all $j$ such that $2 \le j \le k$, then it logically follows that $\map P {k + 1}$ is true.

So this is the induction hypothesis:

- For all $j \in \N$ such that $2 \le j \le k$, $j$ can be expressed as a product of prime numbers.

from which it is to be shown that:

- $k + 1$ can be expressed as a product of prime numbers.

### Induction Step

This is the induction step:

If $k + 1$ is prime, then the result is immediate.

Otherwise, $k + 1$ is composite and can be expressed as:

- $k + 1 = r s$

where $2 \le r < k + 1$ and $2 \le s < k + 1$

That is, $2 \le r \le k$ and $2 \le s \le k$.

Thus by the induction hypothesis, both $r$ and $s$ can be expressed as a product of primes.

So $k + 1 = r s$ can also be expressed as a product of primes.

So $\map P k \implies \map P {k + 1}$ and the result follows by the Second Principle of Mathematical Induction.

Therefore, for all $n \in \N_{> 1}$:

- $n$ can be expressed as a product of prime numbers.

## Sources

- 1971: George E. Andrews:
*Number Theory*... (previous) ... (next): $\text {2-4}$ The Fundamental Theorem of Arithmetic: Theorem $\text {2-5}$ - 1997: Donald E. Knuth:
*The Art of Computer Programming: Volume 1: Fundamental Algorithms*(3rd ed.) ... (previous) ... (next): $\S 1.2.1$: Mathematical Induction: Exercise $5$