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 $5$