Composite Number has Prime Factor

Theorem
Let $a$ be a composite number.

Then there exists a prime number $p$ such that:
 * $p \mathop \backslash a$

where $\backslash$ means is a divisor of.

Proof
By definition of composite number:


 * $\exists b \in \Z: b \mathop \backslash a$

If $b$ is a prime number then the proof is complete.

Otherwise $b$ is composite.

By definition of composite number:


 * $\exists c \in \Z: c \mathop \backslash b$

and so:
 * $c \mathop \backslash a$

Again, if $c$ is a prime number then the proof is complete.

Continuing in this manner, some prime number will be found which will divide the number before it.

This will be a divisor of $a$.

Because if not, then an infinite series of numbers will be divisors of $a$, each of which is less than the other.

This is impossible in numbers.

Hence the result.