(related to Theorem: Fundamental Theorem of Arithmetic)

- Let \(n > 1\).
- According existence of prime divisors, we can find a prime divisor \(p\mid n\).
- In the series of consecutive prime numbers, \(p_1=2, p_2=3, \ldots\), we also can find an index \(i\), for which we can write $n=p_{i}^1\cdot n_1,$ with \(p_i=p\) and some \(n_1 < n\).
- Since \(n\) was either composite or prime, we have that \(n_1 > 1\) or \(n_1=1\). * In the latter case, we are done.
- In the first case, \(n_1\) is another, non-trivial divisor of \(n\).
- Repeating the same argument for \(n_1\) we will find other prime \(q\) and a new divisor \(n_2 < n_1\), whereby we have \[\begin{array}{ccl} n&=&p_i^2\cdot n_2,\text{ if }q=p_i\text{ or }\\ n&=&p_i\cdot p_j\cdot n_2 \text{ else.} \end{array} \]
- Since \(n_1\) was either composite or prime, we have again that \(n_2 > 1\) or \(n_2=1\), so we can repeat the above argument.
- By doing so, we will possibly find even further divisors \(n_2, n_3,\ldots\) of \(n\) and the corresponding primes \(p_k\mid n\), grouping them by the corresponding powers \(p_k^{e_k}\).
- Obviously, the set of these numbers is a non-empty subset of natural numbers.
- Following the well-ordering principle, the set of all \(\{n_k\}\), \(k=1,2,\ldots\), with all \(n > n_1 > n_2 >\ldots > 1 \), for which we repeat the argument, must have a minimum \(n_{p} > 1 \).
- By the corresponding corollary, \(n_p\) must be a prime number.
- Thus, the repetition of the above steps will finally end and we will find a factorization of the form \[n = p_1^{e_1}\cdot p_2^{e_2}\cdot\ldots\cdot p_r^{e_r},\] where \(p_r\) denotes the biggest prime we have found, the \(p_1,\ldots p_r\) denote consecutive primes (i.e. starting with \(p_1=2, p_2=3,\ldots\) and ending at \(p_r\)), and \(e_i\) denote the number of times we have found the prime \(p_i\) in the procedure.

- Assume that, applying the above procedure more then once, we can find two different factorizations of \(n\): \[\begin{array}{cclr} n&=&p_1^{e_1}\cdot p_2^{e_2}\cdot\ldots\cdot p_r^{e_r},&~~~~~~~~~~(1)\\ n&=&p_1^{f_1}\cdot p_2^{f_2}\cdot\ldots\cdot p_s^{f_s}.&~~~~~~~~~~(2) \end{array} \]
- Without loss of generality, assume \(r\ge s\).
- If \(r > s\), then it follows from \((1)\) that \(p_r\mid n\) and from \((2)\) that \(p_r\not\mid n\), which is a contradiction, because a divisor of \(n\) cannot be mutually not a divisor of \(n\).
- By construction, our two factorizations list all consecutive primes, \(p_1 < p_2 < \ldots < p_r\).
- Thus, it must be \(r=s\).
- Now, if \((1)\) and \((2)\) are two different factorizations by hypothesis, then it must be \(f_i\neq e_i\) for at least one \(i\).
- Let, without loss of generality, assume \(f_i < e_i\).
- Then, from \((1)\) it follows that \(p_i^{e_i}\mid n\) and from \((2)\) that \(p_r^{e_i}\not\mid n\), which is again is a contradiction, by the same argument.
- Thus, the two factorizations must be in fact identical.∎