example of well-founded induction
This proof of the fundamental theorem of arithmetic (every natural number
has a prime factorization
) affords an example of proof by well-founded induction over a well-founded relation that is not a linear order.
First note that the division relation is obviously well-founded. The -minimal elements are the prime numbers
. We detail the two steps of the proof :
- 1.
If is prime, then is its own factorization into primes, so the assertion is true for the -minimal elements.
- 2.
If is not prime, then has a non-trivial factorization (by definition of not being prime), i.e. , where . By induction
, and have prime factorizations, the product
of which is a prime factorization of .