# How to Proof?

## Main Question or Discussion Point

How to proof?
A prime number p is a factor of a non-zero product of integers a*b if and only if it is a facotr of a and/or b.

Related Linear and Abstract Algebra News on Phys.org
The "if" direction is obvious. For the "only if", you need to use the fundamental theorem of arithmetic. Because a*b has a unique prime decomposition, if a prime p divides a*b then it must be one of the primes in the decomposition. The prime decomposition of a*b is just the product of the decompositions of a and b, so p must divide a and/or b.

This sounds like a homework question! In which case you will need to be a lot more rigorous when you write it out.

Last edited:
CompuChip
Homework Helper
One side is trivial: if p is a factor of a and/or b, then it is clearly a factor of a * b.

For the converse implication, the easiest way I can think of is using the unique decomposition of any integer into its prime factors.

thx,guys

Caution. Some proofs of unique factorization may use this property of primes. And then it would be illegitimate to use unique factorization in the proof of this...

crd
assume p is a factor of ab. then p|ab. since p is prime, euclid's lemma says p|a and/or p|b, thus p is a factor of a and/or b.