The following is a very simple statement I want to prove:
If a,b,c are non-zero integers, then b∣ac implies b∣(a,b)(b,c)
Here (a,b),[a,b] denote the greatest common divisor and the least common multiple between a,b, respectively.
The symbol ∣ means divisibility.
Attempt: a,b,c∣ac implies [a,b]∣ac,[b,c]∣ac.
Then ac=tab(a,b)=ubc(b,c) for some integers t,u.
Now, it follows that (a,b)c=tb,(b,c)a=ub.
Multiplying we get (a,b)(b,c)ac=tub2.
Since b∣ac, it follows the existence of an integer q s.t. ac=bq, so that (a,b)(b,c)bq=tub2.
Clearly (a,b)(b,c)q=tub.
Here I stuck.
Can I argue something more or should I follow a different strategy?
Thank you in advance for your help.
Update I would like to avoid Bezout identity, whenever possible, because I'm interested in applications to GCD domains, where a Bezout identity does not always hold. Some answers using Bezout were posted before this update.
Answer
(b,a)(b,c)=((b,a)b,(b,a)c)=(bb,ab,bc,ac)=b(b,a,c,ac/b)
No comments:
Post a Comment