Saturday, 7 January 2017

elementary number theory - GCD property: bmidac implies bmid(a,b)(b,c)



The following is a very simple statement I want to prove:




If a,b,c are non-zero integers, then bac 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,cac 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 bac, 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)


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