Sunday, 19 February 2017

elementary number theory - Given gcd(d,d)=1,dmidn,dmidn, show that ddmidn



Given d,d are in Z>1, and gcd, and d \mid n and d'\mid n,



Show that d\cdot d'\mid n.



I pretty much have it but I think it could be made more clear. I have:



d \mid n so n=q*d and d'\mid n so n=q'*d'




So q*d*q'*d'=n^2



So (q*d*q'*d')/n=n



So (q*q'/n)*(d*d')=n ... so the only thing left to do is show that q*q'/n must be an integer.



I think it has something to do with \gcd(d,d')=1 because d and d' cannot both divide an integer n unless n is a multiple of d*d', but I'm not sure how to show this elegantly.



Thanks!



Answer



Hint: since \gcd(d, d') = 1, there exist \alpha, \beta \in \mathbb{Z} such that \alpha d + \beta d' =1. Now multiply both sides by n, and use the fact that d \mid n and d' \mid n. Feel free to comment if you need further clarification!


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