(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

Show that {1,2,3} under multiplication modulo 4 is not a group but that {1,2,3,4} under multiplication modulo 5 is a group

2. Relevant equations

a mod n=r ;a=qn + r

3. The attempt at a solution

I'm going to assume when the problem says modulo 4, the problem is read as: a mod n=4 . I don't want you tutors to do the problem for me, I just want you to help me where I should start because I have no idea how set {1,2,3,4} is related to modulo 4 . I know a for modulo four is equivalent to: U(4)={1,2,3,4} I have no idea how to proved that U(4) ={1,2,3,4} is not a group under multiplication.

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# Abstract algebra question

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