# Primes and Associates in Rings

#### Zoe-b

1. Homework Statement
Let a, b be members of a commutative ring with identity R. If a is a a prime and a, b are associates then b is also prime. True/False

2. Homework Equations
Definitions: a is prime if a|xy implies a|x or a|y
a and b are associates if there exists a unit u s.t a=bu

3. The Attempt at a Solution
First off I'm not sure whether its even true or not; so I've tried to both prove it and find a counter example:
Attempt at a proof:
let b|xy.
Then if b = au for some unit u, au|xy, that is for some k in R auk = xy.
so ak divides xy(u^(-1))
but k does not necessarily have an inverse so this doesn't really get me anywhere.

I've shown in an earlier question that if a is irreducible and a, b are associates then b is irreducible. Prime implies irreducible but not vice versa so if there is a counter example it will depend on an element of R that is irreducible but not prime. I know examples of these can be found in rings such as Z(5i) (the set of numbers a+5bi for a, b in Z). But I don't know how to find units of this ring.
Consider (a+5bi)(c+5di) = 1.
This gives the simultaneous equations 1+25bd-ac = 0 = ad+bc
But so far I have not managed to find any integer solutions (and to be honest I don't know any way of doing this other than trial and error).

Thanks for any help!

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#### micromass

First off I'm not sure whether its even true or not; so I've tried to both prove it and find a counter example:
Attempt at a proof:
let b|xy.
Then if b = au for some unit u, au|xy, that is for some k in R auk = xy.
OK, so stop here. We know that auk=xy. So this implies that $ak=u^{-1}xy$. Thus a divides $u^{-1}xy$. Now use that a is prime.

#### Zoe-b

Thank you! Done it now :)

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