Question #106438
Use the ring Z(underroot -2) to show that (1) the quotient ring of a ufd need not be a UFD. (2)an irreducible element of a UFD need not be a prime element
1
Expert's answer
2020-03-27T13:35:50-0400

The given ring is Z[2]\Z[\sqrt{-2}] ={a+b2:a,bZ}=\{ a+b\sqrt{-2} :a,b\in \Z \} .

Claim: The Quotient ring of a UFD need not be a UFD.

Since RR is an Euclidean domain ,so RR is PID.

Again , we known that every PID is UFD.

Hence, RR is a UFD.

Now ,consider the ideal <6>={<6>=\{ 6(a+(2)b ):a,bZ}6(a+(\sqrt{-2})b \ ):a,b\in \Z \}

Then ,RI={(a+b(2))+<6>:a,bZ}\frac{R}{I}=\{ (a+b(\sqrt{-2}))+<6> :a,b\in \Z \}

Clearly,2<6>,2\notin <6>, and 3<6>3\notin <6> but 6<6>6\in<6>

Hence ,RI\frac{R}{I} is not a domain.

But we known that ,to became a UFD ,it must be first integral domain.

Hence , it is not a unique factorization domain.

(Proved).

Since RR is a PID , Every irreducible elements is prime .

Hence, 2nd part of the question is wrong. That is ,we can't find a irreducible element in the given ring R which is not a prime.

But in place of RR if you take R=R'= Z[5]\Z [\sqrt{-5}] then there are beautiful results,

For example:

The ring Z[x]\Z[x] is a UFD but when we quotient by the idealI=<x2+5>I=<x^2+5> then,

RI=Z[x]IZ[5]\frac{R'}{I}=\frac{ \Z[x]}{I}\approx\Z[\sqrt{-5}]

Which is not a UFD.

Since ,46=2.2346=2.23

=(1+35)(135)=(1+3\sqrt{-5})(1-3\sqrt{-5})

We can easily verify that 2,23,(1+35)(135)2,23,(1+3\sqrt{-5})(1-3\sqrt{-5}) are irreducible.

Define a norm on Z[5]\Z[\sqrt{-5}] by N(a+b5)=a2+5b2N(a+b\sqrt{-5})=a^2+5b^2

Then xR is a unit iff N(x)=1x\in R' \ is \ a \ unit \ iff \ N(x)=1

Suppose x=a+b5x=a+b\sqrt{-5}is a unit     \iff there exit y=a+b5y=a'+b'\sqrt{-5} such that xy=1 \

Suppose x is a unit    yRs.t. xy=1x \ is \ a \ unit \implies\exists y \in R' s.t . \ xy=1

So,N(xy)=N(1)     N(x)N(y)=1N(xy)=N(1)\implies \ N(x)N(y)=1

Hence ,N(x)=1. sinceN(x)=1. \ since norm gives non negative integer.

Now ,assume N(x)=1     a2+5b2=1N(x)=1\implies\ a^2+5b^2=1

Since,a2 and 5b2are positive number    a=+1 or 1a^2 \ and \ 5b^2 are \ positive \ number\implies a=+1 \ or \ -1

Hence,x=ax=a

Which is a unit .

Also it follows that the only unit in RR' are +1 and -1.

If possible let 2=xy2=xy for some x,yRfor \ some \ x,y\in R' ,where neither x nor y is a unit.

Then N(2)=N(x)N(y)    N(x)N(y)=4N(2)=N(x)N(y)\implies N(x)N(y)=4

N(x)=N(y)=2\therefore N(x)=N(y)=2 ,Since N(x) and N(y)areN(x) \ and \ N(y) are non negative integer.

Hence,2=42=4

Which is not possible .Hence 2 is irreducible.

Similarly 23 is not irreducible.

If possible let 1+35=xy1+3\sqrt{-5}=xy ,for some x,yRx,y\in R' where neither x nor y is a units.

Then N(xy)=N(1+35)=46=N(x)N(y)N(xy)=N(1+3\sqrt{-5})=46=N(x)N(y)

N(x)N(y)=2×23     eitherN(x)=2 or N(x)=23N(x)N(y)=2×23\implies \ either N(x)=2 \ or \ N(x)=23

Therefore, N(x)=a2+5b2=2 or 23N(x)=a^2+5b^2=2 \ or \ 23

Since,a2,5b2 are positive number soN(x)2a^2,5b^2 \ are \ positive \ number \ so N(x)\neq2

If N(x)=23 then 5b2=23a205b^2=23-a^2\geq0

Which is not possible because a is an integer .

Hence, 1+351+3\sqrt{-5} is irreducible.

Similarly,1351-3\sqrt{-5} is irreducible.


Also ,we can prove (1+35)(1+3\sqrt{-5}) is irreducible but not a prime

just using the definition of prime in an integral domain.

If possible ,let this number is a prime.

Then,

(1+35)(135)=46=2.23(1+3\sqrt{-5})(1-3\sqrt{-5})=46=2.23

     (1+35)2.23\implies \ (1+3\sqrt{-5}) |2.23

But neither (1+35)2 nor (1+35)23(1+3\sqrt{-5})|2 \ nor \ (1+3\sqrt{-5})|23

Suppose,(1+35)2(1+3\sqrt{-5})\mid 2 then there exit a number (a+b5) such that (a+b5)(1+35)=2(a+b\sqrt{-5} ) \ such \ that \ (a+b\sqrt{-5})(1+3\sqrt{-5})=2

    a+15b=2 and a+b=0\implies a+15b=2 \ and \ a+b=0

Which is impossible.

Hence(1+35)2.(1+3\sqrt{-5})\nmid 2.

Similarly,(1+35)23(1+3\sqrt{-5})\nmid23

Therefore,(1+35)(1+3\sqrt{-5}) is not a prime.




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