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
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Expert's answer
2020-03-27T13:35:50-0400
The given ring is Z[−2]={a+b−2:a,b∈Z} .
Claim: The Quotient ring of a UFD need not be a UFD.
Since R is an Euclidean domain ,so R is PID.
Again , we known that every PID is UFD.
Hence, R is a UFD.
Now ,consider the ideal <6>={6(a+(−2)b):a,b∈Z}
Then ,IR={(a+b(−2))+<6>:a,b∈Z}
Clearly,2∈/<6>, and 3∈/<6> but 6∈<6>
Hence ,IR 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 R 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 R if you take R′=Z[−5] then there are beautiful results,
For example:
The ring Z[x] is a UFD but when we quotient by the idealI=<x2+5> then,
IR′=IZ[x]≈Z[−5]
Which is not a UFD.
Since ,46=2.23
=(1+3−5)(1−3−5)
We can easily verify that 2,23,(1+3−5)(1−3−5) are irreducible.
Define a norm on Z[−5] by N(a+b−5)=a2+5b2
Then x∈R′isaunitiffN(x)=1
Suppose x=a+b−5is a unit ⟺ there exit y=a′+b′−5 such that xy=1 \
Suppose xisaunit⟹∃y∈R′s.t.xy=1
So,N(xy)=N(1)⟹N(x)N(y)=1
Hence ,N(x)=1.since norm gives non negative integer.
Now ,assume N(x)=1⟹a2+5b2=1
Since,a2and5b2arepositivenumber⟹a=+1or−1
Hence,x=a
Which is a unit .
Also it follows that the only unit in R′ are +1 and -1.
If possible let 2=xyforsomex,y∈R′ ,where neither x nor y is a unit.
Then N(2)=N(x)N(y)⟹N(x)N(y)=4
∴N(x)=N(y)=2 ,Since N(x)andN(y)are non negative integer.
Hence,2=4
Which is not possible .Hence 2 is irreducible.
Similarly 23 is not irreducible.
If possible let 1+3−5=xy ,for some x,y∈R′ where neither x nor y is a units.
Then N(xy)=N(1+3−5)=46=N(x)N(y)
N(x)N(y)=2×23⟹eitherN(x)=2orN(x)=23
Therefore, N(x)=a2+5b2=2or23
Since,a2,5b2arepositivenumbersoN(x)=2
If N(x)=23 then 5b2=23−a2≥0
Which is not possible because a is an integer .
Hence, 1+3−5 is irreducible.
Similarly,1−3−5 is irreducible.
Also ,we can prove (1+3−5) 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+3−5)(1−3−5)=46=2.23
⟹(1+3−5)∣2.23
But neither (1+3−5)∣2nor(1+3−5)∣23
Suppose,(1+3−5)∣2 then there exit a number (a+b−5)suchthat(a+b−5)(1+3−5)=2
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