Question #104882
Show that if :f Q->Q is a ring homomorphism, then f(x) = x ∀x ∈ Q. Would
this still be true if f were a group homomorphism? Why, or why not?
1
Expert's answer
2020-03-09T13:00:38-0400

If f ⁣:QQf\colon \mathbb Q\to\mathbb Q is a ring homomorphism, then f(x)=f(x1)=f(x)f(1)f(x)=f(x\cdot 1)=f(x)f(1) for any xQx\in\mathbb Q, so f(1)=1f(1)=1 or f(1)=0f(1)=0.

If f(1)=0f(1)=0, then f(x)=f(1x)=f(1)f(x)=0f(x)=f(1\cdot x)=f(1)f(x)=0 for all xQx\in\mathbb Q.

So if ff is a nonzero homomorphism, then f(1)=1f(1)=1.

Prove by induction that f(x)=xf(x)=x for any xNx\in\mathbb N

Indeed,

1)It is true for n=1n=1: f(1)=1f(1)=1

2)Assume that f(k)=kf(k)=k for some k1k\ge 1 and kNk\in\mathbb N

3)Then f(k+1)=f(k)+f(1)f(k+1)=f(k)+f(1).

f(k)=kf(k)=k by the inductive hypothesis, and f(1)=1f(1)=1, so f(k+1)=k+1f(k+1)=k+1

By the principle of mathematical induction we obtain that f(x)=xf(x)=x for all xNx\in\mathbb N.


Moreover f(x)=f(x+0)=f(x)+f(0)f(x)=f(x+0)=f(x)+f(0) for all xQx\in\mathbb Q , so f(0)=0f(0)=0 as additive identity. And 0=f(0)=f(x+(x))=f(x)+f(x)0=f(0)=f(x+(-x))=f(x)+f(-x), so f(x)=f(x)f(-x)=-f(x).

Then f(x)=f(x)=xf(-x)=-f(x)=-x for any xNx\in\mathbb N.

So we obtain that f(x)=xf(x)=x for any xZx\in\mathbb Z.

Let xQx\in\mathbb Q, then x=mnx=\frac{m}{n} for some m,nZm,n\in\mathbb Z. Then m=f(m)=f(xn)=f(x)f(n)=f(x)nm=f(m)=f(x\cdot n)=f(x)f(n)=f(x)n, and we obtain that f(x)=mn=xf(x)=\frac{m}{n}=x.


Therefore f(x)=xf(x)=x for any xQx\in\mathbb Q.


If ff is a group homomorphism, then ff have not to be the identity homomorphism. For example, f ⁣:QQf\colon\mathbb Q\to\mathbb Q, were f(x)=2xf(x)=2x, is a group homomorphism.


Need a fast expert's response?

Submit order

and get a quick answer at the best price

for any assignment or question with DETAILED EXPLANATIONS!

Comments

No comments. Be the first!
LATEST TUTORIALS
APPROVED BY CLIENTS