If f:Q→Q is a ring homomorphism, then f(x)=f(x⋅1)=f(x)f(1) for any x∈Q, so f(1)=1 or f(1)=0.
If f(1)=0, then f(x)=f(1⋅x)=f(1)f(x)=0 for all x∈Q.
So if f is a nonzero homomorphism, then f(1)=1.
Prove by induction that f(x)=x for any x∈N
Indeed,
1)It is true for n=1: f(1)=1
2)Assume that f(k)=k for some k≥1 and k∈N
3)Then f(k+1)=f(k)+f(1).
f(k)=k by the inductive hypothesis, and f(1)=1, so f(k+1)=k+1
By the principle of mathematical induction we obtain that f(x)=x for all x∈N.
Moreover f(x)=f(x+0)=f(x)+f(0) for all x∈Q , so f(0)=0 as additive identity. And 0=f(0)=f(x+(−x))=f(x)+f(−x), so f(−x)=−f(x).
Then f(−x)=−f(x)=−x for any x∈N.
So we obtain that f(x)=x for any x∈Z.
Let x∈Q, then x=nm for some m,n∈Z. Then m=f(m)=f(x⋅n)=f(x)f(n)=f(x)n, and we obtain that f(x)=nm=x.
Therefore f(x)=x for any x∈Q.
If f is a group homomorphism, then f have not to be the identity homomorphism. For example, f:Q→Q, were f(x)=2x, is a group homomorphism.
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