- #1
mahler1
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1. Homework Statement .
Prove that the only homomorphism between Z5 and Z7 is ψ(x)=0 (the trivial homomorphism).
3. The Attempt at a Solution .
I wanted to check if my solution is correct, so here it goes:
Any element x in Z5 belongs to the set {0,1,2,3,4}
So, I trivially start by saying that ψ(1)=ψ(1) and then, using the fact that ψ(x+y)=ψ(x)+ψ(y), I get:
ψ(2)=ψ(1+1)=ψ(1)+ψ(1)=2ψ(1)
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ψ(5)=ψ(1+1+1+1+1)=ψ(1)+ψ(1)+ψ(1)+ψ(1)+ψ(1)=5ψ(1)
But 5=0 (mod 5) and, as ψ is a homomorphism, it must be 0= ψ(0)=ψ(5).
By transitivity, 0=5ψ(1) (mod 7) but 5≠0 (mod 7), then ψ(1)=0. But ψ(x)=xψ(1)=0 for all x in Z5, hence Hom(Z5,Z7)=0.
Prove that the only homomorphism between Z5 and Z7 is ψ(x)=0 (the trivial homomorphism).
3. The Attempt at a Solution .
I wanted to check if my solution is correct, so here it goes:
Any element x in Z5 belongs to the set {0,1,2,3,4}
So, I trivially start by saying that ψ(1)=ψ(1) and then, using the fact that ψ(x+y)=ψ(x)+ψ(y), I get:
ψ(2)=ψ(1+1)=ψ(1)+ψ(1)=2ψ(1)
.
.
.
ψ(5)=ψ(1+1+1+1+1)=ψ(1)+ψ(1)+ψ(1)+ψ(1)+ψ(1)=5ψ(1)
But 5=0 (mod 5) and, as ψ is a homomorphism, it must be 0= ψ(0)=ψ(5).
By transitivity, 0=5ψ(1) (mod 7) but 5≠0 (mod 7), then ψ(1)=0. But ψ(x)=xψ(1)=0 for all x in Z5, hence Hom(Z5,Z7)=0.