- #1
kingwinner
- 1,270
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"Given a metric space (X,ρ), define a new metric on X by σ(x,y)=min{ρ(x,y),1}. The reader can check that σ is indeed a metric on X.
Claim 1: xn->x in (X,ρ) iff xn->x in (X,σ).
Claim 2: {xn} is Cauchy in (X,ρ) iff it is Cauchy in (X,σ). (Hence completeness is the same for these two metrics.)"
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Right now, I'm trying to prove claim 1.
xn->x in (X,ρ) means:
for all ε>0, there exists N s.t. if n>N, then ρ(xn,x)<ε
xn->x in (X,σ) means:
for all ε>0, there exists M s.t. if n>M, then σ(xn,x)<ε
Now if 0<ε<1, then ρ(xn,x)<ε <=> σ(xn,x)<ε, so taking N=M above works in the definition of limit. Am I correct so far?
But if ε>1, I think it is NOT true that ρ(xn,x)<ε <=> σ(xn,x)<ε, right? So what should I take N or M to be in the case when ε>1? What should I do for this case?
Thanks for any help!
Claim 1: xn->x in (X,ρ) iff xn->x in (X,σ).
Claim 2: {xn} is Cauchy in (X,ρ) iff it is Cauchy in (X,σ). (Hence completeness is the same for these two metrics.)"
=================================
Right now, I'm trying to prove claim 1.
xn->x in (X,ρ) means:
for all ε>0, there exists N s.t. if n>N, then ρ(xn,x)<ε
xn->x in (X,σ) means:
for all ε>0, there exists M s.t. if n>M, then σ(xn,x)<ε
Now if 0<ε<1, then ρ(xn,x)<ε <=> σ(xn,x)<ε, so taking N=M above works in the definition of limit. Am I correct so far?
But if ε>1, I think it is NOT true that ρ(xn,x)<ε <=> σ(xn,x)<ε, right? So what should I take N or M to be in the case when ε>1? What should I do for this case?
Thanks for any help!