- #1
Hodgey8806
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Homework Statement
The function g:ℝ→ℝ defined by "g(x) = 0 for x being rational and g(x) = 1 for x being irrational" does not have a limit at zero.
Homework Equations
I have to use the definition of the limit of a function at a point--I can't use the sequential criterion this time.
So the definition of the limit of a function at a point is: (assuming function and 0 being the cluster point)
A real number L is said to be a limit of f at c if, given any ε>0, there exists a δ>0 s.t. if x is in the domain, and 0<|x-c|<δ, then |f(x) - L|<ε.
Thus the negation of the definition is:
A real number L is NOT a limit of f at c if there exist ε>0 s.t. for all δ>0, x is in the domain, 0<|x-c|<δ, AND |f(x)-L|≥ε.
The Attempt at a Solution
I am going to prove this true by using the negation of the definition.
Proof:
Assume the limit of L ≤ 0. Choose ε=1/2.
By denseness of the irrationals, for all δ>0, there exists x in the irrationals s.t. 0<|x|<δ, but |f(x)-0|=|1-L|≥1 ≥ 1/2=ε. Thus L≤0 is not a limit of g.
Assume the limit of L ≥ 1. Choose ε=1/2.
By denseness of the rationals, for all δ>0, there exists x in the rationals s.t. 0<|x|<δ, but |f(x)-L|=|0-L|≥1 ≥ 1/2=ε. Thus L≥1 is not a limit of g.
Assume the limit of 0≤ L ≤ 1/2. Choose ε=1/2.
By denseness of the irrationals, for all δ>0, there exists x in the irrationals s.t. 0<|x|<δ, but |f(x)-L|=|1-L|≥1/2=ε. Thus 0≤ L ≤ 1/2 is not a limit of g.
Assume the limit of 1/2< L ≤ 1. Choose ε=1/2.
By denseness of the rationals, for all δ>0, there exists x in the rationals s.t. 0<|x|<δ, but |f(x)-L|=|0-L|>1/2=ε. Thus 1/2< L ≤ 1 is not a limit of g.
Thus, we have proved for ever L in the reals that L cannot be a limit of this function.
Q.E.D