- #1
zigzagdoom
- 27
- 0
Hi guys,
I attempted to prove this theorem, but just wanted to see if it a valid proof.
Thanks!
1. Homework Statement
Prove that x is an accumulation point of a set S iff there exists a sequence ( s n ) of points in S \ {x} that converges to x
N * ( x; ε ) is the x - deleted ε - neighbourhood of x
Suppose that x is an accumulation point of a set S. Then N * ( x; ε ) ⋀ S ≠ / Ø
Now consider any y such that y ∈ N * ( x; ε ) ⋀ S. Since N * ( x; ε ) ⋀ S ⊆ S \ {x} ,we know that S \ {x} ≠ Ø .
Since y ∈ N * ( x; ε ) , 0 < | y - x | < ε , which means | y - x | < ε for any y. Now consider any sequence of y ∈ N * ( x; ε ) and call this ( s n ).
For every n > N, where N can be any number of the index, |( s n ) - x|< ε which means ( s n ) converges to x.
Conversely, suppose that there exists a sequence ( s n ) of points in S \ {x} that converges to x .
Then for ∀ ε >0, ∃ N such that ∀ n>N, it is the case that |( s n )-x|< ε . But this is the ε - neighbourhood of x, so that ( s n ) consists of all y such that y ∈ N * ( x; ε ) ⋀ S . It follows then that N * ( x; ε ) ⋀ S ≠ Ø , and so x is an accumulation point
I attempted to prove this theorem, but just wanted to see if it a valid proof.
Thanks!
1. Homework Statement
Prove that x is an accumulation point of a set S iff there exists a sequence ( s n ) of points in S \ {x} that converges to x
Homework Equations
N * ( x; ε ) is the x - deleted ε - neighbourhood of x
The Attempt at a Solution
Suppose that x is an accumulation point of a set S. Then N * ( x; ε ) ⋀ S ≠ / Ø
Now consider any y such that y ∈ N * ( x; ε ) ⋀ S. Since N * ( x; ε ) ⋀ S ⊆ S \ {x} ,we know that S \ {x} ≠ Ø .
Since y ∈ N * ( x; ε ) , 0 < | y - x | < ε , which means | y - x | < ε for any y. Now consider any sequence of y ∈ N * ( x; ε ) and call this ( s n ).
For every n > N, where N can be any number of the index, |( s n ) - x|< ε which means ( s n ) converges to x.
Conversely, suppose that there exists a sequence ( s n ) of points in S \ {x} that converges to x .
Then for ∀ ε >0, ∃ N such that ∀ n>N, it is the case that |( s n )-x|< ε . But this is the ε - neighbourhood of x, so that ( s n ) consists of all y such that y ∈ N * ( x; ε ) ⋀ S . It follows then that N * ( x; ε ) ⋀ S ≠ Ø , and so x is an accumulation point