# Limits and Limit Superior proof

• Jan 25th 2010, 03:10 PM
kingwinner
Limits and Limit Superior proof

By definition,
a_n->a iff
for all ε>0, there exists an integer N such that n≥N => |a_n - a|< ε

[note: also under discussion in Math Links forum]
• Jan 25th 2010, 08:11 PM
Drexel28
Quote:

Originally Posted by kingwinner

By definition,
a_n->a iff
for all ε>0, there exists an integer N such that n≥N => |a_n - a|< ε

[note: also under discussion in Math Links forum]

What is the definition of supremum?
• Jan 25th 2010, 10:56 PM
kingwinner
Quote:

Originally Posted by Drexel28
What is the definition of supremum?

After some more thought, I proved that 1) is true using proof by contradiction. (I always forgot to use the fact that it's the LEAST upper bound). Then taking limits, I got the result because the inequality holds for ALL epsilon>0, so we can get rid of the epsilons.

How can I prove the other direction? (I am trying to modify my proof but I got stuck)
Given ε>0.
a(n)->a => there exists N1 s.t. if n≥N1, then a(n)>a-ε. <-----this is true for sure
b=limsup b(n) => there exists N2 s.t. if n≥N2, then b(n)> ???