Question: Let {an} be a sequence and L  is any real number. Prove that {an} =1 converges to L  if and only if every monotone subsequence of {an} converges to L .

Elements Of Modern Algebra
8th Edition
ISBN:9781285463230
Author:Gilbert, Linda, Jimmie
Publisher:Gilbert, Linda, Jimmie
Chapter2: The Integers
Section2.2: Mathematical Induction
Problem 55E: The Fibonacci sequence fn=1,1,2,3,5,8,13,21,... is defined recursively by f1=1,f2=1,fn+2=fn+1+fn for...
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Question: Let {an} be a sequence and L  is any real number. Prove that {an} =1 converges to L  if and only if every monotone subsequence of {an} converges to L .

 

Tip:  prove the contrapositive by using a) in image, and the Bolzano-Weierstrass Theorem.

(a) Let {an} be any sequence and L any real number. Prove that if {a,} does
not converge to L, then there exists an e> 0 and a subsequence {a} such
that an - L > e for all k.
Tip: Start by writing down what it means that {am} does not converge to L,
i.e. write down the formal negation of the statement {a,} converges to L. Use
this formal negation and induction on k to construct an increasing sequence of
natural numbers {ng} such that |an - L| > e for all k.
3D1
Transcribed Image Text:(a) Let {an} be any sequence and L any real number. Prove that if {a,} does not converge to L, then there exists an e> 0 and a subsequence {a} such that an - L > e for all k. Tip: Start by writing down what it means that {am} does not converge to L, i.e. write down the formal negation of the statement {a,} converges to L. Use this formal negation and induction on k to construct an increasing sequence of natural numbers {ng} such that |an - L| > e for all k. 3D1
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