- #1
FaroukYasser
- 62
- 3
Homework Statement
Prove that if a bound sequence ##\left\{ { X }_{ a } \right\} ## is divergent then there are two sub sequences that converge to different limits.
Homework Equations
None.
The Attempt at a Solution
Ok so I am not sure if my attempt for a solution is correct or not, but I have no ideas except this one.
Since ##\left\{ { X }_{ a } \right\} ## is bound, Let ##x## be the highest lower bound and ##y## be the lowest upper bound.
##\Longrightarrow \quad x<\left\{ { X }_{ a } \right\} <y\quad ,\quad x<y##
Next, Let ##\left\{ { X }_{ \alpha _{ k } } \right\} ## be a sub sequence of ##\left\{ { X }_{ a } \right\} ## where
##\frac { x+y }{ 2 } <\left\{ { X }_{ \alpha _{ k } } \right\} <y\quad and\quad { X }_{ \alpha _{ 1 } }\quad \le { \quad X }_{ \alpha _{ 2 } }\le { \quad X }_{ \alpha _{ 3 } }\quad \le ...##
Since ##\left\{ { X }_{ \alpha _{ k } } \right\} ## is an increasing sequence and is bounded then it converges.
Assume ##\lim _{ x\longrightarrow \infty }{ { X }_{ \alpha _{ k } } } ={ L }_{ 1 }\quad (*)##
Next, Let ##\left\{ { X }_{ b_{ k } } \right\} ## be a sub sequence of ##\left\{ { X }_{ a } \right\} ## where
##x<\left\{ { X }_{ b_{ k } } \right\} <\frac { x+y }{ 2 } \quad and\quad X_{ { b }_{ 1 } }\quad \ge \quad X_{ { b }_{ 2 } }\quad \ge \quad X_{ { b }_{ 3 } }\quad \ge \quad ...##
Since ##\left\{ { X }_{ b_{ k } } \right\} ## is a decreasing sequence and is bounded then it converges:
Assume ##\lim _{ x\longrightarrow \infty }{ { X }_{ b_{ k } } } ={ L }_{ 2 }\quad (**)##
##(*)-(**)\quad =\quad { L }_{ 1 }-{ L }_{ 2 }\quad =\quad \lim _{ x\longrightarrow \infty }{ { X }_{ \alpha _{ k } } } -\lim _{ x\longrightarrow \infty }{ { X }_{ b_{ k } } } ## , Since ##{ X }_{ \alpha _{ k } }>{ X }_{ b_{ k } }## for all ##\alpha _{ k },b_{ k }##, then by the order rule ##{ L }_{ 1 }-{ L }_{ 2 }\quad >\quad 0\quad \Longleftrightarrow \quad { L }_{ 1 }\quad >\quad { L }_{ 2 }##, Therefore the two sub sequences converge to two different limits. Q.E.D
Any idea if what i wrote is correct or not? if not, any idea on how I can approach this?