# Least Upper Bounds

by elizaburlap
Tags: least upper bound
 P: 8 1. The problem statement, all variables and given/known data Find the least upper bound and greatest lower bound (if they exist) of the following sets and state whether they belong to the set: a. {1/n:n$\in$"Natural Number"} b. {x$\in$"Rational Number":0≤x≤√5 c. {x irrational:√2≤x2} d. {(1/n)+(-1)n:n$\in$"Natural Number"} 2. Relevant equations Not applicable. 3. The attempt at a solution a. least upper bound does not exist; greatest lower bound is 1 and does belong to the set b. least upper bound is √5 and does not belong to the set; greatest lower bound is 0 and does belong to the set. c. least upper bound is 2 and does not belong to the set; greatest lower bound is √2 and does belong to the set. d. I am not sure about this one, I don't know what the graph would look like. Am I getting the right idea here? Any ideas for d.? Thanks! 1. The problem statement, all variables and given/known data 2. Relevant equations 3. The attempt at a solution
 HW Helper P: 3,220 For a. Is 1 <= x, for all x in your set? Revise the definitions of "greatest lower bound", i.e. "infimum".
 P: 8 Well, I thought it was, because x has to be a natural number.
HW Helper
P: 3,220
Least Upper Bounds

 Quote by elizaburlap Well, I thought it was, because x has to be a natural number.
Unless I'm missing something, n has to be a natural number. Numbers of the form 1/n, where n is natural, are rational numbers.
Math
Emeritus
Thanks
PF Gold
P: 39,682
 Quote by elizaburlap Well, I thought it was, because x has to be a natural number.
??? There is NO mention of an "x" in problem (a). There is mention of a natural number, n, and the problem talks about the number 1/n for each n:
if n= 1, 1/n= 1
if n= 2, 1/n= 1/2
if n= 3, 1/n= 1/3
if n= 4, 1/n= 1/4
...

I strongly recommend that you write out at least a few of the numbers in each problem.
 P: 8 This is why I thought that the greatest lower bound was one. Because n=1, 1/n=1. as far as I know n cannot be smaller than one as a natural number. Doesn't this make 1 the greatest lower bound??
HW Helper
Thanks
PF Gold
P: 7,717
 Quote by elizaburlap This is why I thought that the greatest lower bound was one. Because n=1, 1/n=1. as far as I know n cannot be smaller than one as a natural number. Doesn't this make 1 the greatest lower bound??
As Halls mentioned, ##S = \{1,1/2,1/3,\, ...\}##. Plot a few of those points on the ##x## axis. Then remember that a greatest lower bound of a set is at least a lower bound. Then ask yourself if 1 is a lower bound for ##S##.
 P: 8 Okay! Thanks!
HW Helper
Thanks
PF Gold
P: 7,717
 Quote by LCKurtz As Halls mentioned, ##S = \{1,1/2,1/3,\, ...\}##. Plot a few of those points on the ##x## axis. Then remember that a greatest lower bound of a set is at least a lower bound. Then ask yourself if 1 is a lower bound for ##S##.
 Quote by elizaburlap Okay! Thanks!