MHB Supremum and Infimum of $S$: $a < b < c < d$

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$S = \{x : (x - a)(x - b)(x - c)(x - d) < 0\}$, where $a < b < c < d$

This questioned shouldn't be to difficult but would it be best to multiply out?

And how is the $a < b < c < d$ going to affect it?
 
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dwsmith said:
$S = \{x : (x - a)(x - b)(x - c)(x - d) < 0\}$, where $a < b < c < d$

This questioned shouldn't be to difficult but would it be best to multiply out?

And how is the $a < b < c < d$ going to affect it?
What is asked?
 
dwsmith said:
$S = \{x : (x - a)(x - b)(x - c)(x - d) < 0\}$, where $a < b < c < d$

This questioned shouldn't be to difficult but would it be best to multiply out?

And how is the $a < b < c < d$ going to affect it?

Hi dwsmith, :)

It's clear that the set \(S\) contains elements \(a<x<b\) or \(c<x<d\). Otherwise, \((x - a)(x - b)(x - c)(x - d) >0\). That is,

\[S=\{x : a<x<b \mbox{ or }c<x<d\}=(a,b)\cup(c,d)\]

Now I suppose it is obvious as to what is the supremum and what is the infimum. Isn't? :)

Kind Regards,
Sudharaka.
 
Sudharaka said:
Hi dwsmith, :)

It's clear that the set \(S\) contains elements \(a<x<b\) or \(c<x<d\). Otherwise, \((x - a)(x - b)(x - c)(x - d) >0\). That is,

\[S=\{x : a<x<b \mbox{ or }c<x<d\}=(a,b)\cup(c,d)\]

Now I suppose it is obvious as to what is the supremum and what is the infimum. Isn't? :)

Kind Regards,
Sudharaka.

$\text{inf} \ S = a + c$ and $\text{sup} \ S = b + d$
 
dwsmith said:
$\text{inf} \ S = a + c$ and $\text{sup} \ S = b + d$

\(a+c\) may not be a lower bound and \(b+d\) may not be an upper bound. A simple example to contradict your supremum and infimum would be, \(a=1,b=2,c=3,d=4\). Then,

\[S=(1,2)\cup(3,4)\]

Now it is clear that, \(1+3=4\) is not a lower bound of \(S\). \(2+4=6\) although an upper bound for this example is not the least upper bound.

The simplest way to think about this would be to draw the two intervals \((a,c)\) and \((b,d)\) on a real line(Note that, \(a<b<c<d\)) and see what are the upper bounds and lower bounds of \(S\).

Kind Regards,
Sudharaka.
 
A sphere as topological manifold can be defined by gluing together the boundary of two disk. Basically one starts assigning each disk the subspace topology from ##\mathbb R^2## and then taking the quotient topology obtained by gluing their boundaries. Starting from the above definition of 2-sphere as topological manifold, shows that it is homeomorphic to the "embedded" sphere understood as subset of ##\mathbb R^3## in the subspace topology.
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